Measuring the impacts of traffic flow improvement programs with instrumented vehicles

The emission impacts of traffic flow improvement programs (TFIPs) have long been estimated with models. These model estimates are often subject to coarse assumptions which limit model accuracy. Development and greater availability of on-road vehicle portable emissions monitoring systems (PEMS) provides a technology with which the efficacy of emissions modeling of TFIPs impacts can be examined. To better understand the limitations of TFIPs modeling, a case study was undertaken comparing modeled emissions with those collected during on-road operations. HOV (high occupancy vehicle) lanes were selected as the TFIPs to be examined, contrasting operations in these lanes with those in GP (general purpose) lanes. Results of on-road carbon dioxide (CO2) and carbon monoxide (CO) emissions were found to vary with speed as across GP and HOV lanes and the operational period. No clear differences in total on-road emissions of nitrogen oxides (NOx ) and hydrocarbons (HC) were measured across the study variables. When compared with the on-road data, the model failed to detect differences in estimated mean emissions in the majority of cases. Despite this, the model did provide emission trends reasonably comparable to mean emissions reflected in the on-road data. The PEMS technology demonstrated its benefit as a research tool, and as effective technology for collecting data to support further refinement of emissions models. Based on this study, it is expected that for the foreseeable future emissions modeling will remain the most cost effective and responsive method of forecasting changes in vehicle emissions associated with TFIPs.