Role of Heavy-Duty Freight Vehicles in Reducing Emissions on Congested Freeways with Elastic Travel Demand Functions

Abstract

This paper investigates the effect of heavy-duty (HD) vehicles (primarily road freight) on the traffic congestion–emissions relationship. Unlike previous studies, this research explicitly considers the effects of travel demand elasticity by vehicle class on total emissions. Modeling results show that, even as a small share of the traffic volume, HD vehicles can contribute a large share of total pollution emissions, especially for particulate matter and nitrogen oxides. HD vehicle emission rates are more sensitive to congestion than are light-duty (LD) vehicle emission rates, and thus greater emissions benefits may result from mitigating congestion for these vehicles. Potentially lower travel demand elasticity with respect to speed for HD vehicles further indicates vehicle class–specific benefits from congestion mitigation. Differences between LD and HD vehicles suggest greater air quality benefits from vehicle class–targeted congestion mitigation or lane and capacity management strategies. HD vehicle travel demand elasticity is a key parameter for predicting the net emissions effects of congestion. It is strongly recommended that analysis of emissions effects from congestion mitigation strategies include class-specific volume forecasts. However, the estimation of HD vehicle travel demand elasticity values has received scant attention in the literature.