This study examined changes in vehicle emissions as a result of electronic toll collection at three major toll stations (Fort McHenry Tunnel, Baltimore Harbor Tunnel, and Francis Scott Key Bridge) outside Baltimore, Maryland. The system used toll tag readers and in-vehicle transponders to automatically execute toll transactions as vehicles passed through toll booths. Computer models were used to quantify emissions of hydrocarbons (HC), Carbon monoxide (CO), and Nitrogen oxides (NOx), before and after the system was deployed.

The WESTA (Weigh Station) microscopic simulation model and a custom deterministic model were used to estimate traffic patterns during morning peak-hour periods to estimate total travel times and determine typical driving cycles for vehicles at each toll station. The zone of influence was modeled as the area between the point of lane transition for upstream traffic and the point of lane transition for downstream traffic. The data derived from these models was input into the Mobile5b model to quantify mobile emissions.

Field observations were conducted in order to validate the data generated by each model. On-site observations of travel times, average speeds, and arrival volumes were collected at each toll station between 7-8 am on three weekdays (Tuesday through Thursday) in the spring of 2000. The maximum difference between observed and simulated values was found to be approximately 15 percent.

In order to examine the air-quality impacts of electronic toll collection, the following three scenarios were simulated:

• The first scenario involved pre-deployment conditions.
• The second scenario evaluated conditions following the initial deployment of electronic toll collection with a 21 to 28 percent market penetration rate.
• The third scenario represented the current (2001) market penetration rate of 50 percent as determined by Maryland Transportation Authority (MdTA) toll station throughput data.

RESULTS

The results showed that electronic toll collection reduced hydrocarbons and Carbon monoxide emissions by 40 to 63 percent, and reduced emissions of Nitrogen oxides by 16 percent.