The variable speed limits (VSL) impact on emissions reduction was initially based on traffic data collected by the Texas Department of Transportation on February 6, 2003. This data was used to randomly generate vehicle speeds for the Monte Carlo simulation. The EPA's MOBILE6 model was used to generate the emissions rates for the vehicles. The study was then repeated to examine the effect of VSL over multiple days using data collected over a month.
When comparing the NOx emissions under the regular, baseline conditions with a 65 mph speed limit, the 55 mpg speed limit for a single "Ozone Action Day" a reduction of emissions was seen in several cases, morning off-peak, daytime off-peak, and evening off-peak. Emissions reductions were not seen during the peak period because the average speed of the vehicles was well below the speed limit due to congestion. The emission reduction results showed:
- 12.3 percent reduction during morning off-peak
- 8.4 percent reduction during daytime off-peak
- 14.2 percent reduction during evening off-peak
- 10.8 percent reduction daily
When the study was completed using variable speed limits over a longer period of time for "Ozone Alert Days", the 55 mpg speed limit reduced the average daily NOx emissions by 17.3 percent.
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