20 mph speed limit sign on Stanford Street at East Ave in the Town of Chevy Chase, Maryland

20 mph speed limit sign on Stanford Ave. at East Ave in the Town of Chevy Chase, Maryland
Originally uploaded by rllayman

I don't recall seeing a posted 20 mph speed limit sign in the Washington region before, although I don't travel on residential streets outside of Washington very much.

Also see "How London Is Saving Lives With 20 MPH Zones" from Streetsblog.

Recall also that I wrote about this similar policy in Montreal. See the Transport Montreal webpage on the topic.

Which derives from Graz, Austria, which implemented such a policy in 1992. From Literature Review on Vehicle Travel
Speeds and Pedestrian Injuries published by the U. S. Department of Transportation, National Highway Traffic Safety Administration:

In Graz, Austria, a city-wide 30 km/h (19 mph) limit on all residential streets (50 km/h (31 mph) on through "priority" streets) was implemented in September 1992 (Sammer, 1997; Pischinger et al., 1995). The change was implemented in response to increasing requests from citizen groups to participate in an area-by-area traffic calming scheme that was gradually including more areas over a ten-year period. The comprehensive areawide program included traffic regulation (signs, roadway markings), extensive and varied public information and awareness campaigns, and supervision (enforcement and speed display boards). About 75 percent of all roads became 30 km/h (19 mph). Injuries decreased from the year before the change to the year after. Minor injuries declined 12 percent, serious injuries dropped 24 percent, and all pedestrian injuries fell by 17 percent. Economic savings from the injury decreases were calculated to be about $6,000,000, a 26 percent drop. Mid-block average and 85th percentile speeds dropped immediately, then gradually recovered to a level slightly below pre-law speeds. Intersection speeds also dropped, by 2.5 km/h (1.5 mph) on average, and the proportion of extreme speeds dropped sharply. Drivers exceeding 50 km/h (31 mph) dropped from 7.3 percent "pre" to 3.0 percent "post." Surveys showed that approval of the reduced speed limits increased steadily after implementation, reaching 68 percent after 18 months for private car drivers, who were the least enthusiastic group throughout. Noise levels, measured on 30 km/h (19 mph) streets, decreased; overall air pollution did not change.