Increased speed saves time only linearly but requires a with the square increase in power to overcome wind resistance, the major factor. With the power increase comes a similarly large increase in crash damage and mortality particularly for pedestrians. This is in addition to the increased crash risk simply due to the increase in speed while mental capabilities of drivers remain constant(ly poor).<p>See the data here:<p><a href="http:&#x2F;&#x2F;humantransport.org&#x2F;sidewalks&#x2F;SpeedKills.htm" rel="nofollow">http:&#x2F;&#x2F;humantransport.org&#x2F;sidewalks&#x2F;SpeedKills.htm</a><p>As this makes clear, even 30mph is too fast for a residential area in the sense that the tradeoff makes no sense; 20mph brings a massive reduction in deaths but only a comparatively much smaller increase in travel time (in a grid city  with constant lights anyway).