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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.
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”
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)
Potential Problems• A major safety concern at high speed signalized intersections
Common Treatments• Advance Warning (AW) Flashers – Flashing signal heads and warning signs • Activated at predetermined time before end of green• “Mixed” results regarding effectiveness
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
NDOR’s Actuated AdvanceWarning (AAW) System• Combines advance detection and advance warning – Single detector – Shorter maximum allowable headway – Lower frequency of max-out
Issues• Results positive but mostly anecdotal• Guidelines for installation – When do they need to be removed (if ever)?• Motivation for study
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
Treated Intersections: Table2.2 Simple example ignores regression to mean, changes in AADT… Need to compare to untreated intersections…
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
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
Study Site: Lincoln• Highway 77 and Saltillo Road
Study Site: Omaha• Highway 370 and N 132nd Street
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
Microsimulation Model• GA Calibration Procedure
Sensitivity Analysis• Simulation runs – 480 total factor combinations – 1-hour simulation run for each – 10 replications each• Output – Waiting times – Number of conflicts
Sensitivity Analysis• Effect of turn percentage – On average waiting times
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
Conclusions• Simulation model – Developed framework for modeling system – Successfully applied to two sites• Sensitivity analysis – Site specific – Can be used to perform sensitivity analyses
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.