Average speed system needed

Dear group,

I am still on the trail of system that will help us to slow traffic down. We have incidentally started monitoring speed with a plastic bag marker  and stopwatch over distances of 700 metres and have results highlighting the extent of the problem. Some vehicles are doing twice teh speed limit of 80, and whilst we still need an instantaneous speed camera, there is now greater need for someone to help us with basic system so that we can reliably determine average speed over a distance of 70 km.  Reliable is key as we are getting closer to having law enforcement agencies assist.  

So what do we now need? We  envisage a camera that will read the number plate, Record the time, take a picture, and transmit these data via a cell network to enforcement of the other end who will have a similar system to match the plate number and have the average speed calculated.

Any ideas welcome.