The syncopated clops of horse shoes hitting the ground. At 4 mph and below the gait would be a walk. At speeds from 5 mph to 10 mph the sound should transition to a trot. From 10 mph to 17 mph it should be a canter, and speeds from 18 mph to 30 mph the sound should be a gallop. In this way, sight-impaired people will know how fast a vehicle is approaching so they can adjust accordingly.
I have a serious comment that I'd like to send to this National Highway Transportation Safety Administration study. Do you know how I can submit such feedback to them?
In particular, I want to recommend to them that they not make recommendations not specific to EVs or hybrids, but to any vehicles that make less than so-and-so much sound volume at low speed, regardless of why that may be the case.