How to design a city?

Speed does not cause accidents, it only determines the magnitude of an collision after it has occured the majority of accidents are caused by dangerous driving, because you can't quantify dangerous driving, and you can quantify speed, it's much easier to say that speed causes accidents, as that way it is easier to "solve" the problem.

The real solution is to improve driver skills and reactions.

Michael, this is definitely not true. Speed is most certainly a major contributory factor in many (though of course not all) collisions and this is backed up by copious research, e.g. a much-misquoted TRL report.

It is really not a solution to just improve driver skills and reactions. It needs to be at least coupled with a commitment to drive slowly. High skill levels among drivers, paradoxically, can cause higher speeds. What matters is the degree of commitment that they have to driving in such a way that does not cause fear and road danger. Plus, some highly skilled drivers, e.g. racing drivers driving in normal traffic, exercising their skills can scare the living daylights out of other, less skilled drivers.

The key is always, always, to reduce road danger at source. That means avoiding high speeds in the first place, not attempting to manage them using second-best sticking plasters.

Someone hit by a car doing 20mph has a 95% chance of survival and only a 55% chance at 30mph. The vast majority of those who survive a 30mph crash will sustain serious injuries.

Also beware of the word 'accident'. It doesn't do justice to the fact that rates of collision are almost built-in system features rather than accidental. Collision rates have gone down gradually but often at the expense of people shying away from cycling and walking and using some sophisticated statistical comparisons haven't improved much in the last fifteen years.