This paper describes the measured effectiveness of a transit signal priority system installed in the city of Eindhoven, the Netherlands (population 300,000), in reducing the deviation of transit vehicles from their scheduled running times. The system installed in all local transit vehicles monitors the adherence of the vehicle to its schedule, notifying the vehicle operator of "early" or "late" status and on several routes transmitting a request for signal priority to traffic controllers when the bus is behind schedule. The on-board devices determine the vehicle’s location based on inductive loop sensors embedded in the pavement; these loops also allow communication between the vehicle and the local traffic controller. Each on-board computer also provides a daily performance record for the vehicle that can be used, along with the records from all other vehicles on the same route, in several route-planning strategies further described in the paper. These planning strategies are designed to create route schedules that account for the conflicting goals of minimizing route transit time and increasing schedule reliability.

The study described in this paper measured the impact of conditional signal priority on schedule deviation by measuring the difference in the deviation of individual vehicles from their schedule as they passed through a signalized intersection. Conditional signal priority refers to a system that only provides transit vehicles with extended green time of an extra green phase when the in-vehicle computer indicates that the bus is behind schedule. Field measurements at one intersection along a route in Eindhoven found that the average absolute value of schedule deviation decreased by 17 seconds on the day when conditional priority was active, and increased by an average of 11 seconds on the day without conditional priority. This indicates that conditional priority was effective in bringing the vehicles closer to their scheduled running time at this particular interseciton.

In addition, the study investigated the impact of several operational control strategies on schedule reliability. A simulation of a simplified transit route was used to assess the impact of four strategies on the performance of vehicles:

• No control – operating without intervention based on the deviation of the vehicle from schedule.
• With holding – holding vehicles at each stop, if early, to wait for scheduled departure time.
• With conditional priority – buses given signal priority when they are running behind schedule.
• With holding and conditional priority.

The simulated route consisted of four stops separated by route segments with an average running time of 10 minutes each with a standard deviation of 2 minutes. Scheduled running time was assumed equal to the average running time. The performance measure used to assess the impact of each control strategy was the central schedule deviation band, which the authors define as the band between the 15th percentile and 85th percentile of schedule deviation. Without control strategies, the simulation found this band to be [-3.5, 3.5], indicating that 15 percent of the transit vehicles arrived more than 3.5 minutes ahead of schedule and 15 percent arrived more than 3.5 minutes late. Holding early buses at stops until their scheduled departure time improved this range to [0, 4.1]. Conditional priority without holding further improved performance to a range of [-1.9, 1.9]. The best operational strategy, incorporating both holding and conditional priority, reduced the range to [0, 2.0]. When compared to conditional priority alone, this strategy reduces early arrivals while only increasing the late end of the range by 0.1 minutes.

Field measurements confirmed the positive impact of conditional signal priority. The central deviation band at the intersection described previously improved from [-3.3, 2.5] without priority to [-1.0. 2.2] when priority was operational.