Well there are two options wrt controlling speed, one are the limits so that everyone kniws what to do and expect, and the second is bumps, e.g. London where driving on tarmac is as comfortable as driving down on steps. It's not reasonable to trust people driving tonnes of steel with liters of flammables inside impeccably at high speeds, especially in residential areas where roads are more like a tightly knit cobweb than a straight line from a to b.