Headway

Headway is a measurement of the distance or time between vehicles in a transit system. The precise definition varies depending on the application, but it is most commonly measured as the distance from the tip of one vehicle to the tip of the next one behind it, expressed as the time it will take for the trailing vehicle to cover that distance. A "shorter" headway signifies a more frequent service. Freight trains might have headways measured in parts of an hour, metro systems operate with headways on the order of 1 to 5 minutes, and vehicles on a freeway can have as little as 2 seconds headway between them.

Headway is a key input in calculating the overall route capacity of any transit system. A system that requires large headways has more empty space than passenger capacity, which lowers the total number of passengers or cargo quantity being transported for a given length of line (railroad or highway, for instance). In this case, the capacity has to be improved through the use of larger vehicles. On the other end of the scale, a system with short headways, like cars on a freeway, can offer very large capacities even though the vehicles carry few passengers.

The term is most often applied to rail transport, where the number of tracks is limited and signalling capabilities require long headways between trains. Newer signalling systems and moving block controls have dramatically reduced headways in modern systems compared to the same lines only a few years ago. In principle, automated personal rapid transit systems and automobile platoons could reduce headways to as little as fractions of a second.