Behind High-Resolution Traffic Data: Discovery, Modeling, and Control

Applications of High-Resolution Traffic Event Data: Managing Oversaturated ArterialsDr. Xinkai Wu, Assistant Professor

Department of Civil EngineeringCalifornia State Polytechnic University PomonaITE Western District 2012 Annual MeetingHigh-Resolution Event Data

ITE Western District 2012 Annual MeetingSMART-SIGNAL

Terminal Box

DACITE Western District 2012 Annual MeetingTrunk Highway 55 and Boone Ave (Golden Valley, MN)

ITE Western District 2012 Annual MeetingOversaturationGazis (1963): An oversaturated intersection is defined as one in which the demand exceeds the capacity.

Little research has been conducted on the identification and quantification of oversaturated conditionsMostly qualitative and incomplete

5However, traffic arrivals are usually hard to predict or measure.

Therefore, a network of intersections would become oversaturated when the system is overloaded with heavy demand which exceeds the total capacity of the networkDetrimental EffectsTemporally, characterized by a residual queue at the end of cycle. Residual vehicles cannot be discharged due to insufficient green splitsCreating detrimental effects on the following cycle by occupying a portion of green time.

Spatially, characterized by a spill-over from a downstream intersection. Vehicles cannot be discharged even in green phase due to spill-over Creating detrimental effects by reducing useable green time for upstream movements6Oversaturation Severity Index (OSI) OSI: the ratio between unusable green time and total available green time in a cycle.

Further differentiate OSI into T-OSI and S-OSI. Temporal dimension (T-OSI) The unusable green: because of the residual queue from the last cycle

Spatial dimension (S-OSI)The unusable green: because of the downstream blockage

7Measure T-OSI & S-OSIT-OSI: Estimate the length of residual queue at the end of cycle

S-OSI: Identify spillover Calculate the reduction of green time of upstream intersections

8T-OSI & S-OSI Measure Using High-Resolution Traffic Event DataITE Western District 2012 Annual Meeting

Queue Length Estimation

Instead of traditional input-output approach, we estimate queue length by taking advantage of queue discharge processBased on LWR shockwave theory

10Queue Length EstimationUtilize the data collected by advance detectorIdentify Critical Points: A, B, C

Point A: Shockwave v1 arrives to detector, indicating traffic state changes from (qa,ka) to (0,kj) Point B: Shockwave v2 arrives to detector, indicating traffic state changes from (0,kj) to (qm,km) Point C: Shockwave v3 arrives to detector, indicating traffic state changes from (qm,km) to (qa,ka)Identify from high-resolution event-based dataVehicle GapDetector occupied time Point A: Long occupy time (>3 sec) or occupancy > 1 for 3 secPoint B: Occupy time changes to normal value (< 1 sec) or occupancy changes to less than 1Point C: Large gap time (>2.5) or occupancy = 0 for about 3 sec

Break Point Identification from High-Resolution Detector Data

12Field TestsTest Site: TH55 (6 intersections)Independently evaluated by Alliant Engineering, Inc.At Rhode Island Ave.Three morning peaks (7:00am-9:00am) Jul. 23rd, 2008Occ. 29th, 2008Dec. 10th, 2008

ITE Western District 2012 Annual MeetingSOSI: Identify Queue-over-detector (QOD) Caused by Spillover

14Due to cyclic signal timing, vehicles slow down and stop for red phase or joining the queue, and then resume travel as the light turns green or queue is clear. Such deceleration-stop-acceleration process rapidly changes the occupancy at some locations. In other words, if a vehicle stays on the detector during the queuing process, the occupancy is significantly changed because of the relatively prolonged detector occupation time. We call this phenomenon as Queue-Over-Detector (QOD). This phenomenon is clearly indicated in the high-resolution data by large occupation time (2 sec, for example) or occupancy being 100% for several seconds. Generally, there are two kinds of QOD in signalized arterials: the first is caused by cyclic signal timing, i.e. the red phase; the second is because the queue spills back from downstream intersections, i.e. spillover. If the second QOD has been identified, the oversaturation at an arterial/route can be diagnosed.S-OSI: Identification of SpilloverIdentify QOD-II.High-resolution data.

ITE Western District 2012 Annual Meeting15Rhode Island phase 6Managing Oversaturation: A Simple Forward-Backward ProcedureITE Western District 2012 Annual MeetingA Simple Forward-Backward ProcedureBased on TOSI and SOSI measurementsRespond and mitigate traffic congestion quickly Simple and effectiveReactive

ITE Western District 2012 Annual MeetingProblem SettingN intersections along an oversaturated pathAt control period t, decisions are made according to the average TOSI and SOSI values at the control period t-1, i.e.,

ITE Western District 2012 Annual MeetingBasic Mitigation StrategiesThe TOSI and SOSI values can help identify the causes of arterial traffic congestionPositive SOSI indicates the spill-back of downstream queuePositive TOSI indicates that the available green time is insufficient for queue dischargeTherefore for a single intersection, three basic strategies can be applied.ITE Western District 2012 Annual MeetingTOSI > 0

Extending greenITE Western District 2012 Annual MeetingSOSI > 0Reducing red at the downstream intersection

ITE Western District 2012 Annual Meeting21SOSI > 0Gating (Reducing traffic arrivals & giving green to other approaches)

ITE Western District 2012 Annual MeetingHandling Spillover

ITE Western District 2012 Annual MeetingForward-Backward ProcedureForward Process (Seeking the available green)Follow the flow direction to eliminate spillovers and residual queues

Boundary condition

ITE Western District 2012 Annual MeetingForward-Backward ProcedureBackward Process (Gating or metering)Follow the opposing flow direction to check the arc capacity

Boundary condition

ITE Western District 2012 Annual MeetingSimulation Test22 intersections, Pasadena, CAOffline control

Simulation TestTOSI/SOSI Changes

Fair Oaks Ave SBColorado Blvd. WBITE Western District 2012 Annual MeetingFuture WorkThe Fundamental Diagram: Congestion

Safety

Environment

Control

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