1. DILETI{MA ZONE RESPONSE AT SIGNALIZED INTERSECTIONS
BY
DON W. LEWIS
A REPORT PRESENTED TO THE GRADUATE COI,TMITTEE OF THE
DEPARTMENT OF CIVIL ENGTNEERTNGTN PARTTAL FULFILLMENT OF
THE REQUTREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING
UNIVERSITY OF FI,ORIDA
Spring L993

2. TABLE OF CONTENTS
I. fntroduction
1. l- Problem Statement
L.2 Project Objectives
1. 3 Summary of The Report
fI. Background Discussion
IfI. Model Development
3.1 fntroduction
3.2 fnput Requirements
3.3 Data Collection
3 .4 ModeL fmplementation
IV. Application of Model using AutoScope
4.I Input Requirements
4.2 Site Selection
4.3 Measurement Technique
4 .4 Results
V. Conclusions
Vf. Bibliography
Vff. Appendix
2
3
4
2.L Introduction 5
2.2 Design Speed and Dilemma Zones 1-3
2.3 Yellow, All-red, and Uniform YeIIow fntenrals L7
2.4 Traffic Signal Controllers 20
2.5 Clearance Intervals and Safety 23
2.6 Vehicle Detection Concepts 27
2.7 Speed Monitoring and Vehicle Classification 35
2.8 Loop Detector Maintenance 40
2.9 Video Surveillance 42
2 . lO Controller Considerations 48
5L
52
59
62
64
65
67
75
80
8L

3. LTST OF FIGURES
Title
1. Yellow Interval Length for Clearing the Intersection, P9 9.
2. Yel1ow Interval Length for Not Entering the intersection
on Red, pg l-0.
3. Minimum Yell-ow Interval for Clearing the Intersection , Pg l-3.
4. Effect of Speed and Distance on the ability to Clear the
Intersection , pg l-5.
5. AII-red Required to Safely Clear the Last Collision Point,
pg 19.
6. Plots of Collision Rate Versus Three Parameters , pg 23.
7 . Intersection Control Components, p9 28 .
8. Construction Details of an inductive Loop , pg 29.
9. Loop Location for Extended-Call Design , pg 32.
10. Loop Location for Non-Locking Controller Operation r Pg 33.
11. Typical Loop PLacement for Speed Measurement , Pg 36.
L2. Speed Actuated Warning Sign, p9 37 .
13 . Detector Layout for Vehicle Classification, p9 38.
L4 . Depiction of Detection and Tracking of Vehicles, pg 42 .
15. Typical AUTOSCOPE-2OO3Setup, pg 44.
16. Location of Phase B Indication on Load Rack, p9 49.
L7 . Saf e Stopping during Cl-earance Interval, pg 52.
18 . Decision Not to Stop During Clearance Interval, pg 54.
L9 . Free Body Diagram of Vehicle Braking, pg 55.
20. Time-Space Relationship for Dilemma Zone Response, pg 58.
2I. Determination of Vehicle Velocity using ETS program, pg 59.
22. Intersection Geometrics , pg 60.
23 . Dilemma Zone Analysis Program FIow , pg 52.
24 . Dilemma Zone Analysis Model, p9 63.
25. Flashing Warning Sign, p9 66.
26. Extended-CaLl Detector Placement, p9 66.
27 . Cross Section of Source/Sensor Pair, p9 67 .
28. Sensor Applied to LED on Load Switch, pg 68.
29 . Optoisolator inside Controller Cabinet, pg 68.
30 . Optoisol-ator Schematic , pg 69 .
31. Data Collection Setup for VIP Technique, p9 70.
32. Observation Point, pg 7L.
33. Data CoLlection Equipment, p9 7I.
34. Camera Angle for Dilemma Zone Detection, p9 72.
35. Creation of a Combined Event File using AUTOSCOPE,pg 73.
36. Playback Analysis of Video Tape with Phase Status r pg 74.
37 . Data Reduction using AUTOSCOPEVideo Image Processing , pg 74.
38. Screen Display of Vehicle Processing, pg 76.
39. Screen Display of Program Output, p9 77 .
LIST OF TABLES
TitIe
1. Coefficients of Friction, pg 57 .
2. Combined Event File Output, p9 78.
I 1

4. DIT,EMMA ZONE RESPONSE AT STGTALIZED TNTERSECTIONS
I. INTRODUCTION
1. I Problem Statenent
Most signal controllers provide for a change plus all-red
clearance interval in their timing plan. The clearance tine
requi-rement is based on a dilemma zone analysis with assumed
constant parameters. Due to the randomness of traffic flow the
actual parameters vary. Because of the limited capability of
conventional vehicle detectors, this fluctuation is usually not
taken into consideration.
The change interval programmed into the controller is based on
one of two l-imiting case scenarios, the clearance or the not
entering criteria. Extensions or holds on the green phase are
provided to minimize the possibility of vehicles entering at the
end of the yefl-ow. Dilemma zone incursions still occur, horuever,
and contribute to accident problems due to the termination of the
green when its maximum green extension is reached.
video image processing (VIP) provides for improved detector
capability without the headaches of installing and maintaining
multiple inductive loop detectors. A scheme for reducing clearance
interval confl-icts without sacrificing the efficiency of the
controller is desirable. A model for controlling the length of the
change plus clearance interval using VfP to provide adeguate
clearance for vehicles entering on the yellow could be beneficial-.

5. L.z proJcot ODtrottr,t*
t
The specific objectives of thG ptrolrct
l,l"__lpply video inagr prom&f
cttlernmazone Lncurslgnr.
*_Develop a model for analyztry tt*iil
clearance internrar baacd dn vi&*c H
* Determine t|" capablfity of a t-ttF$:
dilemna zone incuriioRt.
2

6. 1.3 Summary of the Report
A data collection plan will be presented that incorporates VIP
and optoisolation to allow an in-depth study of controller
operations and individual vehicle response. The results will
include a technique for determining if dilenma zone incursions have
occurred during the operation of the controller. The current
methods of deal ing with problems rel-ated to clearance interval-s
wiII be presented as part of the background discussion.
The objective was to develop a model that could classify, then
anaJ--yzefor possible dilemmas, the vehicles approaching the yellow
and all red cl-earance interval. Tirned observations are to be made
of oncoming traffic near an intersection. Stopping distances for
each vehicle will be cal,culated based on a velocity estimate and
compared to the distance from the stop line
The purpose was to monitor the response of drivers to a
change in the traffic signal from green to yellow phasing.
Using the computer to convert actual field observations into
understandable results, an algorithn would be developed to
consider only the situation in which a driver was making an effort
to stop. The phase being displayed and the individual vehicle
classification were required to perrnit the study. The amount of
braking effort required to stop the vehicle at the stop line would
be compared to the phase being displayed and dilemma zone
incursions recorded. The vehicles would also be tested for
violation of the red light.

7. rT. BACKGROUNDDISCUSSION
2. L Introduction
Clearance intervals are an essential part of traffic signal
timing. They allow a smooth transition between phases so that
approaching vehicles can either clear the intersection without
having a collision or stop before entering the intersection using
a reasonabl-e deceleration rate. To accomplish this function, the
intervals must start with a cLear signal to approaching traffic
that the phase is about to change. The signal used almost
universally is the ye1low (or amber) light.
After the yefl-ow Iiqht is displayed, the clearance interval
may continue with other intervals which, while not being so obvious
to approaching drivers, may improve intersection operation and
safety. The most common of these other intervals is the all-red
interval, in which traffic on both the ending and beginning phases
is simultaneously presented with a red signal.
For clearance intervals to be successful, they must have the
proper duration. fn establishing this length several factors must
be considered. one of these factors is the physics associated with
vehicle stopping. Vehicl-es traveling at a given speed require a
given amount of space and time to stop depending on when and how
strongly deceleration is appfied. Maximum deceleration depends on
road, tire, and brake conditions. ft is also necessary to
consider, however, that drivers do not want to use maximum
decel-eration and prefer a value that is more comfortable.
Drivers also have finite reaction times so that their

8. deceleration can not begin innediately upon receiving the
appropriate stimuLus.
A second factor in establishing clearance interval length is
the law that mandates driver behavior when facing a clearance
interval-. This law should be consistent with the physics of
driver/vehicle behavj-or so that drivers do not find themselves
faced with the problem of either being inadvertent criminals or
having to take an unsafe action.
Possibly the most important consideration for setting the
Iength of the clearance interval is what the driver has come to
expect the clearance interval will be. In the United States the
use of the yell-ow light as part of the cLearance interval is
unequivocally required in the Manual on Uniform Traffic Control
Devices (1):
A steady CIRCULAR YELLOW indication:
tt. Shall- be given following a CIRCULAR GREEN
indication in the same signal face.
Two possible ways of choosing the length of these intervals
are implicit in two of the types of laws regarding driver behavior
at yellow lights. One of the older types of laws can be called the
clearance criterion. ft specifies that vehicles cannot enter the
intersection on the red liqht and any vehicles that did enter the
intersection during the yellow, must have completely cleared it by
the tirne the red l ight comes on. This is stated in the L944
Uni f orm Vehicl-e Code (2) .

