This document evaluates the Nebraska Department of Roads' (NDOR) Actuated Advance Warning (AAW) system at signalized intersections. It analyzes safety and operational data from 26 intersections with AAW systems and 29 comparison intersections. Statistical models show the AAW system likely reduces crashes by over 90%. Operational analyses found the system reduces the number of vehicles trapped in the "dilemma zone" and decreases the frequency that lights reach maximum time. Microsimulation models were developed and validated for two test sites. A sensitivity analysis examined how factors like turn percentage affect average wait times and conflicts. The conclusions recommend the AAW system for other high-speed intersections and provide guidelines for when to install or remove the systems based on measures like

1. Evaluation of NDOR’s Active
Advance Warning System
Laurence R. Rilett Ph.D., P.E.
University of Nebraska-Lincoln

2. Presentation Outline
• Background
• Analyses
– Safety
– Operation
– Simulation
– Sensitivity
• Conclusions
• Recommendations

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)

6. Potential Problems
• A major safety concern at high speed signalized
intersections

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

11. Part 1
Crash Data Analyses

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

15. Safety Effectiveness
• Crash Reduction Rate

16. Safety Effectiveness
• Model

17. Safety Effectiveness Results

18. Safety Effectiveness Results

19. Safety Effectiveness Results

20. Safety Effectiveness Results

21. Safety Effectiveness Results

22. Safety Effectiveness Results

23. Safety Effectiveness Results

24. Safety Effectiveness Results

25. Part 2
Operational Analyses

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

27. Study Site: Lincoln
• Highway 77 and Saltillo Road

28. Study Site: Omaha
• Highway 370 and N 132nd Street

29. Operational Analyses
• Data

30. Operational Analyses
• Max-out probabilities

31. Operational Analyses
• Waiting time on minor road

32. Operational Analyses
• Waiting time on minor road

33. Lincoln (Figure 3.13)
• Acceleration/deceleration- lead flash

34. Omaha
• Acceleration/deceleration- lead flash

35. Operational Analyses
• (Average) speed profile- lead flash

36. Operational Analyses
• Acceleration/deceleration- yellow

37. Operational Analyses
• Acceleration/deceleration- yellow

38. Operational Analyses
• (Average) speed profile- yellow

39. Operational Analyses
• Vehicles in dilemma zone- yellow

40. Part 3
Microsimulation Analyses

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

42. Microsimulation Model
• GA Calibration Procedure

43. Microsimulation Model
• Calibration Results

45. Microsimulation Model
• Measures of performance
– Average waiting time

46. Microsimulation Model
• Measures of performance
– Speed profile

47. Sensitivity Analysis
• Experimental Design

48. Sensitivity Analysis
• Simulation runs
– 480 total factor combinations
– 1-hour simulation run for each
– 10 replications each
• Output
– Waiting times
– Number of conflicts

49. Sensitivity Analysis
• Effect of turn percentage
– On average waiting times

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.

53. CREDITS
Dr. Laurence Rilett, Ph.D., P.E.
University of Nebraska-Lincoln
Slide design © 2009, Mid-America Transportation Center. All rights reserved.