Methodology for Developing Transit Bus Speed–Acceleration Matrices for Load-Based Mobile Source Emissions Models

An emissions model for transit bus based on road load estimates emissions as a function of transit bus power demand for given transit bus activities and environmental conditions. Transit bus speed and acceleration rates are key activity parameters and are the most important parameters in the estimation of transit bus power demand, also known as engine load. Once the transit bus engine load is calculated for a given speed and acceleration, emissions in grams per vehicle hour can be calculated with grams per brake-horsepower hour emission rates. However, collecting speed and acceleration data on various road types and times of day requires extensive efforts for use in load-based mobile source emissions models. To quantify Atlanta regional transit bus speed and acceleration rates, the Georgia Institute of Technology research team installed trip data in a transit bus operated by the Metropolitan Atlanta Rapid Transit Authority. The team collected second-by-second speed and location data for 3 weeks on a variety of routes and created speed–acceleration matrices by roadway facility type and time of day. This paper focuses on developing a methodology to create transit bus speed–acceleration matrices for use in load-based modal emissions models for the Atlanta metropolitan area. Once a bus service route is specified by roadway facility type and time of day, engine power demand for each speed–acceleration matrix bin can be calculated, weighted by activity frequency on each corresponding matrix bin, and then multiplied by baseline emissions rates that can be obtained from engine dynamometer or chassis dynamometer test results.