As every user knows buses tend to bunch. To alleviate this problem, transit agencies introduce slack into their schedules and then hold buses back to schedule at pre-established control points along their routes. Unfortunately, this practice retards buses and only works with low frequency systems; i.e., when the headways are long. For higher frequency systems, which effectively operate without a schedule, headway-based control strategies show promise but unfortunately, they also retard buses. To alleviate bus retardation in all its forms, transit signal priority (TSP) is commonly used. Curiously however, the potential of TSP to enhance bus control practices has not been explored in detail.

This paper evaluates a form of TSP designed for this purpose. In this version of TSP, which we call conditional signal priority (CSP), buses send priority requests only when the requests improve reliability. The evaluation includes both scheduled (low frequency) systems, and unscheduled (high frequency) systems operated by headways. A mathematical model based on Brownian motion is proposed for the former. This model allows us to optimize the CSP strategy and to develop formulas for the expectation and variance of the steady state deviations from the schedule–with CSP, with conventional TSP and with neither. Simulations confirm the accuracy of these findings. It is found that CSP improves reliability considerably and that it is better for traffic than TSP because it reduces the number of priority requests. For high frequency systems operated with a fixed number of buses CSP is shown to not just improve reliability and reduce the number of priority requests of TSP (by about 50%) but also to reduce the average headway by speeding up the buses.

每个用户都知道，公交车容易拥挤。为了缓解这个问题，运输机构将松弛时间引入了他们的日程安排，然后在沿其路线建立的控制点将公交车拖回时间表。不幸的是，这种做法会延迟总线，并且仅适用于低频系统。也就是说，当车头很长时。对于无需调度即可有效运行的更高频率的系统，基于行进的控制策略显示出希望，但不幸的是，它们还会使总线减速。为了减轻各种形式的总线延迟，通常使用过渡信号优先级（TSP）。然而，奇怪的是，尚未详细探讨TSP增强总线控制实践的潜力。

本文评估了为此目的设计的TSP形式。在此版本的TSP（我们称为条件信号优先级（CSP））中，总线仅在请求提高可靠性时才发送优先级请求。评估包括计划的（低频）系统和由车道运行的非计划的（高频）系统。提出了基于布朗运动的数学模型。该模型使我们可以优化CSP策略，并开发出针对计划的稳态偏差的期望值和方差的公式-使用CSP，使用常规TSP或不使用两者。仿真证实了这些发现的准确性。发现CSP大大提高了可靠性，并且由于减少了优先级请求的数量，因此它比TSP的通信性能更好。