Geometric design

Mekelle University
Ethiopian Institute of Technology-Mekelle
School of civil engineering
RTEC

INTRODUCTION
• Geometric design is the stage of road design
process where the dimension and layout of
road elements are proportioned to meet the
needs of road users.
• According to ERA geometric design is the process
whereby the layout of the road through the terrain is
designed to meet the needs of the road users. i.e.
Safety, Comfort, Efficiency, Economy, Environment
etc.
8/16/2016 Highway Geometric Design 2

INTRODUCTION
Highway geometric elements:
– Cross-section
– Sight distance
– Horizontal alignment
– Vertical alignment
– Vertical and lateral clearance
– Intersections etc.

INTRODUCTION
• In geometric design of highways, the following points
should be considered:
– Volume and composition of traffic
– Consistent and compatibility
 The standards proposed for the different elements should be compatible
with one another.
– include all aspects of geometry of the road
 Signs, markings, proper lighting, intersections etc.
– Environmental friendly
– Economic consideration
– Safety
– The highway should enable all the road users to use the facility.
– Aesthetics

ROAD CROSS SECTION ELEMENTS
• Cross sections define the configuration of a
proposed roadway at right angles to the
centerline.
• Typical sections show the width, thickness
and descriptions of the surfacing courses as
well as the geometrics of the graded
roadbed, side ditches and side slopes.

Right-of-Way
• It is the width of land secured and
preserved to the public for road purposes.
• should be adequate to accommodate all the
elements that make up the cross-section of
the highway and may reasonably provide for
future development.

Carriageway
• The part of the road constructed for use by
moving traffic, including traffic lanes,
auxiliary lanes such as acceleration and
deceleration lanes, climbing lanes, width of
median strip and passing lanes.

Shoulders
• The portion of the roadway adjacent to the
carriageway.
Purpose
• Serves for an emergency stop of vehicles
• Used to laterally support the pavement structure
• Parking facility
• Pedestrian walkway
• Recommended shoulder width for paved road is in
the range of 0.5 to 3 m.

ROAD CROSS SECTION ELEMETS
Roadway
• consists of the carriageway and the shoulders, parking lanes
and viewing areas.
Median
• section of divided road that separates lanes in the opposite
directions.
• Functions:
• Provide recovery area during emergency
• Provide stopping area for left and U-turning vehicles
• Provide refuge for pedestrians
• Reduce headlight glare
• Median can be either raised, flush or depressed

Crown slope/camber
• Is the cross slope provided to raise middle
of the road surface in the transverse
direction to drain rain water from the road
surface but not being so great as to make
steering difficult.
• Normal cross fall should be 3% on paved
roads and 4-6% on unpaved roads.

Side Slopes & Back Slopes
• The graded area immediately adjacent to the
graded roadway shoulder and side slope toe or
ditch.
• Side slopes should be designed to insure the
stability of the roadway and to provide a
reasonable opportunity for recovery of an out-of-
control vehicle.

• Three regions of the roadside are important when evaluating
the safety aspects of side slope
 the top of the slope (hinge point),
 the side slope, and
 the toe of the slope
• Embankment or fill slopes parallel to the flow of traffic may
be defined as recoverable, non-recoverable, or critical.

Curbs
• Raised structures used mainly on urban roads to
delineate pavement edge and pedestrian walkways.
• Curbs are also used:
• To control drainage (part of gutter)
• Improve aesthetic
• reduction of maintenance operations
• Reduce right-of-way
– Are classified as
• Barrier curbs – relatively high designed for preventing vehicles
from leaving the road
• Mountable curbs – are designed so that vehicles can cross them

Side walks
– provided on urban or sub urban roads
• When pedestrian traffic is high along main or high
speed roads
• In urban areas, sidewalks are provided along both sides
of streets to serve pedestrians access to schools, parks,
shopping centers, and transit stops.

