15ft @ 15mph is a lot easier than 20ft @ 24mph. This standard was developed by the bike lobby to replace the stupid "must be able to skid the rear wheel".
I think: acceleration = 0.5*v^2/d
Right? According to the article cited, the best brakes were able to stop from 40kph in 6.44m, for a deceleration of 9.58 m/s^2. (almost 1g!)
And indeed, the math checks out
0.5*(40kph*1000/3600)^2/6.44 = 9.585
And the worst brakes only managed 0.6g, which caused the reviewers to write:
It was the only brake in this test where we had a true, “I don’t think I’m going to be able to stop,” close call.
So the legal standard of 15ft (4.6m) from 15mph (24.2kph) works out to:
0.5*(24.2*1000/3600)^2/4.6 = 4.8 m/s^2
That's less than 0.5g. Is my math wrong?
That's even worse than the reviewers' "I don't think I'm going to be able to stop" brake -- the one that they said required them to "grossly adjust their braking technique".
@netviln (below, sorry): I don't think that's equivalent to "stand on the front brake and hope you don't endo". I do think that you are right that you can't manage 0.5g braking with a rear brake alone. At best, it would be close. But you can do it with a front brake alone, at least in theory.
Note that the law doesn't actually require that a cyclist has the skill to perform such a stop, only that the equipment must be capable of it.
But I would hope that the denizens of this board actually can do it, and if not, practice!