9. The more recent Law can be c?Iled the not-entering
criterion. It specifies, more sinply, that when the light turns
red, vehicles cannot enter the intersection. This is the current
standard specified in the Uniform Vehicle Code. These laws
make it j-ncumbent on traffic engineers to provide yellow intervals
of sufficient length that will a1low drivers to comply with the
laws . The yef l-ow interval length f or the clearance criterion is
given below as Equation 1.
Y : (S + W + L ) / V t1l
Where:
Y _ Yellow interval length (s)
V _ Approach speed (fps)
W _ Intersection width (ft)
L - Vehicl-e length (ft)
S - Distance reguired to stop using a
comfortabLe deceleration rate. (ft)
The parameter w in Equation L can be measured for a given
intersection and representative values of L and V selected. To
complete the calculation of the yell-ow interval length, it is only
necessary to know the stopping distance S. There have been a
number of empirical studies of this distance. One approach has
been to observe the relationship between the probability of vehicle
stopping and its velocity and distance from the intersection when
the yellow light appears (3) .
These tests were conducted using a vehicle runway simul-ator
(simil-ar to a f l ight simulator) . The authors found that for the
same speed and distance from the intersection, road drivers

10. were more reluctant to stop than the test runway drivers. ft was
felt that this difference was due to the runway drivers being more
attentive and Less motivated to proceed than were the road drivers.
Therefore, it was concluded that basing a yellow interval on the
probability of a vehicle stopping will give too low a yetlow
interval length.
In determining the stopping distance S, it is possible to
proceed f rom the f ol-lowing two assumptions :
* Drivers have a reaction time Rt to noticing the
beginning of the yellow Iight.
* They then decelerate at a constant and comfortable
deceleration rate a.
The stopping distance can then be calculated from kinematic
equations and this relationship for S can be inserted into the
equation for yellow interval length as shown in Figure 3. This
equatj-on is currently recommended by the Transportation and Traffic
Engineering Handbook (4) for the total (yellow plus all-red)
clearance interval. The Handbook further recommends the following
parameter val_ues:
Rt : 1.0 seconds
a_10 feet per second squared
L = 20 feet
Figure 1 shows the terms of this eguation on a time-space diagram
using a yellow interval- length for clearing the intersection
assuming constant deceleration.

11. (w+L)/v
o
E
tr ComfortableStop
y vl2a
Rt
Non ,7'
Stopping
<- Position
L w v'lza
I p)Rt
|.-
S
)
(v)v
risure 1
::ll:: ,
-l:?il:t""""":?t?";:"?1""'ins the rntersection
There have been criticisms that making the clearance
interval Iong enough for non-stopping vehicles to clear the
intersection is overly conservative. Michigan reguires only that
the rear of the vehicle be past the nidpoint of the intersection
(s). Knoflacher (9) and Williams (7) suggest that when the
clearance interval- ends, a non-stopping driver must be at a point
that will allow him to just pass the farthest collision point
before a cross-street driver arrives. The amount of reduction
in the clearance interval time that this allows depends on
intersection geometry and the behavior of the crossing traffic.

12. Stopping
€- Position
r----------tl l
t l
t-_._.-.-._.-.-._._._i
Figure 2 YeIlow Interval Length for Not Entering the
Intersection on Red
Source: Reference 8, Page L2.
The second criterion for setting the yellow length insures
that drivers can either enter the intersection before the light
turns red or else stop before entering with a comfortable
deceleration rate. To derive an equation for the clearance
interval, consider the time-space diagram of Figure 2. As can be
seen
v(Y) - s
Y : S/V
9
or

13. Stopping distance equations gan be inserted into the above
equation to get a relationship for the yellow interval that is a
function of speed as shown in Equation 2.
S_Rt(V) +Yn2/2a
Dividing by V:
Y _Rt+V/2a L2)
This equation is mentioned in the Transportation and Traffic
Engineering Handbook (4) as satisfying one of the purposes of the
cl-earance interval:
rrTo advise motorists that the red interval is
about to commence and to permit the motorists to come
to a safe stop.tl
The handbook, however, states that since the other purpose is to
aIlow vehicles that entered on amber to clear the intersection, the
total (yellow plus all-red) clearance interval length should be
calcul-ated f rom the complete equation.
Although appearing less conservative than the clearance
criterion, a number of arguments can be made for the not-entering
criterion . Pr j-marily, it is not necessary for drivers to have
cleared the j-ntersectj-on in order to be out of danger of a
collision. They need only be beyond the point which will allow
them to clear the last collision point before the arrival- of the
cross street traffic.
Secondly, it can be argued that what a driver who is faced
with a yellow light wants to know is when that light will turn red,
as concluded on page L4, VoLume 2, of the FHWAreport on Clearance
fnterval Studies (B). The not-entering criterion a1lows the
10

14. driver to estimate this tlno ncrrG.r€ffil.ilffif'
clearance criterion. Finally, lt can bc
rs a reasonable on since it gorkcl ft tr
jurisdictions (6) without an Lnordtnttr
accidents.
t1

15. 2.2 Design Speed and Diienma Zones
The two criteria discussed previously use the single speed V
in the equations for clearance interval lengrth. But, since
approaching traffic has a range of speed, traffic engineers must
choose a design value that is safe for virtually all drivers. The
task of sel-ecting a design speed for the clearance criterion can be
inferred from Figure 3, which shows a graph of the clearance
interval equation as a function of speed. As can be seen, there is
a point of minimum yelIow, which is arrived at from setting the
derivative equal to zero.
%=Rt .il?g
g$*
Rt
vo=Ea(,rru
ApproachSpeed- v
Figure 3 Minimum YeIIow Interval for Clearing the intersection
Source: Reference g, page L6.
I
6
-
oF
s
=
-gE
o
yoF[ "nu
2(w+t-;
L2

16. The figure shows that for speeds higher than Vo, the required
cl-earance interval increases with increasing approach speed. For
speeds Less than Vo, however, the required clearance interval
incre'ases with decreasing speed. To select a design speed,
therefore, it is necessary to Look at both ends of the speed range
and choose the speed that corresponds to the higher clearance
interva I .
Even if a conservatively selected design speed is chosen, that
value is correct only for those vehicLes going exactly at that
speed. To determine the effects of vehicles going at a higher or
Iower speed, the clearance criterion eguation can be shown on a
graph of distance from the intersection and velocity when the light
turns yellow as shown in Figure 4.
on the graph the cl-earance interval equation is a straight
Iine, the slope of which is the clearance interval length Y. Also
shown on the same graph is the stopping distance equation. The two
equations partition the graph into severar regions.
Approaching vehicles that have a position and velocity that
I ie on or above the stopping distance eguation can stop before
entering the intersection at a comfortable deceleration rate. As
shown in the figure, there may be an area of overlap where drivers
can either successfully cl-ear the intersection or comfortably stop
before entering it. The name tfoption zonetr has been given to this
region ( 6) . There are also two regions on the graph where
approaching vehicles can neither clear the intersection before the
t-3

17. ._P8
f"of
Can
Stop
tr'igure 4 Effect
fntersection
Source:
DifemmaZone
end of the clearance interval nor stop before entering the
intersection. These regions have been called ftdilemma zonesrr or
rrcritical sectionsrr (3) .
Exactly what proportion of approaching vehicles will be in
each zone depends on the speed distribution and arrival pattern of
the vehicles. As the clearance interval length (slope of the line
in Figure 7) increases, the size of the option zone increases and
size of the dilemma zones decreases. With decreasing clearance
interval- Iength, there eventually comes a point where the clearance
interval line just touches the stopping distance cun/e and no
Ch
I
o
c
=
CL
oF'
CD
E
oL
IL
o
o
c
(6
f,
U'
i5
can
Clear
DifemmaZone Velocity- v
of Speed and Distance on the Ability to Clear
Reference I, Page l-8.
L4

18. option zone remains. The clearancg intenral length and velocity at
this point are those at the same minimum clearance internral shown
in Figure 4 - As the clearance interrral is decreased further beyond
this point, it is possibte for vehicles traveling at any speed to
be in a dilemma zone, depending on their position when the light
turns yeflow.
Because the design speed is usually based on a certain
percentile of the speed distribution, there are bound to be some
drivers caught in the dilemma zone. This situation is not
automatically hazardous since drivers can usually see the
opposing traffic. rf a driver decides not to stop, waiting
cross street traffic should be able to delay their start
accordingly.
L5

19. 2.3 YeIIow, AII-f,€d and Unifom Yellow Intenrals
An important aspect of signal timing is determining what part
of the clearance interval should be yellow and what part (if any)
should be al-I-red. To provide adequate clearance, the yellow
length could be sel-ected from any reasonable value as long as the
total clearance interval was adequate. It is possible for a
yellow interval- to be longer than necessary for a given
intersection.
In states where the law reguires the driver to have cleared
the intersection before the light turns red, if the yellow is
chosen according to the equation, and if drivers stop when they
comfortably can do so, then no all-red interval should be
necessary. In reality, drivers in certain locales will not
necessarily stop at a yellow light, just because they can do so
comfortably. fn those instances, it may be necessary to add an
al-l-red interval to achieve the necessary level of safety.
However, there may still be a clearance problem at wide
intersections, dt strangely shaped intersections, if the crossing
traffic tends to arrive just as it light turns green, or if the
crossing traffic is particularly aggressive. The problem is made
worse if many main street drivers enter near the end of their
yeflow (B). The Transportation and Traffic Engineering Handbook
(4 ) impl ies that the al-L-red interval should be the dif f erence
between the cLearance criterion and the selected yellow interval.
The Handbook adds that the yellow interval should be in the range
L6

20. of 3 to 5 seconds and that 2 to 3 seconds are appropriate for all-
red intervals.
A more precise procedure is obtained when non-stopping drivers
are only required to clear the last collision point before the
arrival of cross street traffic (7). The equation is defined
below.
A = Necessary all-red interval length
A - TL T2 dY = (x1 + L)/v T2 dY t3l
Where:
Xl: The distance on the main street from the stop line
to the farthest collision point (ft)
L : Vehicle Length (ft)
V _ Speed of main street vehicle (fps)
T1 _ Time for main street vehicle to travel from its stop
Iine to the farthest collision point (s)
T2 : Time for cross street vehicle to travel from its
stop line to the farthest collision point (s)
dY - The part of the yellow interval which vehicles will
not use to enter the intersection (s)
The tirne-space diagram developed by Knoflacher (9) and Williams (7)
is shown in Figure 5. The parameters should be determined from
local observation. The worst cases are when vehicles tend to use
aII the ye1low and cross street vehicles arrive in a platoon as
their light turns green.
L7

