There is an art to this traffic engineering. You can design so that the majority of people drive slowly. You have narrow roads, shared spare with pedestrians. So the edge of the highway is not clear. All of this leans to 85 percentile speed dropping with a much better environment.

But (there is always a but), for visually impaired people it does not work that well. For example no kerb edges. For idiots (think stronger word than that), they will drive at the still high speed - at then the accident are worse (severity of outcomes) so it fails.

Speed bumps just are not good - better with a chicane or shuttle working which allows an emergency vehicle to go straight through the obstacle.

>If you want people to drive 20mph, make it hard to navigate at 30mph. There is a wide range of driving skills - design sometime has to account for the worst. Making driving test harder - is a political problem not a technical problem. Driver assist means road get easier to navigate. Driver assist enforcing the speed limit ... is a political problem not a technical problem.