National Traffic Speeds Survey I: 2007

A field survey was conducted during spring and summer 2007 to measure travel speeds and prepare nationallyrepresentative speed estimates for all types of motor vehicles on freeways, arterial highways, and collector roads across the United States. Over 10 million vehicle speeds were measured at more than 700 sites included in the geographic cluster sample of 20 primary sampling units (PSUs). Each PSU was a city, county, or group of two or three counties representing combinations of regions of the United States, level of urbanization, and type of topography (flat, hilly, mountainous). Speeds were acquired on randomly drawn road segments on limited access highways, major and minor arterial roads, and collector roads.

 http://www.nhtsa.gov/staticfiles/nti/pdf/811663.pdf