The 20mph campaign/s and arguments...

Different vehicles are already treated differently by the law, trucks and busses have different speed limits on motorways and national speed limit zones, the ones signed are for cars as there is a fuckload more of them.

This is an important factor, HGV for example, have a longer braking distance than a says, saloon cars at the same speed.

Found this handy chart showing the stopping distance of each speed (including the thinking time).