Unreliability causes extra waiting or delay, the later of which is more heavily weighted. Reliability can be partly considered as a “weakest link” factor as well. However, the arrival time distribution of the last link requires intense analytical work. As when the risk of missing the next link becomes part of our concern, the risk of delay performs rather non-linearly. The scenario also depends on the degree of coordination between modes. The most common example exists when the user need to transfer between flights. If the two connected flights belong to the same airline, the connection can be considered as perfectly coordinated (comparing with other types of connection). Whatever accidental events occur to the first flight, the airline company is responsible for arranging the connected flight. A certain level of service is guaranteed. Whereas if the flights are from different companies, there is no guarantee exist. If the first flight is delayed, the user is responsible for all the monetary and time cost that rises from it. The difference can be also referred to as “inside the system” versus “get out of the system”, where system refers to the condition when two connected modes are coordinated. However, coordination means investment cost. In some cases, coordination may not necessarily improve the situation. For example, to transfer between the airtrain and the flight does not require coordination, especially when the airtrain has a high service frequency. We will further demonstrate this quantitatively in the following section. 3.3.5 Flexibility In our definition of a multimodal transportation alternative, the illustrative structure is a corridor. When it comes to a transportation network, the overlapping between alternatives raises complication in numeration. The number of available alternatives may even exceed the total number of links involved. The network structure may provide convenience, especially in the case when one link in an alternative is shut down, the user may switch to another alternative without sacrificing much of the utility. For example, to transfer from a flight to the train, which means from an airport terminal to the air train station serving it, a user may choose from several options, such as walking, taking a shuttle bus, or though a linking bridge. Even though the modes proceeding and following this link are the same. Therefore each of these choices indicates a transportation alternative, as we defined. From the point of view of utility, one of the alternatives may be dominant (shuttle bus), and the rest may be considered as redundancy (walking). But in case when shuttle bus is not in service (due to accidents or service hour limits), users planning to choose that alternative have the flexibility of switching onto another one (walking) without much delay. In other words, if there are not walking routes or any other substitutes available, the user will need to go all the way back to the directory to search for a taxi. We define flexibility as an indicator of the level of service It describes the ability of a network to adapt to changes (either due to user’s behavioral change or the network setup change). It is overlapping with the reliability issue, as they are both concerned with accident possibility. To some extent, flexibility may contradict with other indicator. For example, direct service provides better LOS in terms of time cost, but sacrificing flexibility. This is also why when direct service and indirect service are usually both offered. The concern of flexibility also accounts for the fact that some transportation modes, especially public transportation, has limited service hour. And therefore, the flexibility of using the mode, or the alternative, is measured by the service hour. Flexibility has similar properties as accessibility, which is also an alternative specific “weakest link” factor.