On-demand systems in which several users can ride simultaneously the same vehicle have great potential to improve mobility while reducing congestion. Nevertheless, they have a significant drawback: the actual realization of a trip depends on the other users with whom it is shared, as they might impose extra detours that increase the waiting time and the total delay; even the chance of being rejected by the system depends on which travelers are using the system at the same time. In this paper we propose a general description of the sources of unreliability that emerge in ridesharing systems and we introduce several measures. The proposed measures are related to two sources of unreliability induced by how requests and vehicles are being assigned, namely how users’ times change within a single trip and between different realizations of the same trip. We then analyze both sources using a state-of-the-art routing and assignment method, and a New York City test case. Regarding same trip unreliability, in our experiments for different fixed fleet compositions and when reassignment is not restricted, we find that more than one third of the requests that are not immediately rejected face some change, and the magnitude of these changes is relevant: when a user faces an increase in her waiting time, this extra time is comparable to the average waiting time of the whole system, and the same happens with total delay. Algorithmic changes to reduce this uncertainty induce a trade-off with respect to the overall quality of service. For instance, not allowing for reassignments may increase the number of rejected requests. Concerning the unreliability between different trips, we find that the same origin-destination request can be rejected or served depending on the state of the fleet. And when it is served the waiting times and total delay are rarely equal, which remains true for different fleet sizes. Furthermore, the largest variations are faced by trips beginning at high-demand areas.

多个用户可以同时乘坐同一辆车的按需系统具有巨大的潜力，可以提高移动性，同时减少交通拥堵。但是，它们有一个很大的缺点：旅行的实际实现取决于共享旅行的其他用户，因为他们可能会绕行额外的弯路，从而增加等待时间和总延迟；甚至被系统拒绝的机会也取决于同时使用该系统的旅行者。在本文中，我们提出了对拼车系统中出现的不可靠来源的一般描述，并介绍了几种措施。拟议的措施与请求和车辆的分配方式引起的两个不可靠来源有关，即用户的时间在单次旅行中以及同一次旅行的不同实现之间如何变化。然后，我们使用最先进的路由和分配方法以及纽约市测试用例来分析这两个源。关于同一趟旅行的不可靠性，在我们针对不同固定舰队组成的实验中，并且在没有限制重新分配的情况下，我们发现没有立即被拒绝的请求中有超过三分之一面临一些变化，而这些变化的幅度是相关的：用户面临的等待时间增加了，这个额外的时间可以与整个系统的平均等待时间相提并论，但总的延迟是一样的。为减少这种不确定性而进行的算法更改会导致在总体服务质量方面进行权衡。例如，不允许重新分配可能会增加拒绝请求的数量。关于不同行程之间的不可靠性，我们发现，根据车队的状态，可以拒绝或提供相同的起点-目的地请求。而且当服务完成后，等待时间和总延迟很少相等，这对于不同的机队规模仍然适用。此外，从高需求地区开始的旅行面临着最大的变化。