h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

ah but, motorists are exhorted to leave a gap between them and the vehicle in front "such that they can always stop safely in the distance they can see to be clear on their side of the road". According to this they can be infinitely close when stopped, but your 'six foot' is only OK up to [if very rusty arithmetic serves] about 5 mph. Then they have to start backing off again.

Which is why the variable speed limits can increase capacity on the M25 by decreasing vehicle speed [braking distance is sort of proportional to the square of speed, so halve the speed, the cars get there half as fast, but with only a quarter of the gap between them].

This theory would work better if anyone left the correct gap, which they don't, as witnessed by the fact that most times when there's a crash on a motorway, everyone else stays close enough that they join in.

The article says that nobody leaves a gap of 5-6 feet in a queue at the lights. That is true but have you noticed how car drivers actually start moving just a bit too soon in a queue that has just begun moving off from the lights? By this, I mean that most cars start moving and then have to slow right down (or even sometimes brake) because they get too close to the car in front during the first few seconds of motion. Nearly everyone does it.