Simulating the uncertain environmental impact of freight truck shifting programs

Abstract Shifting freight from regular hours to nighttime has been widely used to relieve traffic congestion and reduce diesel emissions. However, there are also several cities that restrict trucks from operating at night to meet the noise regulation. Uncertainty exists in terms of the environmental impact of these truck shifting programs. On one hand, shifting trucks from peak hours can reduce the total emissions by easing congestion, while the impact of moving truck emissions to nighttime will be interacted with more stable atmospheric boundary layer and lower mixing height, increasing the nighttime concentration. On the other hand, shifting trucks from the night to day avoids the emissions with a stable atmosphere at night, while worsening traffic condition at daytime, and hence increasing the total emissions. The research presents a realistic case study and explores a comparative analysis of PM 2.5 concentration originated from a hypothetical freeway corridor and concentrated at a hospital located at 200-m far away from the corridor by implementing four truck strategies applied under various scenarios with traffic demands and heavy-duty trucks (HDTs) proportions. Two meteorological conditions are also considered, with one representing coast region and unstable atmospheric boundary layer, and thus good dispersion condition, and one inland region with a relatively stable atmospheric condition and bad dispersion condition. The results indicate that shifting trucks from peak-period to off-peak period is effective to reduce both emissions and 24-hr average concentrations in scenarios of high traffic demands and high truck proportions, in which the peak period traffic is congested and thus the benefits of reducing congestion and resultant emissions are quite large by altering freight trucks from peak period. It is also observed in the scenarios with lower-traffic demand where the traffic congestion is less severe (in this case, the throughput is less than 75,000 vehicles per day on a three-lane freeway segment, and average speed is higher than 60 km/h), although shifting truck from night to day increase emissions by adding the shifted traffic demand at peak period, it decreases the average concentration by avoiding nighttime emissions. Such a benefit will be further amplified in the meteorological condition of low mixing height at night. Shifting trucks from daytime to nighttime is found to be the strategy that contributes to the highest average PM 2.5 concentration, although sometimes the total emissions are minimized.