Route capacity

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, such as a stretch of road with fewer lanes. Air traffic route capacity is affected by weather. For a metro 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.

Any assessment of the effectiveness of a transport network includes a calculation of what capacity is used, how it is used, and whether it is used effectively. For instance, overloaded routes may need to be upgraded, or capacity provided by other routes. Unused capacity can represent an opportunity to move more people or goods: as the capacity exists no additional investment is needed. Many transport networks have unused capacity.

External factors affect route capacity in different ways. Severely overcrowded highways will reduce the capacity of bus services. Severe snowfalls will reduce the capacity of highways and freeways, and high winds will make landing and departing airports difficult. In many cases route capacity will vary day to day depending on external factors. Rail systems are more rarely affected by external factors.

Routes can become congested where only a fraction of routes can accept certain traffic types. For example, a road may have a low bridge that restricts the height of any trucks (lorries), or a rail line may be unable to accept wagons loaded beyond a certain axle load. This will result in any route that can accept a wider range of vehicles being congested, and other more restrictive routes be underutilised. Rail traffic between the US and Mexico is limited by the types of vehicles, especially grain wagons, and as 2009 the only routes that could accept newer rail wagons passed through Texas.

Bottlenecks play a large role in determining route capacity. Along any route the capacity is limited to the point with the lowest capacity, and long routes may have their capacity compromised by one bottleneck. Where more vehicles enter a route than a single bottleneck can accept, then the route will be free of congestion at all points except at the bottleneck. For this reason bottlenecks are often the focus of transport improvement projects.

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