Drainage Ditches
• Function of collecting and conveying surface water
from the highway right-of-way.
• The depth of channel should be sufficient to remove
surface water without saturation of the subgrade.
• should be located and shaped to avoid creating a
hazard to traffic safety.

Design controls and criteria
• Refers to the constraints and requirements that a
certain highway should fulfill.
• Factors affecting the geometric design of a highway are: (ERA
Manual)
Functional classification of the road
The nature of the terrain
Design vehicle
Traffic volume (expected)
Design speed
Density and character of adjoining land use
Economic and environmental considerations
• These considerations are not of course, completely
independent of one another.

Design controls and criteria
So these considerations can be reduced to:
(principal design criteria)
• Functional classification of the road
• Topography
• Traffic
• Design speed
• Design vehicle

Functional classification of the road
• Functional classification is the process by which streets
and highways are grouped in to classes or system based
of the character of service they are intended to provide.
• Road can be classified in many ways for instance
Traffic volume
Surface condition
Level of service etc.
However, classification based on the speed and
accessibility is the most generic one.
Mobility: The ability to move goods and passengers
to their destination. (in a reasonable time)
Accessibility: the ability to reach desired destination

Classification(Cont.)

ERA functional classifications
Trunk roads
Access roads
Collector roads
Link roads
Roads linking centers of national or international importance
and have over 300 first year AADT, although values can
range between 75-10,000 AADT.
Roads linking centers of provincial importance and their
AADTs between 25-1,000 .
Roads linking locally important centers to each other, to a
more important center, or to higher class roads and their fist
year AADTs between 25-300 .
Any road link to a minor center such as market and local
locations with AADT less than 150..
Roads linking centers of international importance and roads
terminating at international boundaries and have a present
AADT 1000 and as low as 150.
Feeder roads

Nature of terrain
• The geometric design elements of a road depend on
the traverse terrain through which the road passes.
Traverse terrain properties are categorized into four
classes as follows:
Flat or gently rolling terrain
Rolling terrain
Mountainous terrain
Escarpment

Topography(Cont.)

Traffic Volume and Composition
• Traffic data indicates the service for which the road is
being planned and directly affects the geometric
elements such as width, alignment, etc.
– Traffic volume – AADT, ADT, PHV, DHV
– Directional distribution – the percentage of traffic volume
flowing in each direction
– Traffic composition – the percentage of different types of
vehicles in the traffic stream – different types of vehicles
are converted into passenger car unit to design a road width
– Traffic projection – using the design period of a road (5-20
years)a reliable traffic projection should be made
considering the following elements

Traffic(Cont.)
o Traffic projection (cont’d.):–
• Current traffic – currently using the existing road
• Normal traffic growth – anticipated growth due to population
growth or change in land use
• Diverted traffic – traffic that switches to a new facility from near by
roads
• Converted traffic – traffic resulting from changes of mode
• Change of destination traffic – traffic that has changed to different
destination due to new or improved transport and not changes in
land use
• Development traffic – traffic due to improvement on adjacent land
development that would have taken place had the new or improved
road not been constructed
• Induced traffic – traffic that did not previously exist in a any form
but results when new or improved transport facilities are provided

Design vehicle
• In practice a mix of different vehicle types uses a
road at any time
• However, a road designed for truck-trailers while
it is mainly used by passengers car is
uneconomical, and designing a road for passenger
cars while there are significant number of trucks
using it is again not appropriate.
• Hence, a largest vehicle that uses the road in
frequent bases is considered as a Design Vehicle.

Design vehicle(Cont.)
• Both the physical characteristics and turning
capability of vehicles are controls in geometric
design.
• Vehicle characteristics and dimensions affecting
design include power to weight ratio, minimum
turning radius and travel path during a turn, and
vehicle height and width.
• The road elements affected include the selection
of maximum gradient, lane width, horizontal
curve widening, and junction design.

