Route capacity is the maximum number of vehicles, people, or amount of freight than can travel a given route in a given amount of time, usually an hour. It may be limited by the worst bottleneck in the system,[1] such as a stretch of road with fewer lanes.[2] Air traffic route capacity is affected by weather.[3] For a metro or a light rail system, route capacity is generally the capacity of each vehicle, times the number of vehicles per train, times the number of trains per hour (tph). In this way, route capacity is highly dependent on headway. Beyond this mathematical theory, capacity may be influenced by other factors such as slow zones, single-tracked areas, and infrastructure limitations, e.g. to useful train lengths.