21. s-Gollision
Point
Position
e'"
ime
trigure 5 A1l_red Required to safely clear The Last collision
Point
E6
V O
Q 6
oo - =
o ( t '
6=
(u
eo
c
(6
c
o,
6
1,8

22. 2.4 Traffic Signal Controllers
The capabilities of traffic signal controllers must be
considered in setting clearance intervals since controllers are
linited in the values that can be set on then and not all interval
lengths are possibLe. The state-of-the-art controller in use in
the United States today is the digitally timed actuated controller.
The older pretined controllers must have aII signal intervals in
integral percent multiples of the signal cyc1e. Adding an all-red
interval to an operating pretimed controller requires no expensive
o '
equipment. But, two percent is the practical minimum the
controller can handle. This may be considerably higher than desired
for an all-red interval.
DiqitaIIy timed actuated controllers generally have sufficient
f lexibil ity to meet aII cl-earance interval requirements. The
rang,es of amber and all-red interval lengths are guite broad and
the intervals can be changed in small increments: one quarter
second on some controllers and 0.1 second on others (8).
one of the great advantages of traffic actuated signals is
that they can change signal phases when there are no drivers
present at the critical section of the approach. One strategy for
accomplishing such a phase change is placing a vehicle detector at
the beginning of the critical section which will hold the signal
green long enough for the vehicLe to reach the end of the critical
section. At that time, the light can turn yellow if no other
vehicle has actuated the detector in the interim (8).
19

23. As noted previously, the clearance interval problem is
complicated by the fact that not all approaching vehicles have the
same speed. one approach in dealing with this problem is to place
a series of detectors at various fixed Locations based on approach
speed and assign an extension for each detector that is just long
enough for a vehicle going at that speed to reach the next detector
t:
( 10)'. This concept is called rrshifting presence zone detect j-onrr.
The spacing between detectors decreases closer to the intersection
so that stopping vehicles will allow the extension to run out.
Another way of taking varying speeds into consideration
requires a single speed detector (11). The detector is located
sufficiently far from the intersection so that it is beyond the
acceptable stopping distance for most drivers. When an approaching
driver crosses the detector, the time it will take him to reach
botlr the beginning and ending of his critical section is estimated
by a special controller. A hold is placed on the main street green
during the interval of time that the driver is expected to be in
his critical section. Either before the vehicle reaches the
beginning of its critical section or after it has left the end of
it, the light can turn yeflow, provided no other vehicle is
determined to be within its critical section.
one Iimitation on the ability of traffic actuated signals to
terminate a green phase when no vehicles are in the critical zone I
is that the signals must have a maximum tirne limit. When this
time limit is reached, the light turns yellow irrespective of the
Iocation of approaching vehicles. The abrupt effects of an
20

24. absolute maximum can be mitigated somewhat by the ttgap reductionrl
feature of density controllers. This feature allows the vehicle
interval to be reduced as a function of tine, depending on the
number and waiting tirne of cross-street vehicles. But with a
shorter interval, the probability of a driver being caught in the
Ieading portion of the critical section increasesi nevertheless,
until the maximum green tirne is reached, the signal will be
protected from changing to yellow while vehicles are in the
position of the critical section farthest from the intersection
(g).'
2L

25. 2.5 Clearance fntenrals and Safety
There have, unfortunately, been only a few studies relating
yellow and all-red interval lengths to safety. As a result,
traffic engineers use different eguations for calculating clearance
intervaL length assuming they are being safe. The results of one
cl-earance interval safety study are graphed in Figure 6. The
figure shows that as the ratio of reconmended interrral length to
actual clearance increases the combined collision rate decreases.
From these plots, however, it is clear that collision rate is
1.80
1.60
1.20
1.00
0.80
g
(E
E
c
.o
.o
o
()
E
o
g
.cI
E
o
o
0.90 1.00 1.10 35
Ratlo,Rocommended/AdnalClearance
Figure 5 Plots of Collision
Source: Reference
4045 50556065 45 5055
GrcssS1r€etWldh(O npploeOSpeeO(tps)
Rate Versus Three Parameters
28 , Page 393.
most strongly related to cross-street width, linking the rate to an
22

26. obvi-ous ftexposurefr measure (11) . . Furthermore, as the approach
speed increases the coll-ision rate inversely decreases. Obviously,
higher speed vehicles spend less time exposed to cross street
traffic than slower ones.
Traffic engineers have different opinions about long clearance
interval-s. A majority seems to feel that long clearance intervals
are abused by drivers. For example, the Transportation and Traffic
Engineering Handbook (4) states:
ItOn the general assumption that excessively
short or long yellow intervals encourage driver
d j-srespect, common practice sets yellow periods
between 3 and 5 seconds. When y exceeds the value
selected for the yellow interval and hazardous conflict
is likely, drr all-red clearance interval could be used
for 2 to 3 seconds between the yellow interval and the
start of green for opposing traf f ic. rr
A supporting opinion about the all-red interval is that the
all--red is helpful , but that rr . . . the known presence of the aII-
red encourages intentional abuse of the red signal by some drivers
on occasions when it would have been both possible and safe for
them to have stopped (11) .tt A study was conducted that would tend
to support the view that excessively long cl-earance interval-s are
abused. ft showed that the number of drivers entering the
intersection after the green Iight terminated increased with the
length of the total clearance interval (11).
The State of Michigan (13) and several others (9) made studies
of all--red intervals at locations where right-angle collisions were
evident. The study Iocations generally had either high approach
speeds or poor visibility. Right-angle collisions were found to be
substantially reduced by adding an aIl-red interval, but rear-end
23

27. accidents were increased slightly. The total number of collisions
was reduced ten percent which proved to be statistically
significant. More importantly, the total number of persons injured
was reduced 28 percent.
Based on these statistics, assumed accident costs, and the
assumption that all-red intervals increase delay by an average 2
seconds per vehicle, the following conclusions were made:
* Colfisions can generally be reduced at high volume
l-ocations by the installation of all-red intervals.
* Indiscriminant use of the all-red interval is not
economically justified.
The Michigan Department of Transportation recommended as a criteria
for the use of the all-red interval that there be a minimum of
eight reported collisions , of types susceptible to correction, in
a 12-month period
In contrast, the city of Portland (14) had used the all-red
clearance interval at aII of its signalized intersections for over
15 years. fncreasing traffic demand, that was exceeding capacity
at many locations, led to Portland t s studying whether the cycle
tine consumed by the aIl-red interval might better be used as green
time. The cit.y was also concerned that its policy dif fered from
that of the surrounding county and state which generally did not
use the aII-red interval. The all-red interval was removed from 20
rocations for one year with the followingr results:
* fn the central business district (CBD) where volumes
were exceeding capacity during peak hours, intersection
volumes were increased without an increase in collisions.
24

28. * At locations with high volumes and higher approach
speeds than in the CBD, the volumes and collisions
increased proportionally.
* At isolated locations where capacity was seldom reached
and speeds were high, volumes barely increased and
collisions increased noticeably.
The city of Portland felt that although intersection volumes
were increased, the increased probability of collisions justified
continuing the all-red interval at all city signals. Other safety
aspects of clearance interval lengths are expressed as a concern
that waiting cross street drivers tend to start too early ( ttj ump
the gun") when al-I-red intervals are used (14) and that short
yellows can l-ead to rear-end collisions on the'cross street when
waiting vehicles start up (they have to suddenly stop to allow
vehicl-es on the ending phase to clear the intersection) (5).
Waiting pedestrians may also ttjump the guntt during all-red
interval-s (14).
There are different philosophies for setting the lengths of
yellow intervals. A11 criteria have their linitations. The
clearance criterion often leads to unnecessarily long yellows and
may make the driverr s decision more difficult. The not-entering-
on-red criterion may not always provide sufficient clearance. The
uniform yellow is a compromise of a wide range of infersection
situations. Excessively long yellow intervals definitely are
hazardous. AII-red intervals are known to reduce collisions but,
the Iength and use of the j-nterval has not been firmly established.
25

29. 2.6 Vehicle Detection Concepts
Traffic control- deals with movements of vehicles (and
pedestrians) . The volume of these movements is usually not
constant with time and often fluctuates from minute to minute,
It is desirabl-e to detect (sense) a movement by placing one or more
detection devices (sensors) in the vehiclers path.
At an individual- intersection that has an actuated traffic
signal, a vehicle approaching the intersection passes over (or
under) a sensor. The detector unit (sensor electronics) responds
by sending an actuation (output) into the controller unit, which in
turn either extends the green for that vehicle or else brings the
green to it, at the earl-iest opportunity. This action by the
controller affects traffic movement.
In applications other than intersection control, detectors can
acquire data for offline analysis, perhaps at some later date, for
evaluation of the effectiveness of control strategies on either
streets or freeways. AII detectors operate on one of two basic
principles. The vehicle or pedestrian may close the contacts of a
pressure-sensitive switch by exerting a mechanical force.
Al-ternatively, the vehicle I s motion or mere presence causes a
detectable change in an energy pattern. The inductive loop
detector (fLD) shown in Figure 7 is the most widely used detector
and is an example of an energy pattern-change detector (16).
The National Electrical Manufacturer t s Association (NEMA)
defines a detector as rra device for indicating the presence or
passoge of vehicLes or pedestriansrr (15) . Traff ic detectors may be
26

30. CulbsldeEqulpmenl
Figure 7 Intersection Control Components
applied either singly or in rnultiple installations, to measure
presence, volume, occupancy and speed. These surveillance measures
can be used as control parameters at an individual signalized
intersection, or in a coordinated traffic-responsive signal system,
or for freeway operations.
"
The fLD began to be used in the early Lg60 | s. Like a radar
detector, it uses principles of electromagnetics. The loop
consists of one or more turns of insulated wire wound in a shallow,
rectangular sl-ot sawed in the roadway as drawn in Figure 8. At
curbside the two ends of the wire are carefully spliced to a
factory-twisted and shielded lead-in cable that is led to an
27

