Following the recent tragic death of Kim Briggs, killed by cyclist Charlie Alliston riding an illegal bike with no front brakes, there's been lot of debate over whether or not he could stop even if the bike was legal.
These people often cite the Stopping Distances in the Highway Code which suggest at 20 mph a car should stop in around 6m from when the brakes are applied;
This was last updated over thirty years ago, and vehicles have improved a bit since then, so I conducted a (fairly unscientific) experiment to satisfy my own curiosity.
A modern car travelling around 20mph took around 2.8m to stop, less than half the distance indicated by the Highway Code.
With respect to the Charlie Alliston case, I've seen people argue that "a car couldn't have stopped in that distance" ... yes it could, easily. But, then again, my car has brakes..
There isn't any. As I said, this was a fairly unscientific test to satisfy my own curiosity. Reproducing under test conditions is an exercise for the reader if they have the resources do to so.
However, I believe this is a reasonable indication of a real-world situation and I'm confident that any margin of error is definitely nowhere near 6m (the vehicle itself is 4.6m long)
Various other sources would seem to support these findings;