Percent of traffic driving over the posted speed on rural interstates (by quarter)
The calculations for this graph utilize data collected at 10-speed capable continuous automatic traffic recorders (ATR) located in the 70 mph speed zones that went into effect July 1, 2005, for Iowa's rural Interstate Highway System. The quarterly percentages of traffic exceeding the posted speed limit and average speeds are calculated using all vehicles from all days of the quarter in which each ATR point location in the composite was properly collecting speed data.