Counting cars on the M6

We were travelling north at 60mph. Let's assume that the cars travelling south were doing the same. I counted cars for one minute. That would be the same as me being still and them coming towards me at 120mph. We passed 60 southbound cars in one minute. That would the equivalent of 30 per minute at normal speeds. Not too bad. I once counted about 100 cars per minute south at Leicester Forest East one Sunday night.