Texas Transportation Institute defines medium urban areas as over 500,000 and less than 1 million and large urban areas as over 1 million and less than 3 million. Since the 2009 CTSIP data report, Austin has been redefined as a large city based on this criteria.
The methodology used to calculate Travel Time Index and congestion costs have been developed by the Texas Transportation Institute over several years and several research projects. The congestion estimates for all study years are recalculated every time the methodology is altered to provide a consistent data trend. There were several methodology changes applicable to the most recent data, and CTSIP has included recalculated values for all study years.