The Federal Test Procedure (FTP) for vehicle emissions measurement treats the first 505 seconds of engine operation as the transient mode. Using this definition as the basis, this study developed and used a special technique to trace the elapsed time of vehicles from trip origins during the traffic assignment of zone to zone trips on a highway network and determine the proportions of transient and stabilized operating modes on every link of the network. The travel models of the Sacramento Urban Area were utilized to actually apply this technique and derive operating mode fractions for this urban area. The data from the National Personal Transportation Survey (NPTS) were utilized to distinguish between cold transient and hot transient modes. The operating mode fractions were stratified by both functional class of roadways and the location within the urban area. The time of day of travel also was used for further stratification. The results show wide variations in the operating mode fractions among the different categories of roadways, location, and time of day.


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