There are many variables that do not vary linearly with the speed (e.g. rubber's stiffness) so it could be that the sound is indistinguishable at higher speed.

There are many variables that do not vary linearly with the speed (e.g. rubber's stiffness) so it could be that the sound is indistinguishable at higher speed. The goal is clearly to slow people down, so that would be really surprising if it would work the same way at higher speed, wouldn't it be? Maybe I'm just too naive..