i am in the middle of measuring average headway for 5 minutes time interval, how to compute the avg headway if the vehicle are in platoon with the single vehicle in a platoon for 4 consecutive platoon and duration between them are 20sec, 25sec 108sec and 209sec. except for lat platoon consist of 5 vehicle with 5,4,6,7 sec..
In general average headway is the reciprocal of flow. So, you have to count the number of vehicles in 5 minutes and the divide the time (that is 5*60 s = 300 s) with that number. You can also give the number of vehicles per hour (flow) and then divide 3600 with that flow value.
However, I am not quite sure, what is your problem and how are you going to use those avarage values. In general, the traffic flow consists of vehicles in platoons with short headways and longer headways between platoons. Both theory and measuremnts say that headways aroud two seconds are the most common in any traffic flow. This is in line with the simple theory of random arrivals (poisson arrivals, exponential headway distribution). If you use the average headways, counted as described above, you can test your arrival distribution with a simple poisson distribution or the headway distribution with a (shifted) exponential function, as a start. More complicated headway distributions can be found in litterature.
The Poisson arrival process/exponential time headway assumption is only plausible for light traffic with flows Q significantly below capacity (typically around 2000 vehicles/h/lane on freeways). Otherwise, i.e., if the arithmetic mean 1/Q of the time headway is of the same order than the minimum time headway due to safety reasons, the ensuing "repulsion effect" distorts the exponential distribution (see the attached figure). Notice also that there is a difference between the (bumper-to-bumper) time gap which is relevant for the driver's action, and the time headway:
time headway=time gap+occupancy time
where the occupancy time is the vehicle length divided by its speed. See Treiber/Kesting, "Traffic Flow Dynamics" (Springer, 2013).
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