> slow moving cargo bike has a lot of potential kinetic energy

Not as much as people tend to think, our intuition usually underestimates the importance of that squared velocity component. To illustrate, here are some vaguely-plausible numbers from some quick web-searches:

* A 65kg (~143 lb) person rides a big-bucket cargo bike of 65kg with 100kg of cargo, or 230kg total, moving at 5m/s (~11 mph) for 2875 joules kinetic energy.

* The same person on a 25kg passenger bike traveling at 10m/s (~22mph) is 4500 joules.

To be equal, the second rider would have to slow down to about 8m/s.

> a normal human reaction time can deal with it

True, it's easier for an alerted person to leap out of the way of a slower vehicle... but I think if everyone involved was perfectly alert and poised for action, we probably wouldn't be having this conversation.