31. intersection cabinet housing the unit (16).
Figure 8 Construction Details o an fnductive Loop
The detector unit, lead-in wire and loop wire comprise a tuned
circuit of which the loop is the inductive element. A vehicle
entering the loop will absorb some of the radio frequency energy
because of eddy currents created in the metal chassis. The
inductance is reduced, causing the resonant frequency to increase.
At this point, various designs of ILD electronics process phase,
frequency, amplitude or impedance changes to actuate the detectorfs
output relay (16).
The ILD electronics unit is quite inexpensive, selling for
about a hundred dollars per channel of detection (16).
13.
electronics
II
I
I
RoadwaySection
LoopSlotPlan
28

32. Installation requires traffic to be disrupted and pavement to be
cut. ILD detection is very flexible and can be highly dependable.
The zone of detection also can be varied widely. The ILD is
currently the most popular detector for individual intersections,
signal systems, and freeway surveilLance.
Approaches experiencing speeds of 35 nph or higher are
considered high-speed approaches. f f the yell-ow is displayed when
the vehicle is in a rrdilemma zonett, it may be dif f icult for the
driver to decide whether to stop or go through the intersection as
mentioned earlier. An abrupt stop may produce a rear-end
collision. A decision to go through on the red may produce a right
angle accident. The traf f ic engJ-neer can install a vehicle-
actuated signal controll-er and appropriate detection in an atternpt
to minimize the untimely display of the yellow (lG).
A variety of schemes have been devised for controllers with
locking and non-Iocking detection memory, basic and volume-density
controll-ers circuitry and various approaches to detection. ft is
important that the first detector be placed far enough upstream to
give adequate dilemma zone protection. Attempts to improve the
design in this respect result in an a1lowable gap that is long
enough that the controller would frequently |tmax outrf . This is
unacceptable as a vehicle may well be caught in a dilemma zone if
the green is extended to the maximum interval.
The provision of dilemma zone protection is based on the
detectorrs connected to an auxiliary logic unit that holds the
controll-er in Phase A, until the approaching vehicle had cleared
29

33. the dilemma zone. Such an auxili?ry logic unit is offered
commercially as a 'rGreen Extension Systemtr ( 15) . This system
consists of two or more auxiLiary timers that can disconnect (or
forcd-off) the extended-call-ed detectors and auxiliary electronics
that can monitor the signal display, arm or enable the extended-
call detectors, and control the yielding of the green to the side
street (by activation of hold-in phase circuits) .
The most straight forward conventional design for a high speed
approach uses a "densityt'controller with a single small area
detector at the upstream boundary of the dilemma zone. This scheme
is often used as the baseline design for comparison with new
detector configurations. A density controller is an advanced
actuated model that can count waiting vehicles beyond the first,
because it has a f eature known as trvariable initial interval rf.
Each approach has a small loop at the upstream end of the
dilemma zone, and a small loop calling detector near the stop line
as shown in Figure 9. The upstream detector is located 386 feet
from the intersection, which corresponds to a design speed of 55
mph. As was pointed out earlier, there are shortcomings of this
design with regard to allowable gap. When the green phase ends by
gap-out, slow moving vehicles will not be protected by the density
design if the minimum gap is set 1ow. ff a density design for 55
mph is to also protect the vehicles approaching at only 40 mph the
minimum gap must be increased. However, a longer gap is
undesirable because the green may be well extended by moderate
traffic to the maximum interval, thereby removing the dilemma zone
30

34. protection ( 15)
There are several other weaknesses of this scheme that are
pointed out in References 23 and 24. They are all related to lack
of controLler information about the traffic at the stop line,
Figure 9 Loop Location for Extended-Call Design
because of the lack of a detector at the stop line, and to lack of
control-ler inforrnation for a distance of several hundred feet
upstream from the stop line.
The l-ocation of the calling detector is also important to
these schemes. The loop should be located L0 feet before the stop
line. It should be wired to output calls only during phase red to
prevent locking a caII from a vehicle passing through on yellow.
3L

35. Density controllers usually offer a rrl,ast Car Passageft
feature. rf used, then upon gap-out the signal indication does not
change until the last car has reached the stop line. The next
vehicle, caLled the tttrailing carrr, ildy weII be caught in the
dilemma zone, thereby defeating the objective of the design (15) .
High speed designs using non-locking detection memory always
include a long loop at the stop tine as well as one or more small
detectors upstream as seen in Figure LO. The long loop provides
the controller knowledge of traffic at the stop line, but tends to
increase the allowabl-e gap. Designs for both full-actuated and
density controllers have been devised.
Figure 10 Loop Location for Non-Locking Controller operation
32

36. Basic ful-L-actuated non-locking controllers have been used for
a number of years with an extended-call detector configuration (EC-
Dc) j ust upstream of the dilemma zone. Dif f iculties with rtmax-
outsn under heavy traffic conditions prompted the development of a
design that offered some advantages at no extra cost (L7). At
high speed (55 mph) approaches the upstream detector was located at
the dilemma zone boundary. A middle loop was placed 254 feet
from the intersection, which is the upstream boundary for vehicles
approaching at 35 mph.
The upstream loops utiLtze an amplifier that produces a short
pulse when a vehicle enters the loop, then the unit extension
setting of the controller is selected to be 2.2 seconds. It is
sirnple to show that this setting wiIl carry vehicles approaching
within the speed range through their expected dilemma zones. A
vehicle approaching at 35 mph is also protected because the yellow
wi I I appear bef ore the vehicle has reached its dilemrna zone .
Just as with the minimum gap setting on the density
controller, the stretch setting reguires a compromise. ff only
2.5 seconds are used the result is snappy operation, but poor
protection for slower vehj-cles. f f they are protected by
increasing the stretch time then the green may be extended to
its maxj-muminterval, which, once again, defeats the purpose of the
EC-DC design.
33

37. 2.7 Speed Monitoring and Vehicle Classification
Detectors can be used for other applications besides traffic
detection at signalized intersections. These applications include
speed monitoring, traffic counting of vehicle classification and
for safety applications during sight distance restrictions. This
section j-ncludes descriptions of some of these other applications
that relate to dilemma zones.
Speed monitoring of vehicles can take many forms using
currently available equipment. Of the fLD, pneumatic tubes,
pie zoelectric cabl-es and tapeswitches generally the ILD is
considered the best type sensor (L7). The advantages of inductive
loops for speed detection are as follows:
1. Installed in pavement.
2. Low driver visibility.
3 . No risk of weather or driver damage.
4. Can be used for nulti-lane situations.
5. System accuracy is not compromised by use.
6. Indefinite l-ife.
Its basic disadvantages are:
1. A high initial cost.
2. Lengthy installation time.
3. Longer time delay than tubes or cables.
4. Loop detectors need retuning for accuracy.
The resul-ts of a study ( 18) showed that a cost-ef fective speed
detection system coul-d be built. The system included inductive
loops, commercially avail-able loop detectors, and a small
microprocessor. As a result commercial loop detectors, with minor
modifications, were shown to be capable of speed measurements to
better than plus or minus L mph util :-zing a L6 foot trap as shown
34

38. in Figure 11.
CenterLineofRoad
Edgeof Pavement
Figure Ll Typical Loop Placement for Speed Measurement
For off-the-she1f loop detectors the spacing should be
increased. Detectors and speed determining electronics should be
located near the loops. For speed measurement, digital loop
detectors are preferable to analog detectors. Time requirements
are essentially the difference with the digital detector responding
much faster. If accurate speed is desired, then analog detectors
are better ( 18) .
one application of a speed detector that relates to the
reduction of dilemma zones is the actuated warning sign shown in
Figure L2. Dangerous situations at high speed intersections can be
35

39. STOP AHEAD
WHEN
FLASHING
Figure L2 Speed Actuated Warning Sign
alleviated if the driver is provided with additional information
upon which they can decide their best course of action. In order
to provide this additional information in a manner which will
modify driver behavior the information given the driver should not
be limited to warning the driver of the existence of a signal, but
shoul-d vary according to the indication shown on the signal and the
vehiclers speed (1).
As a part of a dilemma zone analysis it is important to
obtain the speed by class of vehicle. There are presently several
vetiicle classification counters on the market, each with its own
logic. However, most of these recorders use loops and axle
36

40. detectors to obtain the required information to classify vehicles
(18) (1e) (2o) .
Automatic length indication and classification equiprnent was
developed using two loops in tandem, preceding an axle detector,
which was either a pneumatic tube or triboelectric detector (Lg) ,
as shown in Figure L3. The sensor layout uses a simple tine-
Direction
of
Travef
Axle
Detector
Figure 13 Detector Layout for vehicle classification
distance comparator and assumes that the vehicle will traver at a
constant speed while traversing the detection zone. A vehicle
passing through the detection zone is initially detected by roop A
and shortry afterwards by loop B. The time it takes between
actuations is related to the known distance between the loops and
a speed is deri-ved.
37

41. Similar1y, the time between axle actuations is related to
speed and the distance between axles is determined. Vehicle
classification can then be obtained by using a microprocessor to
compare length, wheelbase and chassis height data with a look-up
table stored in memory (19). Other techniques describe vehicle
classification by determining the spacing of the first two axles
and counting the total number of axles (2O) .
fn order to operate signal systems it is necessary to identify
the quantities to be measured and determine where the sensors are
to be located to measure these guantities. Dilemma zone sensors
are normally l-ocated at strategic locations near intersections so
that incursions can be reduced. The selection of the type of
detector, its location and configuration are dependent on the
intersection geometry, design speed and control system.
Because of the flexibility of its design the fLD provides the
broadest range of vehicle detection. Loops are capable of
detecting both presence and passage. Lane occupancy, speed and
volume can be determined from loop detectors. Many think that the
loop is the most satisfactory type of detector. This has been
demonstrated through operating experience based on evaluation of
accuracy and reliability (I7).
38

