headways

Headways are a measure of the separation between successive vehicles traveling on the same route, lane, or track. The term can refer to two related concepts: time headway, the time interval between consecutive vehicles passing a reference point, and spatial headway, the physical distance between vehicles at a given moment. Headways are a fundamental parameter in traffic flow, urban transit operations, and safety analyses.

In transit systems, headway largely determines service frequency and system capacity. For rail and bus networks, the minimum practical headway is constrained by signaling or signaling-free safety rules, vehicle acceleration and braking, dwell times at stations, and disturbances in the network. Shorter headways increase capacity but raise the risk of cascading delays if a vehicle falls behind. Operators study headways to balance efficiency, reliability, and resilience to disruptions.

In road traffic, headway is commonly used to describe following distance. Operators and drivers consider safe time headways to reduce the likelihood of rear-end collisions, with the required distance growing at higher speeds and in adverse weather. Traffic engineers model headways to estimate capacity, manage merging and lane-changing, and design intersection timing plans.

Headways are measured or inferred from fixed sensors, vehicle trajectory data, or automated monitoring systems. They can be expressed as time headways (seconds) or spatial headways (meters) and are affected by mode, speed, vehicle performance, driver behavior, and network conditions. In practice, headway management supports reliable service, safer travel, and efficient use of infrastructure.