3. Background
• Dilemma Zone
– At the legal speed limit, the driver can neither
clear the intersection before the end of the
intergreen period nor stop without entering the
intersection.
4. Background
• Dilemma Zone: NDOR 2002 Report
– “Length of roadway in advance of the intersection
wherein drivers may be indecisive or respond
differently to the onset of the yellow indication.”
– Also known as “option zone” or “zone of indecision”
5. Background
• If an intersection is designed correctly (e.g. NDOR) a
dilemma zone will not exist
– Assuming deterministic system
• Vehicles same characteristics (accelerate, decelerate,
weather, etc.)
– Trucks/braking
• Drivers make the correct decisions
– Stop, proceed
• Assuming: legal maneuvers (not running red light)
7. Common Treatments
• Advance Warning (AW) Flashers
– Flashing signal heads and warning signs
• Activated at predetermined time before end of
green
• “Mixed” results regarding effectiveness
8. Common Treatments
• Advance Detection (AD)
– Series of detectors in advance of intersection
• Extend green on detection
– Effective in reducing crashes and conflicts
– Increases likelihood of extending green to maximum
(max-out)
• Dilemma zone protection is lost
9. NDOR’s Actuated Advance
Warning (AAW) System
• Combines advance detection and advance warning
– Single detector
– Shorter maximum allowable headway
– Lower frequency of max-out
10. Issues
• Results positive but mostly anecdotal
• Guidelines for installation
– When do they need to be removed (if ever)?
• Motivation for study
12. Safety Effectiveness
• Test Sites
– 26 treated intersections
– 29 reference intersections
• “Similar” characteristics as treated
intersections
• Provided by NDOR
– 13 year of crash counts and AADT
• 1996-2008
13. Treated Intersections: Table
2.2
Simple example ignores regression to mean, changes in AADT…
Need to compare to untreated intersections…
14. Safety Effectiveness
• Method
– Full Bayes
– Accounts for uncertainty in data
– Generates a distribution of likely expected number of
crashes
– Combines this distribution with site-specific crash
data to obtain expected crash frequency
– Approach is complex but requires less data
26. Operational Analyses
• Main Characteristics
– Approach speeds
– Acceleration/deceleration characteristics
• Following onset of yellow
• During lead flash
– Frequency of max-outs
– Rate of dilemma zone “entrapment”
– Waiting time on conflicting phases
41. Microsimulation Model
• VISSIM
– Inputs: geometry, traffic counts, timing, speeds, etc.
• Calibration
– Adjust model parameters such that field data
“matches” simulated data
– Measures of performance
• Average waiting time
• Speed profile
48. Sensitivity Analysis
• Simulation runs
– 480 total factor combinations
– 1-hour simulation run for each
– 10 replications each
• Output
– Waiting times
– Number of conflicts
50. Conclusions
• Safety effects
– Greater than 90% probability that installation of
system is beneficial
• Operational effects
– Lower than expected number of vehicles in dilemma
zone
– Low max-out probabilities
– System seems to work well
51. Conclusions
• Simulation model
– Developed framework for modeling system
– Successfully applied to two sites
• Sensitivity analysis
– Site specific
– Can be used to perform sensitivity analyses
52. Recommendations
• System worth considering at other high-speed
signalized intersections
– From a safety perspective
• Guidelines regarding installation
– McCoy and Pesti (2002)
• Guidelines regarding removal
– Simulation study
• Max out, delay, etc.