Operating Mode Fractions on Urban Roads Derived by Traffic Assignment

The federal test procedure for vehicle emissions measurement treats the first 505 sec of engine operation as the transient mode. Using this definition as the basis, a special technique was developed and used to trace the elapsed time of vehicles from trip origins during the traffic assignment of zone-to-zone trips on a highway network and to determine the proportions of transient and stabilized operating modes on every link of the network. The travel models of the Sacramento (California) urban area were used to apply this technique and derive operating mode fractions for this urban area. The data from the Nationwide Personal Transportation Survey were used to distinguish between cold transient and hot transient modes. The operating mode fractions were stratified by 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 roadway, location, and time of day.