For 100% Human Powered, I find is that speed is related to weight.

So, if I'm pulling 400 lbs, I'm lucky to get 5 MPH on the level, sometimes even slower. It is frustrating when pedestrians start passing me.

Although, liability when you're pulling "Precious Cargo" is also more important.

Hills, of course, completely change the brake equation. A major hassle for climbing the hills, but descents can be tricky if a rapid stop is required.

Electric Assist changes the equation, so if you're pulling a load that you should be pulling at 4 MPH, but rather are pulling it at 20 MPH... you need to be prepared to STOP.