headway

noun6

1. •
   A movement ahead or forward.
2. •
   Forward motion, or its rate.
3. •
   (figurative) Progress toward a goal.
4. •
   The interval of time or distance between the fronts of two vehicles (e.g. buses) moving in succession in the same direction, especially along the same predetermined route.

   • The whole of the main lines to be electrified were being equipped with four-aspect colour-light signals, automatically operated, where appropriate, and spaced to give a 5min headway throughout. — 1961
5. •
   The clearance beneath an object, such as an arch, ceiling or bridge; headroom.

   • So as not to impede navigation the two principal spans of the bridge were designed to provide a clear headway of 157 ft. above high water for a distance of 500 ft.; [...]. — 1941
6. •
   A crossheading.