Transport for London (TfL) is investigating the use of GPS technology in public transport for bus location and related services such as selective vehicle detection at Traffic Signals. Although GPS-based systems are perceived to be flexible and versatile, a drawback of this technology is the locational error associated with it. This error could result in some buses missing the given priority actions, particularly extensions. This paper provides analysis and discussion of the effect of GPS error on bus priority taking account of different detector locations, detector combinations and operational conditions. This study was carried out using TRG's microscopic simulation model, SIMBOL. Simulations were carried out with the assumptions that buses are detected using virtual detectors which may be positioned anywhere on the link. GPS location errors were assumed to be random and unbiased and GPS detection was assumed to be available 100% of the time. Bus priority was assumed to be awarded as done in SCOOT. Simulations of single virtual detector at various locations for different levels of GPS error showed that: Increasing GPS error reduces the bus priority benefits. In general, the reduction in benefit due to GPS error was found to quite small when compared with the impacts of other factors such as the junction degree of saturation, using central extensions and using a lower value of the SCOOT parameter bauth (the maximum displacement of a stage change allowed for an extension). The reduction in bus delay savings due to GPS error appeared to be fairly constant over the different detector distances considered. Existing bus journey time variability influences the impact of GPS error on bus delay savings. Simulation results showed that the influence of the GPS error is more noticeable where the journey time variability is low and less noticeable where the variability is high. Results from using an exit detector to end priority extensions when the bus is detected showed that the GPS error influences the optimal location of the exit detector. Assuming the use of local extensions, the GPS error required the location of the exit detector to be shifted downstream of the signals by a distance equal to the maximum detection error anticipated. This was necessary to avoid premature termination of extensions when buses are detected before they pass through the signals. Results from using an exit detector to hold priority extensions until the bus reaches the signals, in addition to the cancelling function, increased bus delay savings slightly. e simulation results showed that GPS error, of the magnitudes expected, has a relatively low impact on bus delay savings when compared to the impacts of other factors such as the junction degree of saturation, use of central extensions and use of a lower value of bauth. For the covering abstract please see ITRD E135207.
Samenvatting