William R McShane

Evaluation of Transit Signal Priority and Optimal Signal Timing Plans in Transit and Traffic Operations

Transit signal priority (TSP) systems have been in place for several years. Traffic simulation models are frequently applied to evaluate the benefits of such treatments before implementation. These benefits can take several forms, including reduced travel time for transit customers, improved schedule adherence, and reduced operating costs for the transit provider. This paper reports a case study that highlighted and embodied the following issues: (a) if signal timing is optimized, the TSP may provide only incremental benefits and not be a cost-effective investment; (b) if signal plans are not updated as often as needed, TSP may provide a way of adapting the base signalization, providing much of the benefit of signal plan updates; (c) side-street traffic does not always suffer because of TSP, nor does main-street nonbus traffic; (d) some critical intersections, with heavy competing volumes, may need to be dropped from the TSP plan to have no excessive adverse impact on the cross street, which may itself be an arterial; and (e) bus travel time reductions can result in fewer buses needed to serve the demand at the same level of service and thus reduce bus transit operating costs. Simulation was an important and cost-effective tool in this case study.

Warranting Traffic Signals on the Basis of Proximity of Railroad Grade Crossings

There were two concurrent paths in this research. Path 1 estimated the probability, pvehicle/train, that a vehicle will be in the train dynamic envelope given that a train arrives. This was done by an extensive simulation effort that included some data collection on relevant gap acceptance patterns. Path 2 identified the exposure factors, selected a risk metric, and established a target or threshold for the risk, which is expressed as an upper bound on pvehicle/train. As a result of the investigation, the warrant is based on the principle that the expected fatality rate at the crossing should not exceed rates found at unsignalized intersections. The two paths converged by expressing the constraint pvehicle/train ≤ ___ (where ___ is in terms of traffic terms in common use—e.g., volumes, composition, track location distances). This constraint led to the warrant, which was then validated with data acquired or collected for the purpose. The draft warrant was prepared as a result of the undertaking, was endorsed by the National Committee on Uniform Traffic Control Devices, and is currently in the formal review process, culminating in final edits and a decision on inclusion in the next edition of the Manual on Uniform Traffic Control Devices.

STOCHASTIC MODELING OF SHARED-LANE LEFT-TURN PROCESS AND ITS EFFECT ON SATURATION FLOW

A shared-lane left-turn model was constructed to investigate the ways in which left turners are impeded during the saturation flow period at the beginning of the green when queues are present. The model takes into account the subject and opposing approaches and the length of the two queues. It is event based, viewing each headway as a discrete discharge opportunity. The number of “extra” headways required because of the left-turning activity (and related missed or blocked opportunities) is defined as the “excess impedance.” The shape of the excess impedance distribution is studied, as is its mean. Above all, the considerable randomness of the excess impedance was noted, as were its implications. For instance, this randomness would imply that the 15-min saturation flow rate has so much variability that it is primarily a stochastic process, not a fixed number. Further, the randomness due to the left-turn effects generally exceeds that due to the fact that discharge headways are themselves random. The net ef...

A revised procedure for the capacity analysis of basic freeway sections

Abstract As part of a contract sponsored by the Federal Highway Administration, the authors developed a set of updated and revised procedures for the capacity analysis of basic freeway segments. These procedures update and revise the material in Chap. 9 of the 1965 Highway Capacity Manual, based upon the results of research and studies in the subject which have taken place since the manuals publication. Revised material includes new Level of Service criteria, revised truck equivalents, and new equivalents for recreational vehicles.

DEVELOPMENT OF PROTOTYPE VIDEO-BASED SENSOR FOR VEHICLE DETECTION FROM STAND-STILL IMAGES

The design, development, and testing of a prototype wide-area traffic detection system are described. The video-based sensor computes the approximate number of vehicles present within an a priori defined observation area from stand-still images. This sensor is mostly oriented toward the traffic detection in congested intersections, in which sensors using existing radar, acoustic, and video-based technology are faced with critical obstacles caused by the automobile stoppage. The prototype system has been tested and found to perform satisfactorily in field studies.