Express bus services are services that skip some of the stops along their routes to provide a faster ride for particularly demanded trips on a corridor. There is a growing literature on express services that focuses on route design and performance evaluation. In this work, we study a simplified transit corridor where a regular service operates in tandem with an end-to-end express service. Assuming that passengers minimize their expected travel and waiting times, we show that, even if the system has enough aggregate capacity, it may present a specific range of frequencies for the express service where it attracts more demand than it can actually fulfill. We call this range the “danger zone” of express services. When frequencies fall within the danger zone, a queue of passengers will form at the station. Applying queuing theory, we obtain expressions to estimate these queues and the associated waiting times, expected travel times and social costs of the system. We show that even when the station has unlimited passenger capacity, the performance of the system can be greatly affected in the danger zone. If the station has indeed limited capacity, the scenario can be much worse: if the queue grows to the point of saturating the station, a vicious circle ensues that amplifies the negative effects of the danger zone.

高速巴士服务是指跳过沿途某些停靠站的服务，以加快在走廊上特别有需求的旅程。关于快递服务的文献越来越多，着重于路线设计和性能评估。在这项工作中，我们研究了简化的运输走廊，其中常规服务与端到端的快递服务一起运行。假设乘客将其预期的旅行和等待时间减至最少，我们将表明，即使系统具有足够的总容量，它也可能会为快递服务提供特定的频率范围，从而吸引更多的需求而不是实际满足。我们将此范围称为快递服务的“危险区域”。当频率落在危险区域内时，车站将形成乘客队列。应用排队论，我们获得表达式来估计这些队列以及相关的等待时间，预期的旅行时间和系统的社会成本。我们表明，即使车站的载客量不受限制，该系统的性能也会在危险区域受到很大影响。如果站点的容量确实有限，那么情况可能会更糟：如果队列增长到站点饱和的程度，则会出现恶性循环，加剧危险区域的负面影响。