 As per ERA Design Manual there are four general classes
of design vehicles

Design speed
• it is a single most important factor that affect most of
the geometric design elements
• it directly governs Sight distance, horizontal curve
radius, length of vertical curve, etc.
• different drivers travel at different speed depending on
their behavior. However, for GD we need a single
representative speed, known as design speed.
• Design speed : is the highest continuous speed at
which vehicle can travel with safety when weather
condition is conducive.
• Usually design speed is taken as the 85th percentile
speed.

Design speed(Cont.)
• It is selected from standards for designing specific
section of road considering the terrain, land use,
classification of the road, etc.
• Using road functional classification selection,
terrain type and design traffic flow, a design class,
or standard, is selected from Table 2-1, with
reference to the design parameters associated with
that class as given in Tables 2-2 through 2-17 the
allowable geometric constraints are fixed.(see
ERA 2013 geometric design manual)

ELEMENTS OF GEOMETRIC DESIGN
»Sight Distance
»Horizontal Alignment
• The tangent, or straight section
• Horizontal circular curve
• The transition curve (spiral) and
• super elevation
• Widening
• Stopping sight distance on horizontal curves
»Vertical Alignment

Horizontal Alignment
• Is a plan view which shows the horizontal
curves and tangents or straights.
• Horizontal alignment deals with the design of
the directional transition of the highway in a
horizontal plane.
• Horizontal alignment consists of:
» Horizontal tangents
» Circular curves and
» Possibly transition curves

 Horizontal alignment should meet these general considerations:
 Alignment should be straight if possible within physical and
economic constraints
 Alignment should be consistent. Try to avoid sharp curves at the
ends of long tangents and sudden changes from gently to sharply
curving alignment.
 Avoid the use of minimum radii whenever possible
 Avoid horizontal curves on bridges
 On minor roads, curves should have a minimum length of 150 m for
a deflection angle of 5° and that this length should be increased by
30 m for every 1° decrease in deflection angle.
 On major roads and freeways, the minimum curve length in meters
should be three times the design speed in km/h.

• Tangent Section
 Straight portion of roads
 Provide better visibility & better appearance
Guidelines
• Long tangent should be avoided as they are monotonous for drivers and
cause headlight dazzle on straight grade
• More pleasing appearance and higher road safety can be obtained by
winding alignment
• Short straight b/n curves should not be used
• Straights b/n curves turning in the same direction should have length
greater than 6V
• Straights b/n the end and beginning of non transitioned reverse circular
curve should have length greater than 2/3 of the total super elevation
runoff

Horizontal Curves
Used to make transition from tangent to
tangent
Used to prevent demolishing of important
places
It is one of the most important features
influencing the efficiency and safety of
highways.

Types of horizontal curves:
» Simple circular curves
» Compound Curves
» Reverse Curves
» Broken back curves
» Switch back curves
» Transitional Spirals

Horizontal curves
• Simple Circular Curves
 Has a constant radius
 Are designated either by the degree of curvature (D) or radius
of the curve (R).
∆=deflection angle
L=Length of Curve
C=Chord Length
R=Radius of Curvature
M=Middle Ordinate
E=External Distance
T=Length of Tangent
P.I.=Point of Intersection
TC=Tangent to Circle
CT=Circle to Tangent

Horizontal curves
Formula for simple curve
• Tangent (T):)
T = R*tan (Δ/2)
• External distance (E):
E = R*(Sec (Δ/2) – 1 or E = T*tan (Δ/4)
• Middle ordinate (M):
M = R*(1‐ Cos (Δ/2))
• Long chord(C): straight‐line distance from PC to PT.
C = 2R*Sin (Δ/2)
• Length of Curve (Lc): distance from PC to PT along the curve.
Lc = 20* Δ/D or Lc = R*π* Δ/180
• Stations of PC, PI, and PT:
PC = PI – T
P T = PC + Lc or PT = PI + T

