START MODES OF TRIPS FOR MOBILE SOURCE EMISSIONS MODELING

An important determinant of vehicle emissions during a trip is the engine temperature at trip start. A trip start may be classified as a cold start or a hot start depending on the emission control equipment and the duration of engine shut-off period before starting the engine. Cold starts are usually associated with higher concentrations of carbon monoxide and hydrocarbons than are hot starts. The emission modeling process uses these start modes as direct or indirect inputs to procedures or models that would be used to determine the portion of vehicle miles traveled in transient and stabilized operating modes. A methodology for determining the operating mode fractions at trip ends is shown. Specifically, a comprehensive analysis of personal travel data available in Nationwide Passenger Transportation Survey data is performed for deriving start mode fractions at trip origins and operating mode fractions at trip destination points. Start mode fractions as cold starts and hot starts are derived for different trip purposes and for each hour of the day. It was observed that the trip purpose is the most important explanatory variable for variance in cold starts, followed by the temporal variables such as the time of day at which the trip is made. The sizes of an urban area and individual metropolitan statistical areas are found to be the two most appropriate spatial variables for which aggregated start mode percentages may be derived. The start mode fractions derived from this methodology will be useful for a variety of mobile source emission modeling exercises.