The likelihood of severe injury or death goes down dramatically with lower speeds. A reasonable speed is a variable concept, but certainly minor residential streets should never have speeds in excess of 40 kmh (25 mph). My city recently reduced the limit to 40 kmh on all such streets. Now they just have to go the other 95% of the work - physically designing the streets for their posted limits, so that the option of speeding is simply removed (unless you're okay with regularly spending money at a body shop).

30 MPH is not in no way a reasonable speed for side streets in a neighborhood that you want to be walkable and child-friendly. Save 30 mph for major arterials. If neighborhood side streets have 30 mph speed limits people are just going to blow through any stopsigns the city puts in to pretend it is pedestrian friendly, and they are going to kill people when they do so.

> If that's the goal is to cause no harm, we can't have cars.

Careful, some people actually want to ban cars.