Horizontal curves
Degree of Curvature
Is the centeral angles subtending a standard
arc or chord length of 20m.
Arc Definition
 Chord Definition
360
220 R
D


R
D
92.1145

R = 10 / Sin (D/2)

8/16/2016
Highway Geometric Design
51
This image cannot currently be displayed.
Compound curves
consisting of a series of two or more simple curves of different radii turning in
the same direction.
180
180
sin)180sin(sinsin
2
tan
2
tan
2
2
1
1
22
11
2121
12
2
22
1
11
21






















R
L
R
L
tHVT
tGVT
ttttHVGV
Rt
Rt

Horizontal curves
Reverse curve
• Are adjacent curves that curve in opposite
directions.
• Should be used for low‐speed roads such as in
mountainous terrain
• Reverse curves are seldom recommended. They
are absolutely NOT recommended for high‐speed
roads. For high‐speed roads, we must provide a
tangent section that will allow full development
of super elevation at both ends.

Horizontal curves

Super‐elevation
• Is the raising of the outer edge of the road along a curve in‐
order to counteract the effect of radial centrifugal force in
combination with the friction between the surface and tyres
developed in the lateral direction.
• To provide safety and comfort while navigating through
curves at higher speeds.

Minimum radius of curve
 The design of radius of roadway curves should be based on an
appropriate relationship between design speeds and
curvature and on their relationship with super elevation
(roadway banking) and side friction.
 Minimum radius from the stability of vehicle
FcRmV /2
mg
f
N
h
Nf
RmVFc
mgN
/
/2



m

Stability on Super‐elevated Surface
Forces & Equilibrium
gR
Wv2
N
F
W
e
1

Super‐elevation
1
1
2
2
22
22
2
























gR
v
gR
v
Tan
gR
v
Cos
gR
v
Sin
WSinCos
gR
Wv
WCosSin
gR
Wv
WSinCos
gR
Wv
N
m
m

m
m

m
m
But the term has a very small value and
could be ignored for all practical purposes.
Check using typical values like V=50km/hr;
m=0.16; and R=100m.
gR
v 2
m

Super‐elevation
 
R
V
gR
v
e
e
gR
v
TanThus
81.9
6.3
,
22
2


m
m V=Km/hr
R=m
e=m/m
m=dimensionless
 m

max127
2
min
e
V
R
Thus, the minimum radius of curve is given by;

Super‐elevation
 Side friction coefficients are dependent on
• vehicle speed;
• type, condition and texture of roadway surface;
• weather conditions; and
• type and condition of tires
Table: side friction for paved roads
Table: side friction for unpaved roads

Super‐elevation
• Maximum value of super‐elevation is controlled by:
» Climatic conditions: frequency & amount of
snow/icing
» Terrain condition: flat vs. mountainous
» Area type: rural vs. urban
» Frequency of very slow moving vehicles
• As per ERA manual 4 % for urban and 8 % for rural.
• A rate of 4 % or 6 % is applicable for urban design in
areas with little or no constraints.

Super‐elevation transition
• super elevation should be introduced and removed uniformly
over the length adequate for likely travel speeds.(comfort and
safety)
 Superelevation transition section consists:
 Superelevation runoff
 tangent run out sections
 Superelevation runoff: ‐ is the length of highway needed to
accomplish the change in cross‐slope in the outside‐lane (flat)
to of fully Superelevation.
 Is a function of the design speed and rate of super elevation.
 Tangent run‐out(crown runoff): ‐ consists of the length of the
roadway needed to accomplish a change in outside‐lane cross
slope from the normal cross slope rate to zero (flat).
 The removal rate is usually the same as the Superelevation
removal rate is usually the same as the Superelevation runoff rate.

• In curves without transition
In the design of curves without spirals the
super‐elevation runoff is considered to be that
length beyond the tangent run out.
67 % of the Superelevation runoff is
developed on tangent and 33 % on circular
curve.