42. 2.8 Loop Detector Maintenance
The rel iable operation of inductive loop detectors can only
be achieved if the maintenance personnel are capable of making good
judgements at different situations and are backed up by technical
know-how and skills. Extensive research was done on maintenance
information for controllers and detectors in a wide range of cities
and States. The data obtained gives an indication of the types of
failure in equipment, along with its installation and maintenance
requirements (16).
The State of Minnesota indicated that of the 8l-1- loop
detectors that were included in their roads it was found that the
loop detectors averaged O.24 failures per year. The city of
Cincinnati, ohio showed very similar detector failure rates as was
found in the Minnesota data. Detector maintenance over 2 years in
Cincinnati indicated that the pressure detector was significantly
more reliable than any other types listed.
These failure rates are Iow especially when compared to the
data from the city of Winston Sa1em, North Carolina. These data
were extracted from one fully actuated controlled intersection
which was fairfy typical of aII the intersections within the city.
The record of the loop detector maintenance showed 33 trips in 4
years to retune, replace , or cut new loops, for an equivalent of
8.46 calls per year.
Almost 500 loop detectors were installed as part of the UTCS
program in Washington, D.C. The installations were made after
thorough studies by the contractor of available electronics,
39

43. procedures, and materials for installing loop wires and lead-ins.
In the first year there were 33 failures of electronics units for
a rate of o.4O7 failures per detector per year. During that period
26 'Ioops f ailed because of utility excavations; if these failures
are removed the total rate becomes O. L3 failures per detector per
year. As a by-product of the survey, it was found that the digital
Ioop detector proved to be more effective than the analog detector.
New York State found in L978 that digital loop electronics could
operate loops in such poor condition that these locations did not
have to be scheduled for reinstallation.
Even though digital- units have the abil ity to operate under
adverse temperature, weather and road conditions; there are six
primary reasons for loop detector failure:
1. Detector unit failure
2. Utility construction
3 . Poor seal-ants
4. Pavement cracking and movement
5. fnadequate splices or electrical connections
6. Lightning surges
AII of these various items will be addressed by the maintenance
technician when anaLyzing a faulty loop system. The technician
normally can quickly isolate a problem within a loop detector
system. An experienced maintenance technician will often be able
to pin-point the troubled area or even the faulty parts by visual
examination.
Detectors are an important element in the operation of an
actuated traffic signal. Many people think that the detector
the weakest link in the traffic control process. The purpose
this section was to identify types of loop detector failures.
1 S
of
40

44. 2.9 Video Sunreillince
The Wide Area Detection System (WADS) concept seeked to bridge
the gap between fLD ttpointrr sensors (which gave quantitative data
but no picture) and video sunreillance. WADSattenpted to use
tel-evision on a standard coaxial cable together with a micro-
processor to focus on the automatic taking of tfarearr measurenents
across several freeway lanes as well as along the traffic stream in
each lane (2L). A depiction of the detection and tracking of
vehicles technique that was used is shown in Figure L4.
,
Trac*ingof
Vehide
Vehicle
/
Motion
Signature
Vehicle
Shadow
DetectionLine
gu L4 Depiction of Detection and Tracking of Vehicles
In the measurement mode WADShad the potential to determine
Iane density, speed, volume, queues, incidents, etc. As of L982
4L
lDe
a
r.ctureF

45. WADSsoftware had been tested in the field and confirmed to detect,
track, and make traffic parameter measurements for vehicles in one
I ane (2L) .
More recently, AUTOSCOPE2OO3| a wide area vehicle detection
system that uses video irnaging to replace inductive loops in
multiple lanes and multiple directions of traffic, has been
introduced. It accepts inputs from up to four video cameras
overlooking the roadway with the detectors drawn graphically on
a video monitor using a mouse. Different types of detectors can be
selected, and the detection zones may be placed anywhere and in any
orientation within the combined field of view of the cameras.
Special speed trap detectors can extract vehicle speed and vehicle
classifications as defined by length (22) .
AUTOSCOPEis designed for outdoor installation in a traffic
contro1 cabinet. It meets NEMATSL/TS? and L7O/L79 environmental
requirements. ft provides up to 64 detector outputs, which can be
"or,.,".ted
directly to a controller. A system with four suitably
Iocated cameras can replace the inductive detection subsystem of
even a large eight-phase intersection, from the loops to the loop
amplifiers (22) .
This technology offers a long-term and cost-effective solution
to the intersection detection, traffic management, surveillance and
control- concerns. The patented technology has been field tested
since early 1-989, and detection accuracy has been documented to be
equal to that of loops. Pattern recognition algorithms assure
proper performance under challenging Iighting, weather and traffic
42

46. conditions, including dusk,
snow and fog.
nighttlme, shadows, reflections, rain,
1to 4 videocameras
RS-l70input
DetectorUO
tocabinet RS-232
RGBsyncvideo
Monitor
Figure ls Typical AUTOSCOPE-2OO3Setup
The present model- is the evolution of technology developed and
tested by the University of Minnesota Center for Transportation
Studies beginning in L984. Model 2OO2, the predecessor product,
has been commercially available since early L99L as a tool for
traffic research. Model 2003 is the result of a joint agreement
between EconoLite and fmage Sensing Systems, Inc. of St. Paul,
Minnesota for the deveLoprnent of an environmentally hardened
version with detector Level outputs.
As shown above in Figure !5, the video detection system
consists of from one to foursynchronous television camera(s) or
-'l
I
I
' r i
t l
t l
t l
Ylro : vto
FonriFilnfi
i
SuperuisorComputer
withdigitizerboard
43

47. other video source(s), the AUTOSCOPE-2003unit, a supervisor
computer and an RGBvideo monitor. ft may be mounted in a traffic
cabinet or in a control room. once a detector configuration has
been downloaded into AUTOSCOPE,the supervisor computer and video
monitor may be disconnected. It will then operate independently,
providing detector outputs and storing traffic data in its internal
memory.
When the supervisor computer and video monitor(s) are
connected, traffic data may be recorded directly on disk and be
displayed on the computer screen in numeric and graphic formats.
Vehicle detections superimposed on the image of traffic may be
observed on the video monitor(s). The potential benefits of this
system are Listed below:
1. Freedom from loops
2. Visual feedback
3. Site flexibility
4. Powerful and economical
5. Easy to install
6. Designed for growth
Video detection avoids the maintenance expense associated with
imbedded loops, which has been described earlier as the least
rel-iable component of a traffic control system. Video detection
shows the detection zones superimposed on images of traffic. The
detection lines change color on a video monitor, thus providing
visual verification of detector actuations.
Video detection is feasibl-e at sites where it would be
expensive or impractical to install inductive loops and their cable
runs. Video detection aIlows the implementation of advanced
detection strategies which require different detector types and a
44

48. Iarge number of detection zones. . Video is more economical than
inductive loops when the number of detection zones is Iarge. The
typical cost of an inductive loop system for intersection control
is $t,Ooo per loop (22) .
Video detection can be installed without disturbing the
roadway or traffic fl-ow. once the basic hardware is on site, the
detection zones can easily be redefined or repositioned on a video
screen to adapt to changing traffic control or data collection
requirements. Many of the unique vehicle detection and data
capture capabilities are provided by software, which will continue
to evolve and make the system even more versatile in the future.
Three current appfications for AUTOSCOPEare listed below:
1. Intersection Control
2. Traffic Surveillance
3. Special Studies
The traffic data stored internally in its memory may be retrieved
at any time using a modem and dial-up telephone 1ines , a private
cable or fiberoptic network, or direct cable connection to a host
computer. If the data is not retrieved, the o1d data is
automatically overwritten by new data. AUTOSCOPEcan count
vehicles in real- time and compute the average of traffic parameters
over user-defined time intervals (or time slices) as follows:
* Volume
* Occupancy
* Vehicle Cl_assification
* Flow Rate
* Headway
* Speed
The duration of the time interval is selectable from I, S, 10, L5,
30 or 60 minutes. The base memory of 2 Megabytes (MB) allows the
45

49. accumulation of LS-minute time interval traffic data for 48
detection zones over a minimum of seven days.
The camera and lens assembly are housed in an environmental
enclosure that is waterproof and dust-tight to NEII{A-4
specifications to withstand high-pressure hosedown. A LS-watt
heater is attached to the lens of the enclosure to avoid ice or
condensation in cold weather. The preferred camera position is a
minimum height of 35 feet above or immediately adjacent to the
roadway. Accurate vehicle counting, speed measurement and length
classification require a steep viewing angle so as to minimize the
possibility of occlusion (where one vehicle covers another) .
The supervisor computer system may be a laptop or portable
computer that is carried to remote locations as needed for detector
setup and data retrieval, dD environmentafly hardened computer that
is installed in the traffic cabinet to store large amounts of
traffic data r or a desktop computer located in a control room
environment where video signals are processed. The computer may be
purchased from a third party as long as it meets certain minimum
specificatj-ons. The Autoscope Supervisor Digitizer Board however,
occupies one full-s Lze AT-compatible expansion slot.
one of the more flexible aspects of the AUTOSCOPEsystem is
the external interface module (EIM) . Not only does it provide NEMA
compatibl-e outputs, but it also features a detector display that
shows the status of each detector using high-output red LEDs. This
allows for external visual verification of vehicle detection.
46

50. 2.LO Controller Considerations
The controLler assembly is a complete electrical mechanism
mounted in a cabinet for controlling the operation of a traffic
control- signal. A type of traffic-actuated controller assembly in
which means are provided for traffic actuation on all approaches to
the intersection is the fulIy traffic-actuated controller assembly.
A pretirned control-l-er assembly is for the operation of traffic
signals with predetermined fixed cycle lengths, fixed interval
durations and interval sequences (18).
The controller unit is that portion of a controller assembly
that is devoted to the selection and timing of signal displays.
A traffic-actuated assembly is for supervising the operation of
traffic control signals in accordance with the varying demands of
traffic as registered with the controlLer by detectors. Solid
state load switches are connected between the alternating-current
Iine power and traf fic signals. The term trsolid statetr means that
the main current to the signal load is not switched by electro-
mechanical- relay contacts (18). A three-circuit load switch with
its socket is shown in Figure l-6.
Indication is provided and appropriately labeled to facititate
the determination of the operation of the unit. These indications
consl=a of the following:
1. Phase or phases in service
3'.;l:::":: 5iTffii:i:'"5i,.1"'ffiil;"3iuacruarjons
4. Presence of pedestrian caII
5. Interval- tirning and controller condition
47

