While I *really* like riding fast in cars and getting to go many places in short amounts of time, an article in Low Tech Magazine suggests that the most sustainable future is at half the speed. “If we cut the average speed of all vehicles by half, fuel consumption would decrease by a whopping 75 percent” the article begins. The article goes on to mention that aerodynamics have not succeeded in mediating this, as a boxy volvo is still far more energy efficient at 60 kph that a honda insight at 120 kph; but energy efficiency is not the only benefit to slowing down. In a nine year old article called Reducing the speed limit to 20 mph in urban areas the following observations are made:

The evidence of increased pedestrian safety at 20 mph is strong. The chance of a pedestrian being seriously injured or killed if struck by a car is 45% if the car is travelling at 30 mph but only 5% at 20 mph.6 Government research showed that 20 mph zones reduced the incidence of traffic accidents by 60% and cut child pedestrian and child cyclist accidents by 67%, while overall vehicle speeds fell by an average 9.3 mph (14.9 kph).7 There was no evidence that accidents increased on surrounding roads. Research by local councils produces similar results. For example, Havant Borough Council has imposed a 20 mph limit on 20 miles of road and has seen traffic accident casualties drop by a significant 40%.

There are signs too that a policy of reduced urban speed limits would be acceptable to the public. Among viewers of a Carlton Television programme who responded to a survey, over 80% favoured a 20 mph limit on all residential roads in London.8 In continental Europe the public response has been largely positive. Graz, in Austria, adopted a 30 kph (18 mph) limit through most of the city, cutting serious casualties by over a quarter and dramatically reducing noise and air pollution. Fewer than 5 people out of 10 supported the initiative when it was first introduced, but 8 out of 10 support it now.

Follow that with an article from StreetsWiki, the Livable Streets Initiative site:

20 mph Zones Improve the Economy

The UK study of best traffic practices across the Europe and the U.K. concluded that 20 mph streets also increased pedestrian activity, bicycling, a sense of safety among residents, and economic activity. The study cited evidence from the city of Horsham, U.K., where 20 mph speed limits, along with a bypass road, public arts, gardens, and other pedestrian amenities have lead to the opening of new shops and restaurants, and a higher level of overall eoconomic activity.

Just sayin.