In curves with transition
In alignment design with spirals the Superelevation
runoff is affected over the whole of the transition
curve. The length of runoff is the spiral length with the
tangent to spiral (TS) at the beginning and the spiral to
curve (SC) at the end. The change in cross slope begins
by removing the adverse cross slope from the lane or
lanes on the outside of the curve on a length of
tangent just ahead of TS (the tangent run out).
Between the TS and SC (the Superelevation runoff)
the traveled way is rotated to reach the full
Superelevation at the SC

 Methods of Attaining Superelevation
• Four methods are used to transition the pavement to
a super elevated cross‐section. These methods include
– Revolving a travelled way with normal cross‐slopes about
the centerline profile.
– Revolving a traveled way with normal cross‐slopes about
the inside‐edge profile.
– Revolving a travel way with normal cross‐slope about the
outside‐edge profile.
– Revolving a straight cross‐slope traveled way about the
outside‐edge profile.

• Undivided highways are usually superelevated
with the axis of rotation at the roadways
centerline.
• Multi‐lane highways with depressed medians are
usually superelevated with the axis of rotation at
the median edges of the traveled way.

Transition curves
• When driving over simple horizontal curves, there is an abrupt
change from a tangent to a circular arc at the PC
• are curves which provide a gradual change in curvature from
tangent to a circular path
• have got a radius of curvature gradually changing from infinity to
the designed radius.
• have constant rate of change of radius of curvature.
• Is convenient for the application of super‐elevation
• Transition curves are not normally required for
» Curves with large radius
» Roads with lower classification

Transition curves
• The length of transition should be determined from the
following two conditions:
 The rate of change of radial acceleration adopted in the
design should not cause discomfort to the drivers. If C is
the rate of change of acceleration,
Ls = 0.0215V3 / (C*Rc)
Where:
V = speed (Km/hr)
Rc = radius of the circular curve (m)
 The rate of change of super elevation (super elevation
application ratio) should be such as not to cause higher
gradients and unsightly appearances.
 Ls=new
where: w ‐is width of road
n ‐is The rate of raising the outer edge above the centre line
e‐ rate of Superelevation

Stopping Sight Distance On Horizontal
Curves

Widening of Highway Curves
• Extra width of pavement necessary on curves.
• Widening of the carriageway where the
horizontal curve is tight is usually necessary to
ensure that the rear wheels of the largest
vehicles remain on the road when negotiating
the curve; and, on two lane roads, to ensure
that the front overhang of the vehicle does
not encroach on the opposite lane. Widening
is therefore also important for safety reasons.

• The turning effect of a vehicle (rear wheel relative to the front
wheel) requires widening of the lane width.
• The traveled way on horizontal curves is widened to accommodate
curve negotiation
• Widening is necessary for smaller radius curves or greater
curvatures and/or narrower road lane width and embankments
• Off tracking: is the characteristic of all vehicles that the rear wheels
do not follow precisely the same path as the front wheels when
they negotiate a horizontal curve or make a turn.
• Total widening is computed by adding the mechanical widening and
psychological widening.

• The widening required can be calculated
R
V
R
L
nWc
102
2

B = length of wheel base
R = radius of curve
V = design speed (Km/hr)
n = number of lanes
Wc=total widening

Widening – Methods
• On a simple curve (i.e. with no spirals) widening
should be applied on the inside edge of a
pavement only. For curves with spirals, widening
could be applied on the inside (only) or could be
equally divided b/n the inside and outside
• Widening should be attained gradually over the
super elevation runoff length but shorter lengths
are sometimes used (usually this length is 30 –
60m).
• Widening is costly and very little is gained from a
small amount of widening.

• Widening on high embankments is recommended for
design classes DC8 through to DC4.
• Curve widening is generally not applied to curves
with a radius greater than 250 meters regardless of
the design speed or the lane width

Geometric design

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