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 pre-determined route
the time interval between passage of consecutive vehicles moving in the same stream, measured between corresponding points (e g front bumper) on successive vehicles
If you make headway, you progress towards achieving something. There was concern in the city that police were making little headway in the investigation. = progress. make headway a to make progress towards achieving something - used especially when this is difficult make headway towards/in/with etc