There are two kinds of safety: collisions between cars and cars, and collections between cars and everything else (pedestrians, bicycles, children). For a limi

There are two kinds of safety: collisions between cars and cars, and collections between cars and everything else (pedestrians, bicycles, children). For a limited-access road or grade-separated road (especially electronic toll roads), simply minimizing the speed difference between cars is enough. For the pedestrian case, you want to minimize the speed difference between cars and pedestrians. Speed limits are only part of a plan to accomplish this goal -- instead, you want affordances that make people naturally drive slower: removing car lanes (replacing them with trees or sidewalk or protected bike lanes), narrower lanes, adding curves to the road (but without adding blind corners), adding pedestrian crossings, and so forth.