The new mayor of Paris, Madame Anne Hidalgo, is downspeeding almost the entire city of Paris to 30 kilometres per hour. There are a few exceptions, like the speedways along the Seine that shouldn't be there in the first place and the ring roads, but otherwise everything goes to the equivalent of 18.6411 miles per hour. (Watch one of the best car chase scenes ever filmed, on those Seine speedways and other Paris streets, with Robert DeNiro at the wheel in Ronin) Eric Britton of World Streets explains why this is so important:
As traffic speeds are significantly brought down across the city, a number of very important things occur as a direct result: substantially fewer accidents, significant reduction in serious injuries and deaths, energy savings, reduce dependence on imported fossil fuels, local air pollution reduction, quality-of-life improvements all those who live and work, and play and study there, improved conditions and local accessibility for local business, significantly reduced carbon stress on climate, and the long list goes on.
He also notes that it is important to be consistent. "the creation of a uniform driving environment offers numerous advantages. It becomes much easier for drivers to respect speed limits where they are not confronted with continuous and confusing changes from street to street, where even the conscientious citizen can find themselves not always respecting the letter of the law."
World-Class cities do this.
While world-class city is a really loaded phrase where I live, Britton applies the term to those cities that are highly agreeable places to live and bring up your kids. He is perhaps a bit over the top when he puts a whole lot of baggage onto a speed limit:
So if your city is today characterized by declining mobility conditions, unnecessary traffic, danger, accidents, air pollution, aggressive mobility environments, traffic noise, and the lack of abundant and agreeable choices of ways of getting around other than by car, it is going to lose out in the international competition. You will lose the best and the brightest of your population, who will move to a more congenial, more excellent environment. And when they move out, so does your tax base. One result will be that your city will become a place of increasing social tensions. And at the end of the day, all that is a choice.