I want to calculate how busy a motorway is from a fixed point, this is usually measured in number of vehicles per hour passing said point
Your problem here is that you are looking at the wrong measurement.
There are two different criteria. 1. Vehicles per hour - volume of traffic - , 2. Average speed of vehicles - rate of traffic flow.
As you are talking about CONGESTION then it is actualy the latter criteria that applies. If you are on a motorway and are doing 70 mph, but the motorway is full of vehicles, no problem as the flow of traffic is unhindered. If you are crawling (the average speed is low) and stop start and the motorway is full because the flow of traffic is held up, then you have congestion.
Both amount of vehicles per hour past a given point and average speed of vehicles past a given point are measured.
Both can then used to calculate an overall view of traffic usage.
For example 1 vehicle per hour @ average 10 mph = some idiot on the motorway who has fallen asleep! 100 vehicles per hour @ average 10 mph = traffic congestion.
This is further complicated by the fact that the number of vehicles over a given area of motorway is calculated at different points. This shows how many vehicles are going into the system. This can be compared with the number of vehicles leaving that stretch of motorway. It is then possible to calculate the number and rate of vehicles leaving the area. This will show if vehicles are being detained in the area and therefore there is traffic congestion.
Well Brian, there you have it.
Keith (must get out on Quicksilver again soon) Hart