Effect of Transit Signal Priority on Bus Service Reliability

TL;DR

This paper evaluates how conditional transit signal priority (CSP) can improve bus schedule reliability and reduce delays, showing it can be as effective as traditional TSP while benefiting other traffic.

Contribution

It introduces a mathematical model for CSP, compares its effectiveness to TSP, and demonstrates its advantages through simulation in various bus operation scenarios.

Findings

CSP can enhance bus reliability and reduce delays.

CSP is as effective as TSP in some cases, better in others.

CSP benefits other traffic by sending fewer priority requests.

Abstract

Buses are known to be an unstable system, and as a result they struggle to stay on schedule and/or to maintain regular headways. In current practice, transit agencies add significant slack to the schedule and hold buses until their scheduled departure time at certain stations along the route. These practices can smooth out small disruptions but are not robust to large disruptions. Various headway-based strategies have been proposed but these also rely on holding buses and therefore reduce their average pace. Transit signal priority (TSP) is commonly used to reduce the signal delay that buses experience but the potential for signals to serve as an additional control agent to enhance bus reliability has not been systematically evaluated. We are especially interested in evaluating conditional signal priority (CSP), in which buses send priority requests only when a condition is met. A mathematical model based on Brownian motion is proposed for bus systems with a schedule where pairing is not a concern and is used to develop formulas for the expectation and variance of steady state deviation. Simulation is used to confirm these findings and to evaluate the case of bus systems operated by headway where pairing is a concern. The analytical and simulation results show that CSP can be used not just to accelerate buses but to improve their reliability. CSP is as good as TSP in some scenarios and better in others. In all scenarios it is better for other traffic because it sends fewer priority requests.