Publication Detail
Implications of Using Real-Time and Estimated Speed Data in Air Quality Analyses
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UCD-ITS-RR-95-08 Research Report Alumni Theses and Dissertations |
Suggested Citation:
Page, Dorriah L. (1995) Implications of Using Real-Time and Estimated Speed Data in Air Quality Analyses. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-95-08
Current air quality modeling procedures rely heavily upon estimates of travel behavior characteristics as input to detailed emission quantification for mobile sources. Numerous input parameters are required for each of the models used in air quality analyses. Region by region these models differ, and despite arduous procedures designed to improve their accuracy, oftentimes they fall short of providing reliable output. On the whole, mobile source emissions are difficult to quantify to a great degree of accuracy. However, this problem is exacerbated by the sorely limited procedures designed to reliably reflect real-world travel speeds. Travel speeds can differ significantly from one region to another, as from one road type (facility) to another. As anyone traveling in California is aware, commuting routes designed to bypass freeway segments can often times reduce travel times significantly. Speeds are also significantly different by time of day. This study specifically addresses these differences in speed. Statistically significant results verify that "average" travel speeds do not exist. In fact, there are significant and highly variable differences in real-world travel speeds across lanes, across zones, between regions and by time-of-day.
These differences are particularly important as research by the Environmental Protection Agency (EPA) and California Air Resources Board (CARB) has established that a "significant relationship" exists between travel speed and emission rates (after controlling for trip-starts, trip-ends, diurnal emissions, acceleration, deceleration, cruise and idling). Within certain ranges, hydrocarbons, carbon monoxide and oxides of nitrogen are sensitive to speed. Emission rates also vary significantly by vehicle type. The heavier vehicles have higher emission rates at all speeds and are also more sensitive to speed. Although research is continuing as to the degree to which each of the types of emissions contribute to the overall motor vehicle emissions, an indication of the magnitude of each shows that for emissions of hydrocarbons (reactive organic gases or non-methane hydrocarbons) cold start emissions and running emissions contribute twice as much as stop and diurnal emissions. Approximately one-third of these are contributed by those Vehicle Miles Traveled (VMT). For hydrocarbons, fifty percent of the emissions result trom the trip being made, a combination of the trip start emissions and the evaporative hot soak emissions that occur at the trip end. For heavy duty diesel engines, emissions (particularly NOx) are highly dependent upon operating mode, temperature and speed. These findings clearly demonstrate the importance of accurately reflecting speed profiles in emission estimates.
For air quality modeling purposes it is common practice to use tree-flow speeds for off-peak periods in regions where there is little or no congestion during the off-peak periods. For emission estimation, however, there can be a significant difference between a free-flow speed and the slightly lower loaded speed that results even under uncongested conditions. This can be highly inaccurate, particularly when tree-flow speeds are not indicative of real-time tree-flow speeds presented region by region. The use of a 55 mph speed limit as the upper tree-flow boundary is highly misrepresentative. For example, free-flow speeds on Los Angeles area freeways often exceed 70 mph, whereas in San Diego or Fresno, free-flow speeds may not exceed 65 mph. At these high end ranges, higher emissions may result. It is also important to note that headway variations may also affect tree-flow speeds across facility types and between regions.
These differences are particularly important as research by the Environmental Protection Agency (EPA) and California Air Resources Board (CARB) has established that a "significant relationship" exists between travel speed and emission rates (after controlling for trip-starts, trip-ends, diurnal emissions, acceleration, deceleration, cruise and idling). Within certain ranges, hydrocarbons, carbon monoxide and oxides of nitrogen are sensitive to speed. Emission rates also vary significantly by vehicle type. The heavier vehicles have higher emission rates at all speeds and are also more sensitive to speed. Although research is continuing as to the degree to which each of the types of emissions contribute to the overall motor vehicle emissions, an indication of the magnitude of each shows that for emissions of hydrocarbons (reactive organic gases or non-methane hydrocarbons) cold start emissions and running emissions contribute twice as much as stop and diurnal emissions. Approximately one-third of these are contributed by those Vehicle Miles Traveled (VMT). For hydrocarbons, fifty percent of the emissions result trom the trip being made, a combination of the trip start emissions and the evaporative hot soak emissions that occur at the trip end. For heavy duty diesel engines, emissions (particularly NOx) are highly dependent upon operating mode, temperature and speed. These findings clearly demonstrate the importance of accurately reflecting speed profiles in emission estimates.
For air quality modeling purposes it is common practice to use tree-flow speeds for off-peak periods in regions where there is little or no congestion during the off-peak periods. For emission estimation, however, there can be a significant difference between a free-flow speed and the slightly lower loaded speed that results even under uncongested conditions. This can be highly inaccurate, particularly when tree-flow speeds are not indicative of real-time tree-flow speeds presented region by region. The use of a 55 mph speed limit as the upper tree-flow boundary is highly misrepresentative. For example, free-flow speeds on Los Angeles area freeways often exceed 70 mph, whereas in San Diego or Fresno, free-flow speeds may not exceed 65 mph. At these high end ranges, higher emissions may result. It is also important to note that headway variations may also affect tree-flow speeds across facility types and between regions.
Ph.D. Dissertation