Probe vehicle population and sample size for arterial speed estimation

Equipping probe vehicles with global positioning system (GPS) receivers is a cost-effective way of collecting real-time location and speed information. A large-scale, nationwide travel speed information acquisition and dissemination system has already been in operation in Singapore, using a large fleet of taxis equipped with differential GPS (DGPS) receivers. This paper discusses the use of simulation approach to study the reliability of estimated average arterial link speed from probe vehicles. This study is based on the road network at the Clementi town area in Singapore. Simulation runs were made with a variety of traffic volumes, and with different percentages of probes in the total traffic volume. The reliability of link speed estimate is analyzed with respect to (1) overall probe vehicle percentages, and (2) number of probe vehicles sampled in a link. Results indicate that for an absolute error in estimated average link speed to be less than 5 km/hr at least 95% of the time, the network needs to have 4% to 5% of active probe vehicles, or at least ten probe vehicles must passed through a link within the sampling period.