A straight single-lane road is filled with cars all travelling at 60 mph in the same direction. They are equally spaced at 100 m between the middles of the cars. At a particular point in their journey a speed limit sign for 30 mph causes them all to slow down to this speed, in the same way, at exactly that same point in the road.

What is the new distance between the middles of adjacent cars after the adjacent cars are travelling at the new (30mph) speed?

by Leslie Green