51. Figure 16 Location of Phase B Indication on Load Rack
For determining dilemma zone j-ncursion, the phase in service
indication can be used in conjunction with a speed trap located
suf f iciently downstream.
The outputs of the controller unit include the basic vehicle,
pedestrian, overlap load switch drivers as well as the phase ON and
NEXT logic. The input functions include the vehicle, pedestrian,
and logic inputs. The advanced two through eight phase solid-state
traffic signal controller units also feature a number of inputs
that allow the operation of the controller to be overridden by
external- relay contacts that cause a LOGfC TO GROUND.
The inputs used to affect the operation of the controller in
48

52. ways that were discussed previously, which can increase or decrease
the possibility of dilemma zone incursion, are listed below:
7. Force-off (per ring)
8. Hold (per phase)
13. Inhibit max termination (per ring)
15. Omit red clearance (per ring)
A force off is a command that will force the terrnination of
the right of way. A hold retains the existing right of sray. The
inhibit maximum termination is an input to disable the maximum
termination functions of aII phases in the selected ring. This
input, however, does not inhibit the timing of ITIAXfMUMGREEN.
Lastly, the omit red clearance causes the omission of RED CLEARANCE
interval timing.
The current state-of-the-art methodology for controlling
dilemma zones is cancel-ed by the MiXfMUMor EXTENSfONLIMfT time
out ( 18) . There is currently no system in place to take advantage
of the inhibit max termination feature. As a result, dilemma zone
incursions stiII occur. Furthermore, the omit red clearance input
is usually preset (on or off) depending on local policy or previous
accident experience.
49

53. f f I. DIODELDEVELOPIIIENT
3.1 Introduction
Most vehicle detection systems used for signal control focus
mainly on the presence of a vehicle, not on the velocity of each
vehicle or its classification. When a vehicle approaches an
intersection the signal may changre to the yellow clearance interval
at a time when the vehicle will not be able to clear the
intersecti-on or stop at the I ine . The dilemma zone is a tirne
interval during which the driver can neither stop safely nor
pass through the intersection without running the red light.
The identification of a dilemma zone reguires the calculation
of stopping distance. This quantity is a function of velocity,
deceleration, and individuaL driver reaction time. One assumption
made in the equation is that the deceleration of each vehicle
does not change. Values have been published by the Association of
State Highway and Transportation Officials (AASHTO) that
standardize reaction times for typical drivers (23) .
The velocity estimate of each vehicle is the most critical
piece of information used in the computation. Braking distance
fluctuates quadratically with small differences of this variable.
Drivers who travel- at high speeds while approaching intersections
when the yellow phase is activated are put in a situation
where they must make a difficult decision. This has led to
accidents reLated to cl-earance intervals ( 11) .
50

54. 3.2 Model Reguirements
To understand the dilemma zone and its relation to the yellow
clearance interval, first we will look at the situation in which a
vehicle can come to a safe stop when given the yellow. In this
case, the vel-ocity of the vehicle is low enough so that its
distance to stop is less than the vehicle I s distance from the
intersection as seen in Figure L7. This quantity is called
braking distance and varies based on the type of braking used (air
brakes, disc brakes, etc. ) and the amount of deceleration that the
driver can withstand.
Vf=0
Safe Stopping during Clearance fntenral
V=Vi
-------s
tl
Figure L7
5L

55. Vf :0_Vi (a)T
T = Yt/a
x : vi^2/2a
Where:
t4l
a = Deceleration (g t s)
Vf - Fina1 velocity (fps)
Vi : Initial velocity (fps)
T : Time to stop (s)
X _ Distance to stop (ft)
The air brakes used on semitrailers and some single axle
trucks in particular do not brake as effectively as disc or drum
brakes, which are found on passenger cars. This means that large
trucks have longer braking distances than automobiles. The dilemma
zone is also highly dependent on the length of each vehicle as can
be seen below in Figure l-8. In order to compute a vehicle I s
braking distance the classification must be determined and used to
adjust the quantities in the equation.
The time allowed for vehicles to clear the intersection
duritr'g the yellow interval is determined by Equation L. However,
if d is less than X then there is a dilemma zone, dD area from
which a vehicle can neither stop safely nor cross completely in the
time al-lowed. In this case the stopping distance may be greater
than the vehicle distance from the intersection and less than this
distance plus the sum of the width of the intersection and vehicle
Iength. The driver t s perception reaction tirne can play a critical
52

56. u'1 Vf-V
DILEMMA
ZONE
=-
X
Figure 18 Decisi-on Not to Stop During Clearance fnterval
role in determining whether a vehicle will be caught in the
critical section.
To calculate the stopping distance of a vehicle at a specific
velocity, a free body diagram was developed to relate the forces of
friction to vehicle deceleration. The work done in braking was set
equal to the kinetic energy of the vehicle and an eguation
invol-ving three unknowns was obtained. After applying the
appropriate conversion factor of L.47 fps/mph and rounding off
to two decimal places, a generalized braking distance eguation
resulted as shown below in Equation 4.
53

57. Figure 19
Force
FrictionForces
Free Body Diagram of Vehicle Braking
Force : Friction forces
work done in braking = Kinetic energy
(r) (wt) (D) = (wt) (1-47v)^2/29
F(D) = V^2/30
v".z = 30(F) (D)
D : V^2/30(F) t4l
Where:
v = Velocity (fps)
V(fPs) = L'47V(mPh)
wt = Vehicle weight
D : Braking distance (ft)
F = Coefficient of friction (g t s)
54

58. The total distance required to stop is more than just
braking distance alone. The driver will first mentally perceive
the change in signal phase, then react and respond muscularly by
pressing the brake pedal. OnIy after these three things have
occurred can the vehicle begin mechanical braking. According to
AASHTO, a one second percepEron/reaction time is the average for
most drivers (23) . The combined ef fect of reaction tirne and
braking distance is shown below in Eguation 5.
Drt : L.47 (Rt) V
Db : Vn2/ 30(F+G)
Ds _ Drt + Db t5l
Where:
Drt _ Distance traveled during perceptLon/reaction
Db : Distance traveled during vehicle braking
Ds _ Total distance traveled
c : Grade (Z) / 100
Rt : Reaction time (s)
V = Velocity (mph)
F : Coefficient of friction (g t s)
The coefficient of friction value in the braking distance
equation is the relative decel-eration rate as compared to grravity.
This val-ue varies greatly based on roadway frictional resistance,
weather conditions, and vehicle velocity. The values in Table L
illustrate the various coefficients of friction used in this
dilemma zone study. The values represented in the table are for
55

59. Coefficientof Friction BrakingType
0.08to0.10
0.30
O.7Oto 0.80
EngineBraking
Maximumfor
Comfort
AsphalUConcrete
0.60
Maximum
Maximumfor
Semitrailer
0.30to0.50 WetCurve
Table L Coefficients of Friction
well worn roadways. Selecting a conservative value for the
coefficient wiII help ensure accurate and reliable results.
The shaded region of Figure 20 illustrates the area in
which a vehicl-e will not come to rest at the stop bar, possibly
blocking the cross street traffic or passing thru during the red
interval. Nonstopping vehicles maintain a linear trajectory
through the intersection since no deceleration takes place. The
stopping vehicles decelerate during the clearance interval and
create the curvalinear trajectory which defines the dilemma zone.
To avoid this, the clearance interval must begin before a vehicle
is positioned closer than its comfortable stopping distance.
56

60. Stoppingvehide
DILEMMAZONE
Nonstopping
vehicle
Distancefromthestopline ------>
tr'igure 20 Time-Space Relationship for Dilemma Zone Response
57

61. 3.3 Data Collection
The data collected during the model development was gathered
using a program called Event Time Series (ETS) . A small sized
portable computer was used for intervals of about an hour to mark
arrival times of oncoming vehicles at mathematically selected
points along the road as seen in Figure 2L. These marks were
estabLished using the 50th percentile perception reaction time (r-
sec) and the posted speed limit.
V=Vi
------*
Vf=0
Vi = Vactual(fps)
[i = Drt/T2 -T1
Phase
Glassification
B
Observation
Figure 2L Determination of Vehicle Velocity using ETS program
The first point that the time lras marked was at the total
stopping distance using the posted speed lirnit. The second point
was located at the shortest distance for a maximum braking effort.
58

62. These two points were chosen becatise when the signal turns yellow
and a vehicle is located between these two distances, a vehicle
wiII not be abLe to stop if its approach speed is higher than is
posted.
NE23rdAvenue
@
Waldo Road
W=lntersectionwidth
w-18ft
Pg!-@
Figure 22 f ntersection Geometrics
The roadway chosen for development of the dilemma zone
model- was Northeast wal-do Road in Gainesville, Florida.
The exposure of a stopped vehicle to cross street traffic
control l ed the va lue chosen f or i-ntersection width . The
intersections operated with leading protected reft turn bays.
The time, date, and location of the study are input
as the first three entries in the ETS program on the notebook
computer' The time intervals were appended into the same file and
stored on a small hard disk. A11 the collected information was
rater copied to a diskette. The ETs textfile was then read
into a database for analysis.
,,-.- fcoffi
r r- ,f[@ Wffi
59

