Impact of horizontal geometric design of two-lane rural roads on vehicle CO2 emissions

[EN] In 2014, highway vehicles accounted for 72.8% of all Greenhouse Gases emissions from transportation in Europe. In the United States (US), emissions follow a similar trend. Although many initiatives try to mitigate emissions by focusing on traffic operations, little is known about the relationship between emissions and road design. It is feasible that some designs may increase average flow speed and reduce accelerations, consequently minimizing emissions. This study aims to evaluate the impact of road horizontal alignment on CO2 emissions produced by passenger cars using a new methodology based on naturalistic data collection. Individual continuous speed profiles were collected from actual drivers along eleven two-lane rural road sections that were divided into 29 homogeneous road segments. The CO2 emission rate for each homogeneous road segment was estimated as the average of CO2 emission rates of all vehicles driving, estimated by applying the VT-Micro model. The analysis concluded that CO2 emission rates increase with the Curvature Change Rate. Smooth road segments normally allowed drivers to reach higher speeds and maintain them with fewer accelerations. Additionally, smother segments required less time to cover the same distance, so emissions per length were lower. It was also observed that low mean speeds produce high CO2 emission rates and they increase even more on roads with high speed dispersions. Based on this data, several regression models were calibrated for different vehicle types to estimate CO2 emissions on a specific road segment. These results could be used to incorporate sustainability principles to highway geometric design.
The study presented in this paper is part of the research project titled "CASEFU - Estudio experimental de la funcionalidad y seguridad de las carreteras convencionales" (TRA2013-42578-P), subsidized by the Spanish Ministry of Economy, Industry, and Competitiveness and the European Social Fund. In addition, the authors would like to thank the Center for Studies and Experimentation of Public Works (CEDEX) of the Spanish Ministry of Public Works for subsidizing the field data collection and the General Directorate of Public Works and Transportation of the Valencian Government, to the Road Department of the Valencian Provincial Council, and to the Spanish Ministry of the Interior, especially the General Directorate of Traffic of Spain, for their cooperation in field data gathering. Finally, we would like to thank Professor Hesham A. Rakha, Virginia Tech (USA), for providing the VT-Micro model and assessing the authors in its use to obtain outcomes.