An optimal speed limit investigation on highway for emission reductions

Speed limit has significant effect on the emission reductions of urban and highway traffic. This paper studied the relationship between speed limit and vehicle exhaust emissions with the aim of finding the optimal speed limit which would result in the reduction of emissions in the study area. A test section of S342 in Wuxi, China was selected for this study. An integrated model was deployed which combined the microscopic traffic simulation model VISSIM with the emission model established by the data collected from on-board portable emission measurement system OBEAS-3000. VISSIM recorded the second-by-second speed and acceleration of individual vehicle under different speed limit scenarios as the input data. The emission model outputs vehicle emissions through relating emissions to the instantaneous speed and acceleration of the vehicle. Speed limit scenarios were set from 90 to 40 km/h at a decrement of 5km/h and heavy vehicles weighted above 2t in total occupied near 20%. Distance-based emission rate (g/km) was used as the performance measure. The simulation results showed that, vehicles released least CO emissions with a speed limit of 70km/h, and 80km/h was optimal for HC and NOx emissions.