When does highway construction to mitigate congestion reduce carbon emissions? A Case Study: The Caldecott Tunnel

Abstract:

The construction of the fourth bore of the Caldecott Tunnel, which connects Oakland and Moraga, CA on State Route 24, was the second largest roadway construction project in California last year with a total cost of $417 million. The objective of the fourth bore was to reduce traffic congestion before the tunnel entrance in the off-peak direction of travel, but the project was a source of conflict between policy makers and environmental and community groups concerned about the air quality and traffic impacts.

We analyze the impact of the opening of the fourth bore on CO₂ emissions associated with traffic. We made surface observations of CO₂from a mobile platform along State Route 24 for several weeks in November 2013 incorporating the period prior to and after the opening of the fourth bore on November 16, 2013. We directly compare bottom-up and top-down approaches to estimate the change in traffic emissions associated with the fourth bore opening. A bottom-up emissions inventory was derived from the high-resolution Performance Measurement System (PeMs) dataset and the Multi-scale Motor Vehicle and Equipment Emissions System (MOVES). The emissions inventory was used to drive a box model as well as a high-resolution regional transport model (the Weather and Regional Forecasting Model). The box model was also used to derive emissions from observations in a basic inversion.

We also present an analysis of long-term traffic patterns and consider the potential for compensating changes in behavior that offset the observed emissions reductions on longer timescales. Finally, we examine how the results from the Caldecott study demonstrate the general benefit of using mobile measurements for quantifying environmental impacts of congestion mitigation projects.