America's Fastest and Slowest States, by TomTom
The results of the study, which you can sweep through below, are based on data from Speed Profiles, TomTom's historical speed database. The data were organized to show the average speeds on interstate highways and to reflect only free-flow speed data, or average speeds when there is no traffic present.
The study found that Mississippi has the fastest roads, with average speeds of just over 70 mph, this while posted speed limits ranging from 65-70 mph. The top five fastest states in the US is completed by New Mexico (70 mph actual average, 70-75 mph posted speed limits), Idaho (70 MPH actual average, 65-75 mph posted speed limits), Utah (70 mph actual average, 65-75 mph posted speed limits) and Alabama (70 mph actual average, 60-70 mph posted speed limits).
The fastest road in the US is the I-15 in Utah and Nevada, with speeds averaging 77.67 mph, while the biggest concentration of fast roads can be found in the middle of the US, where 15 states with the fastest highways are located.
At the opposite end, Washington D.C. has the slowest average mph on its interstate highways (46 mph), not because the drivers here are worse, but because of the traffic congestions. Trailing Washington are Hawaii (53 mph), Delaware (61 mph), Rhode Island (63 mph), and Oregon (63 mph).