Over on the comments to this post, there’s a discussion about the merits of lowering the speed limit to 25 MPH in Newton when drivers are comfortable — on nearly all roads — going significantly faster. It’s a fair question.
One answer is easy: while it is comfortable — and even, likely, safe — for the driver to go faster, it’s definitively less safe for pedestrians on the road. As speeds go up — even within the range of what folks are comfortable driving on Newton roads — the risk of death and serious injury goes up dramatically. Just look at the chart, from a Federal Highway Administration document. The figures are for risk of death and serious injury from a crash. The numbers are even more dramatic when you consider that lower speed increase the likelihood of avoiding a crash in the first place.
If all we cared about were drivers and passengers in cars, lowering the speed limit would make little sense. But, we also care about pedestrians and cyclists. Not only do we care about their safety, we care about their comfort. If our roads have slower traffic, cyclists and pedestrians will feel safer (because they are) and bike and walk more.