63. Although the points on the road were marked only with spray
paint, the onboard cLock on the computer measured the time
interval to the tenth of a second. This precision provided the
sensitivity needed to accurately compute the speed of each vehicle.
The changes in signal phase were recorded simultaneously with the
vehicle velocities. This combination of information made dilemma
zone incursions detectable.
The posted speed limit, weather, and roadway surface were
observed and used to adjust the coefficient of friction. The
grade of the road was assumed to be zero percent (0?) . An
average velocity was computed and compared to the posted speed.
The average speed was less than the posted speed and this is
understandabl-e since slowing or braking vehicles were sampled.
The oncoming vehicles were not part of a synchroni zed
vehicle progression from an adjacent intersection, therefore
platoons did not usually arrive during the green. The spacing
between intersections was long enough to allow for wide dispersion.
Several more vehicles were seen caught in the dilernma zone than
the technician was able to record because two lanes were sampled
at the same time.
60

64. 3. 4 Model fmplementation
The modeL was developed in the form of a computer program
called Dilemma Zone Analysis. The text file generated by the ETS
program was read into a random access data structure. The arrival
times were preprocessed into a database of vehicle velocities with
the corresponding phase status. This database was then reprocessed
to produce a report and textfile containing each vehicle I s velocity
and braking distance as diagrammed below in Figure 23.
Database
Figure 23 Dilemma Zone Analysis program Flow
DilemmaZone
Analysis
SignalEvents
BOG
BOY
BOR
Vehicle
Classification
lntersectlonWldth
Distanceto stop bar
ReactionTime
Grade
6L

65. After computation of the vehible I s stopping distance based on
comfortable and maximum coefficients of friction. The lengths are
compared to the phase being displayed and the distance to the
intersection. From this information a comment is generated that
describes the position of the driver (i.e. uncomfortable,
comfortable, dilemma, etc. ) when considering the not entering
criterion. The flow chart diagram of Figure 24 shows what
conditions are met to generate each comment.
Figure 24 Dilemma Zone Analysis Model
dbc=F(X,T,Fc)
dbm=F(X,T,Fm)
ThruVehicle
d<dbm<d+w+l
62

66. IV. APPLICATION OF ITiODEIJUSING AUTOSCOPE
4. L Input Requirements
To accurately determj-ne whether or not problems exist with the
clearance interval at a signalized intersection, a microscopic
analysis of vehicle movements combined with the phase being
displayed is required. Vehicles ignoring the red light could be
counted as well as vehicles passing through during the yellow. One
task of this proj ect was to develop a way of combining the
signal events at an intersection with video surveillance to enabl-e
a more detailed study of the clearance interval timing and possibty
decide if an all-red interval is needed.
A circuit would be built that would enable the status of the
light emitting diode (LED) on the load switch to be sensed and
recorded on the audio channel of a video tape. Access to the
inside of the controller cabinet would be gained to observe the
signal during normal operations. An intersection with a high
approach speed and multiple accidents related to clearance
intervals would be selected as a reguirement of the study.
once both the signal events and vehicular movements are
recorded onto one source, ind j-vidual response to changes in the
signal phasing from green to yellow and then to red can be
analyzed in split second detail. The previous model wiII be
applied to analyze the change plus clearance interval based on Vrp
measurements. A detector for measuring the speed and determining
the classification of each vehicle will be drawn on the roadway.
63

67. 4.2 Site Description
Since dilemma zone occurrences are sporadic and depend
primarify on high approach speed, it was irnportant to select a site
that had previous accident experience related to its clearance
interval in order to adequately test the model. The location
chosen for observation was Waldo Road and Northeast 53rd Avenue.
The high collision rate at the intersection warranted a flashing
sign (see Figure 25) that warned approaching drivers of the
signal- 's existence in both directions. The posted speed lirnit
dropped from 55 mph to 45 mph just upstream of the warning sign.
The intersection was equipped with two extended-ca1l detectors
that stretched across the through lanes (see Figure 26) on the main
approaches. They were approximately 4OO ft from the stop bar and
2oo ft behind each sign. The yellow interval length was measured
and found to be four seconds. The change interval sequence was
observed to include a one second all-red clearance interval. The
geometry was a two-way two-lane T-intersection with littIe or no
sight restrictions.
The controller was a locking tdensityt type with an auxiliary
green extens j-on system. The operation of the controller was not
coordinated with any other due to the extremely long spacing
between adjacent signals. This separation allowed drivers to reach
the high speeds necessary for long and difficult stopping with
frequent dilemma zones. The traffic was widely dispersed and had
a wide speed range due to vehicles entering from driveways.
64

68. Figure 25 Flashing Warning Sign
Figure 26 Extended-Call Detector Placement
65

69. 4.3 Measurement Technique
The status of each phase displayed by the controller is
represented by an LED on the back of a load switch. ft is
possible to detect the activation of a particular phase and isolate
its status by means of an optical sensor applied to the LED
as shown in Figure 27 . fnside the controller cabinet a
phototransistor was attached to the LED indicator on the load
switch by evenly covering a rubber grommet with cement, pressing it
firmly to the surface and later placing the sensor inside (see
Figure 28) . The l-ens on the sensor was placed parallel and in
contact with the LED to insure rapid detection. Care was taken to
not allow the glue to drip between the pair.
LoadSwitch
Glue
DisplayPanel
RubberGrommet
(lnside) (Outside)
LED Photo-
Transistor
Source SY32PT
Cross Section of Source/Sensor PairFigure 27
66

70. Figure 28 Sensor Applied to LED on Load Switch
6P
1
' t 2 2 P
r'"'-L
[,h-._,
J 4 t + Y c
nrqIHL t
ffi
ffiffi)
rnn
,=
lt .rEpFpffi[
.#:-/;j
4s=ry:=
Figure 29 Optoisolator
(Bottorn Right Hand
Detectors )
Controller Cabinet
Next to Vehicle
ins ide
Side
67

71. Sensor
Photo-
Transistor
SY32PT
Potentiometer
+ 9 VoltsDC
2N2222A
Transistor
Source
LED
1Kohm Output
Optocoupler
Figure 30 Optoisolator Schematic
The voltage leveL generated by the source/sensor was shifted
to a higher level using a transistor to power an optocoupler as
shown schematically in Figure 30. A phototransistor was selected
to be the opticaL sensor because of its great sensitivity to
reLatively small amounts of light. It was wired in series with a
variable resj-stor to act as a photoconductive detector. The
optoisorator is shown on the bottom right of Figure 29.
The sensitivity of the circuit is adjusted by the variable
resistor. Since LEDs emit narrow bands of low level light, it is
important to fine tune the isolator to the color and brightness of
the source. To collect accurate phase information, the sensor must
Input
2
68

72. detect the source before an experienced driver can respond by
ref lex reacti_on.
The optocoupler was used to switch on a nultiplexer, also
located inside the controller cabinet (as drawn in Figure 31) , to
al-l-ow for the recording of the yellow change interval onto the
audio channel of a video tape recorder (VCR). The nultiplexer
generated input sel-ectable tones based on the status of the
isolator- This tone, simultaneously recorded with the video irnage
as seen in Figure 33, enabled playback analysis of driver response
to the signal controller using the AutoScope. The observation
point sel-ected for the camera was high up on the side of the
prestressed pole used to suspend the signal heads (see Figure 32).
Video
Camera
Controller
Video
Input
Data Col-lection Setup for Vfp Technigue
T-
I
I
/--
I
L
Cabinet
Source/Sensor
I01 02 03,1U 05 06
W
lnvcfter 12Volt Batcry
Figure 31
69

73. Figure 32 observation Point
Figure 33 Data Collection Equipment
70

74. Figure 34 camera Angle for Dilernma zone Detection
To capture the dilemma zone the camera was aimed at traffic
approaching the signal at approximately the location of the speed
trap created by the clearance interval . The bottom of the irnage
was chosen to be the point of maximum braking effort with the
distance traveled during perceptton/reaction included in the
picture as shown in Figure 34. These points hrere chosen because
they are l-ocated in the center of the critical section. A presence
detector was placed at the point of braking distance and a velocity
detector was stretched over the perception length. This allowed
the pinpoint accuracy required to correctly identify conflicts with
the cl-earance interval.
7L

75. VideoCamera
tr'igure 35 Creation of a Combined Event Fil-e using AutoScope
Reduction of the data was accomplished using the system
diagrammed in Figure 35 and shown in Figure 37 . Since the
AutoScope can operate using a camera or a VCR, the tape made with
the signal events audibly recorded was played back. The same
multiplexer used in the field to encode the phase status, also
decoded the status of the controller using the audio signal. This
allowed simultaneous analysis of intersection phasing and vehicle
movements. The audio/video interface linked the information
together and two separate textfiles were generated. The speed and
classification data was provided by the AutoScope software during
playback analysis (see Figure 36) .
D"./
Multiplexer
AutoScope
c
o
o
o
o
c
o
o
- l o
Ardioln ArdioOrt
ffi
n
72

76. Figure 36
Video Tape
Playback Analysis
with Phase Status
of
Figure 37 Data
AutoScope Video
Reduction using
Image Processing
73

77. 4.3 Results
After the study was completed, text files were created
containing the output of the AutoScope software and the
corresponding status of the controller load switch. The two
separate fiLes were appended into databases and analyzed using
the model developed on the laptop computer. The velocities of
individual vehic1es were combined with the phase being displayed
and distance from the stop bar to categorlze driver response.
The Dilemma Zone Analysis program was able to show that
vehicles were caught in a conflict with the clearance interval.
Reviewing the tape confirmed that they arrived with high speed at
the end of the yellow or entered the intersection during the red.
The incursions occurred even though a green extension system was
provided and a warning sign was installed above the roadway.
Three tapes were made and no problerns were found during
the early morning (Bam to 10am) and late at night ( L0prn to L2am) .
However, during the afternoon peak (4pn to 5pm) at least five
vehicles were caught in a dilemma with three clearly running the
red light. The time these incursions occurred was noted and that
segment of tape was replayed inside of the AutoScope interval time
sl-ice f or analysis .
The Dilemma Zone Analysis program first prompts the user for
site specific parameters such as intersection width and grade of
the approach. Numbers (or codes) are entered corresponding to the
the inputs on the multiplexer. The two filenames of the textfiles
74

