Estimating brake-wear emission reductions with new speed-dependent correction factors
On-road mobile sources are still considered a major pollution source, especially in high-density urban areas. Regulatory measures have significantly decreased the emissions from tailpipes but other uncontrolled emissions from brake-wear, tire-wear and road dust resuspension have remained. Models such as U.S. EPA’s Motor Vehicle Emission Simulator (MOVES) and CARB’s Emission Factor model (EMFAC) emissions models project an increase in vehicle miles traveled suggesting that these uncontrolled emissions will continue to increase. However, these emission inventory models have not accounted for changes in emissions due to the introduction of advanced technology vehicles and new materials. It is assumed that regenerative braking will significantly decrease brake-wear emissions but quantitative data on this reduction is not yet available. Additionally, the emission factors used as inputs to the EMFAC and MOVES models are from older studies and do not reflect current technologies. Finally, the EMFAC model does not account for the speed-dependent behavior of these uncontrolled emissions. Brake-wear is estimated to contribute a larger fraction compared to tire-wear and a standardized method for quantifying brake-wear is being established by a coalition of government and industry leaders, led by the European Joint Research Centre. CARB, the U.S. EPA and Caltrans are currently funding two laboratory research projects which use the recommended enclosed brake dynamometer method to establish new emission factors for light-duty and heavy-duty vehicles. Two of the vehicles being tested are light-duty regenerative braking capable vehicles, which will aid in estimating future projections in brake-wear emission reduction. These new emission factors will be calculated over cycles with varying average speeds which will help establish speed-dependent behavior. The project being proposed here would consist of using new speed-dependent emission factors developed from the current research projects discussed above and funded by CARB, EPA and Caltrans, and combine them with real-world and micro-traffic simulations to assess roadway facility designs that minimize brake-wear emission factors. The awardee would reference the American Association of State Highway and Transportation Official’s (AASHTO) Green Book and assess a range of typical facility types. Facilities could include on and off-ramps, intersections, urban streets and arterial roadways. The awardee would be required to identify facility designs that minimize uncontrolled emissions, specifically those from brake and tire-wear emissions, as well as energy usage in order to minimize greenhouse gas emissions across various types of facilities. This would be of great interest to transportation planners such as those from Metropolitan Planning Organizations (MPO), which conduct conformity modeling on a regional scale, and Departments of Transportation (DOT), which sponsor and conduct project level conformity analysis. The awardee could work with an MPO or DOT to look specifically at future planned projects as case studies to enhance the project outcome. The modeling results would be summarized and turned into a reference, potentially to be included as an appendix in the AASHTO Green Book, and used by transportation planners for reducing uncontrolled emissions. This would be hugely beneficial in areas that are in non-attainment for PM2.5 and which are seeking different methods to reduce emissions.
New transportation projects require extensive planning and need to comply with transportation conformity requirements. The payoff of this research project is two-fold: 1) by assessing existing facility designs, or providing new designs, that minimize uncontrolled emissions, areas seeking to implement new projects can minimize their impact on emissions from brake and tire-wear using modernized emission factors and, 2) this information will be especially useful in non-attainment areas for PM2.5 seeking different ways to reduce emissions to reach attainment.
California Air Resources Board
June 4, 2019
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