Headway

The headway between vehicles in public transit systems is the time between two vehicles passing the same point traveling in the same direction on a given route. This term is most frequently applied to rail transport, where the number of tracks is limited and signalling capabilities control the headway. A shorter headway signifies a more frequent service.

Railway headways

Capacity measured by minimum headway

On a railway system, the potential capacity of a line is measured by calculating the minimum headway at each signal along the line in one direction. The minimum headway is the time taken by a second train to traverse the headway distance running on green signals at line speed behind a preceding train travelling in the same direction.

For the headway at a particular signal, the start of the headway distance is measured from the sighting point of the first signal showing a restrictive (non-green) aspect to a following train, if the preceding train was still occupying the overlap distance of the signal being measured. The end of the headway distance is the front of the first train when it is just emerging from the overlap distance beyond the signal being measured. The number of signal spacings between the first and second trains depends on the aspects which are capable of being shown: in 4-aspect signalling it is three signal spacings; in 3-aspect, two signal spacings; and in 2-aspect (rapid transit), from the first repeater to the stop signal of the next signal section beyond it.

Train-pairs per hour

Some rapid transit systems measure their capacity in "train-pairs per hour", the number of trains that pass a given point in one direction in one hour. (When both directions are considered, considering train-pairs leaves the number unchanged.) Headway and train-pairs per hour are related by the formula:

$headway\; in\; minutes\; cdot\; trainmbox\{-\}pairs\; per\; hour\; =\; 60$

So, for 42 train-pairs per hour, as on some Moscow Metro lines:

$headway\; in\; minutes\; =\; frac\{60\}\{42\; trainmbox\{-\}pairs\}\; =\; 1.42\; min\; =\; 1\; min\; 26\; sec$

A high level of service is indicated a high number of train-pairs per hour or a short headway.