78. Dilemma Zone Analysis
(Esc) - Exit Program
(F1) online help
(F2) Print report
(F3) Create output file
rntersection width ( ft) t 48 l
Grade (Z) t 0l
Drive [C:] Event file [AUTOE0O]-I
Suffix [.TXT] Phase file IDILEM]IA l
Code for red t 0 l
yellow | 2l
grreen t 1l
Reaction (s) [ 1. o]
Distance to i-ntersection (ft) t 751 Autonobile [ 0]
Length of speed trap (ft) t 501 Semitrailer [ 2] Single axle [ 1]
Date | 3/LB/1993 From:08:30232
To:08240226
Camera
Camera
Detector : 0L
Detect or z02
l-
L
Vehicl-e Velocity Braking Distance
Input Phase Classification (mph) ( ft) Result
63 Green Single axle 45 L62 No conflict
Figure 38 Screen Display of Vehicle Processing
created by the image processor and the rnultiplexer are entered
next and use a common suffix supplied by the user. Fina1ly, the
distance from the intersection of the presence detector, the length
of the speed trap drawn on the roadway, and the length
classification codes defined by the AutoScope are aII entered.
After al-l- information has been entered, the two files are
preprocessed for a ye1low interval conflict analysis. The phase
status is added to the event data based on the synchronization of
two computerst internal time clocks during data reduction. The
output generated by the AutoScope includes a banner as the first
record that contains the number of the camera selected, the
corresponding detector identifications (IDs) and the date of
the study. This information is echoed on the screen along with the
75

79. corresponding interval
processing takes place
time slice. as
as shown above
the individual vehicle
in Figure 38.
Dilemma Zone Analysis
(Esc) - Exit Program
(F1) online help
(F2) Print report
(F3) Create output file
I ntersect ion width (ft) [ 48]
Grade (?) t 0l
Event f ile IAUTOEOO]-l
Phase file I DILEMMA ]
Code for red
yellow
green
Reaction (s)
0l
2l
Ll
L.0l
Drive
Suffix
Distance to intersection
Length of speed trap
(fr)
(fr)
751
501
Automobile
Semitrailer
0l
2l Single axle
Date z 3/IB/I993 From:08:30232
To:08240226
Camera: l-
Camera: 2
Detector: 0l-
Detect or z02
Automobiles:
Single axle:
Semitrail-er:
Total vehicles:
dbn (avg) :
dbc (avg) :
Average velocity:
Cycles:
YeLlow length:
Dilemna zones z 2
Ran red light: 1
No conf Iict: l-l-B
Uncomfortable: 0
Safe stop: 1
119
2
l_
L 2 2
85 ft
L44 ft
27 mph
L2
4 S
Figure 39 Screen Display of Program Output
After each vehicle speed and position is compared to the phase
being displayed, the program summarizes the results as shown above
in Figure 39. The total number of vehicles processed is broken
down into an individual count by class. The average velocity,
average comfortable (dbc) and maximum (dbm) stopping distances are
computed. The total number of cycle changes is displayed with the
Iength of the yellow interval recorded on the audio channel. AIso
totaled is the number of dilemmas, red light violations,
uncomfortable and safe (comfortable) stops during the yellow.
Vehicles that passed thru the intersection during the green phase
76

80. or cleared completely during the yeflow are categorized as having
no conflict.
The output of the program consists of the original AutoScope
output combined with information that includes the phase status,
the comfortable and maximum braking distances of each vehicle, and
the results of the analysis (i.e. No conflict, Uncomfortable,
Dilemma, etc.) during the change interval. ft is important to use
the revent data I analysis produced by the AutoScope software and
not the 'interval datar . The event analysis determines the
velocity of each individual vehicle, whereas the interval data is
the average over some specified length of time. The combined event
file output with dilemma zone analysis is shown below in Table 2.
Time ID Occu Speed Phase dbn dbc Response
08
08
08
08
OB
08
OB
08
OB
OB
OB
08
08
08
OB
OB
08
08
OB
OB
32
32
32
32
32
32
32
32
32
33
32
33
33
33
33
33
33
34
34
34
28 . 436 101 466 04 0
28 .637 IO2 268 032
3L.77 4 IO2 3 65 245
34.310 LOz 598 045
41.318 rO2 365 045
42.453 101 698 040
42.71,9 IO2 600 045
45.O23 101 1168040
47 .658 IO2 2526040
13 .554 rO2 600 0r2
57.703 101 167 8038
16.257 101 3736015
L6.924 LOz 3538010
2L . 93L 10 l_ 23 0tOL2
22 .43]. LOz 2168010
3L.575 rO2 432 010
33.244 101 43L OLz
oo.472 101 l_35 0L2
42.489 LO2 3gg 015
43.857 101 444 040
c L34 236 No conflict
G 95 L60 No conflict
G L78 29L No conflict
G ]-62 29L No conflict
Y L62 29L No conflict
Y L34 236 No conflict
Y L62 29L No conflict
Y L34 235 Dilemma Zone
R L34 236 Ran red light
R 24 33 No conflict
G L24 2L6 No conflict
c 32 47 No conflict
c L9 25 No conflict
c 24 33 No conflict
c 1,9 25 No conf I ict
G L9 25 No conflict
G 24 33 No conflict
Y 24 33 Safe stop
G 32 47 No conflict
c L34 236 No conflict
Table 2 Combined Event FiIe Output
The first number is the time of each presence detector
actuation broken down into milliseconds. The next number is the
77

81. detector identification number, ot, detector ID. For this study two
lanes of traffic were analyzed simultaneously. The detector for
Iane one was given ID number l-OL and lane two was given LOz. The
three to four digit number following the detector fD is the
occupancy duration of the detector in milliseconds. The class of
each vehicle is represented by a zero, one, or two for passenger
car, single axIe, or semitrailer respectively. From this
information, the AutoScope then computes the approxirnate speed of
each vehicle based on field measurements taken for calibration.
This is the l-ast piece of information provided by the software.
The results of the Dil-emma Zone Analysis are shown on the
right. The status of the change interval, the vehicle t s braking
distances and a comment generated by the program I s internal logic
are all- combined into a single f ile. As can be seen, during the
interval time slice shown on the previous page two vehicles, one in
each lane, were caught in a dilemma. Their speed and position from
the intersection were such that they could not stop for the red
light using a maximum effort without coming to rest or passing thru
the intersection.
This time interval on the tape was played back several times
in slow motion to confirm the results of the program. Since the
multiplexer created an audible freguency shift in response to the
detector reacting to the LED indicator on the load switch, the
status of the yellow light could be heard while reviewing the tape.
This enabled the situation of each driver to be verified.
78

82. v. coNcLUsIoNs
The individual vehicle processing technique used in this
model gives more insight into traffic signal operations than
ordinary detection methods. Driver response can be monitored and
steps taken to reduce excessive braking. Vehicle speeds are
measured and can be compared to design values for safety studies.
The results of the analysis may provide a traffic engineer
with the appropriate information to decide whether to increase
the yellow clearance j-nterval or install a green extension system
to accommodate those vehicles caught in a dilemma. Further
development may provide a system to detect a dilemma zone as it
actually occurs using more advanced vehicle detection rnethods. f f
no dilemma exists the signal could terminate the yellow without the
inclusion of an all-red j-nterval to give more time to the opposing
movements, thus making the signal system more responsive.
Even though the model requires some knowledge of the
intersection and does just basic computations to arrive at the
conclusions, the program is relatively easy to use and provides
more than just numerical results. The program can show the signal
timer what is actually occurring at an intersection. After further
development, this program could prove to be beneficial to traffic
engineers who are l-ooking at improving existing locations or making
adjustments to signal timings.
with proper camera placement and correct identification of
signal events, video image processing can be used to accurately
79

83. determine if dilemma zone incursions have occurred. Due to the
required steep viewing angle of the AutoScope, it may be impossible
to use the system for green extensions unless the camera itself is
Iocated downstream. However this placement may not be appropriate
for intersecti-on control due to the lack of information about
the traffic at the stop line.
The vi-deo j-mage processing technique used in this proj ect does
enable carefuL speed measurements of vehicles during actual
control-l-er operation and was able to identify vehicles within the
dilemma zone after the controller had decided to terminate the
green. With this information, the length of the yellow interval
may be adjusted to accommodate those vehicles who are routinely
caught in that situation t ot an all-red interval may be added to
increase safety. Whichever approach is taken, the improved
detector capability of the Autoscope allows for a more
thorough study of controller and driver interaction.
This proj ect demonstrated the potential benefit for traffic
operations of the AutoScope. It allowed for multiple detectors to
be placed in the traffic stream without extensive disruption to the
roadway. The information gathered from the detectors was
detailed enough to determine a difficult phase dependent guantity,
the location and speed of an approaching vehicle during the yellow.
80

84. VT. BIBLIOGRAPHY
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Highways, Washington: U.S. Department of Transportation, Federal
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4. Samuel Cass, ItTraf f ic Signals, tt Baerr,rald, John E. , Editor,
Transportation and Traffic Engineering Handbook, Englewood CIiffs,
N. J. : Prentice-Hal1, fnc. , L976.
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Clearance fntervals for Traffic Signals, tt Report TSD-TR-LO4-69 ,
Traffic and Safety Division, Michigan Department of State Highways,
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6. May, Adolf D. , rrCl-earance fntervaL at Traffic Signals, tt
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7 . Will iams , Wil-Iian L. , trDriver Behavior During the Yellow
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8L

85. 13. Conradson, Bruce and Lawrence Bunker, rrEvaluation of an
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Clearance Interval to the Traf f ic Signal Tining Sequenc€, tt Report
TSD-G-208-72, Traffic and Safety Division, Michigan Department of
State Highways, April, L972.
L4. ttRed.overlap Studyr tr Bureau of Traffic Engineering, City
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82