The NCHRP 25-21 project identified and investigated most significant impacts of traffic-flow improvements on travel behavior and air quality suspected or known at this point in time. The impacts of traffic-flow improvements on household trip making, destination choice, time-of-day choice, mode choice, and route choice have been considered and included in a recommended comprehensive methodology for predicting the air quality impacts of traffic-flow improvements. The long-term impacts on the redistribution of future economic activity from less accessible areas of the http://gulliver.trb.org/publications/nchrp/nchrp_rpt_535.pdfregion to more accessible areas have also been considered and incorporated into the methodology. Only two identified impacts of traffic-flow improvements on air quality have been intentionally excluded from the methodology: the potential direct impact on the overall growth of a metropolitan region and the potential indirect impact of traffic-flow improvements on actual or perceived accessibility (via nonmotorized modes) for transit, pedestrian, and bicycle modes. Both impacts were excluded because of the lack of necessary data and limitations of project resources. The NCHRP 25-21 methodology was applied to a series of case studies, and the results were compared with more general results reported in the literature. The facilityspecific results showed travel time and volume changes that were consistent with theory and expectation. However, it was harder to validate the methodology’s predictions for system-level (i.e., regionwide) performance. Some of the results fell within the broad range of results that have been reported in the literature. Other results fell outside the range of results reported in the literature. Indeed, application of the methodology to the same traffic improvement at different locations in the region showed a wide range in predicted systemwide impacts. The same type of project (adding an HOV lane, for example) resulted in net benefits or disbenefits to regional emissions, depending on its location. The NCHRP 25-21 methodology was applied to 10 case studies. The impacts of This report may be accessed by Internet users at individual traffic-flow improvement projects on regional daily vehicle-miles traveled (VMT) were on the order of a few hundredths of 1 percent. A 30-year improvement program impacted regional VMT by less than 1 percent. The impacts varied from a reduction in emissions to an increase in emissions, depending upon the specifics of each case study. The case study results suggest that more applications of each traffic-flow improvement type in different facilities, in different area types, and at different congestion levels are needed to better understand the conditions under which traffic-flow improvements contribute to an overall net increase or decrease in vehicle emissions. (Author/publisher)
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