HEADWAY

n. progress, advance

The headway between vehicles in public transit systems is the amount of time (usually in minutes) that elapses 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 signaling capabilities (usually) control the headway time. It can also be used to denote the distance between two cars; safe headways for automobiles is two seconds.
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Noun
1. vertical space available to allow easy passage under something
(synonym) headroom, clearance
(hypernym) room, way, elbow room
2. forward movement; "the ship made little headway against the gale"
(synonym) head
(hypernym) progress, progression, advance

(n.)
The progress made by a ship in motion; hence, progress or success of any kind.
  

(n.)
Clear space under an arch, girder, and the like, sufficient to allow of easy passing underneath.
  

The time interval between transit revenue vehicles passing a specified location. (FTA1)