When developing transit signal priority (TSP) systems, consider using traffic simulation models as a cost-effective means of comparing the impacts of different TSP strategies on transit and non-transit vehicle travel time.
Guidance emerging from a TSP literature review and from an evaluation of TSP in Burlington, Vermont.
- Consider using traffic simulation models as cost-effective means of comparing TSP strategies in the planning phase. The results of simulations, as revealed by the literature review, were similar to the performance measures obtained from field test evaluations. By comparing the outcome of modeling to data from field measurements, the authors concluded that simulation models such as VISSIM, TRANSYT, NETSIM, INTEGRATION and SCOOT provide valid and economical ways for testing TSP scenarios prior to deployment.
- Conduct a local impact analysis when planning a TSP deployment to estimate the positive and negative impacts. Field tests and microscopic simulation analysis are tools for assessing the impact of transit priority on traffic flow, adherence to transit schedules, non-transit vehicle delay, pedestrian safety, and existing emergency vehicle preemption systems, among other factors. Performance measures most commonly used include bus travel time (the most commonly used measure to assess TSP), side-street queue lengths, side-street person delay, and overall vehicle delay.
- Compare different transit priority strategies during the planning phase. Different strategies are likely to have varying impacts on transit and other vehicles. For example, previous research suggests that green-time extension improves transit bus level of service without impacting travel time of non-transit users. In contrast, green extension in combination with red truncation may have negative impacts on non-transit vehicles. The appropriate strategy may depend upon the characteristics of the corridor, including the frequency and direction of priority vehicles, roadway characteristics, travel demand, the presence of pedestrian phases, transitional and progression strategies, and intersection spacing. Further, existing signal coordination may require employing different types of priority such as queue jumper lanes (that provide priority for transit buses).
- Conduct a pedestrian safety audit when planning TSP for locations that have pedestrian traffic. Corridors with bus routes are frequently characterized by a pedestrian presence that involve people crossing the street, converging on sidewalks, and walking toward and waiting at bus stops. To gain perspective of the location in terms of pedestrian, the audit should consist of a review of historical pedestrian accident data, an examination of the length of the pedestrian cycles and local demographics (including age), and a survey of the location of bus stops, retail areas, and residential housing. Different TSP strategies, such as a green extension only versus green extension in combination with red truncation, may require changes in pedestrian signal phase.
- Conduct an economic analysis that identifies the fixed and recurring costs associated with priority investments. Transit priority, like other ITS projects, may have lower up front investment costs but higher operating costs than non-ITS improvement projects. Consider using a life-cycle cost analysis that includes all life-cycle capital and operational costs, as well as a multi-criteria analysis approach to understanding the benefits and costs of TSP.
- Conduct a pre-installation site survey to incorporate characteristics of the location into the design. The contractor (or agency) should conduct a site survey prior to deployment that determines the corridor's roadway geometry, bus stop placement, line of sight restrictions, pedestrian crossing volumes, existing equipment, and intersection configurations. This information is required for the appropriate placement of detectors to avoid placing a bus in the dilemma zone when the signal turns to the amber phase. For example, for a bus traveling at 15 miles per hour (mph) through an intersection width of 40 feet and a green extension of 10 seconds, a detection distance of 180 feet is sufficient for the bus to clear the dilemma zone.
- Be aware that small- to medium-sized cities may have fewer resources (such as fewer numbers of staff) for TSP planning, procurement, deployment and evaluation activities. Not only will small to medium-sized cities have less resources available for TSP projects, data availability may not be as rich as it frequently is for large cities, requiring that data be collected in the field. Further, the impacts on non-transit traffic may be different for small to medium-sized cities compared to large cities, requiring careful analysis in the planning phases for TSP systems.
The goal of TSP is to reduce transit travel time by facilitating transit movement through signalized intersections. Previous research and field/simulation analyses demonstrates that TSP increases bus transit level of service by reducing delay, tightening adherence to transit schedules, as well as other improvements. Although many evaluations show that TSP has had minimal impact on other users, there are cases in which TSP deployments created additional delay for non-transit vehicles. By following the recommendations listed above, such as conducting local impact analyses, pedestrian safety audits and site surveys, it is possible for agencies to improve transit level of service while having a minimal or neutral impact on other road users.
Author: Vlachou, K., Collura, J., & Mermelstein, A.
Published By: Journal of Public Transportation, Vol. 13, No. 3, 2010, National Center for Transit Research, Center for Urban Transportation Research (CUTR), University of South Florida
Source Date: July 2010URL: http://www.nctr.usf.edu/wp-content/uploads/2011/02/JPT13-3Vlachou.pdf
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