Time Period and Vehicle Class Highway Assignment for Air Quality Conformity Evaluation

Regions that have not attained the National Ambient Air Quality Standard for ozone must demonstrate that their transportation plans and programs conform to a state implementation plan to meet national air quality standards. Vehicle emissions are estimated by metropolitan planning organizations for these conformity evaluations, and the state of the practice is to calculate vehicle emissions from the vehicle distances traveled at different speeds. A highway assignment procedure for air quality conformity has been developed at the Chicago Area Transportation Study (CATS) to provide more detailed estimates of how vehicle distances and speeds vary by time of day and by type of vehicle. The assignment procedure prepares tables of vehicle distances traveled by speed for six classes of vehicles and eight time periods during a typical workday. The eight time period assignments more correctly depict network congestion than the peak—off-peak or average daily highway assignments previously carried out by CATS. The separate vehicle class link volumes reflect the geographic and time period distribution of trips by light- and heavy-duty vehicles. The conformity assignment procedure and the problems encountered in adapting existing highway assignment practices to allow for additional time periods and vehicle classes are outlined. Some conformity assignment results are compared with a daily average assignment to illustrate how the time period assignment procedure affects vehicle emission estimates.