Variable Speed Limits result in 17 percent reduction in NOx on "Ozone Action Days"
Variable Speed Limits reduce emissions in Austin, Texas.
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United States

An Investigation on the Environmental Benefits of a Variable Speed Control Strategy

Summary Information

The safety benefits of variable speed limits (VSL) have already been widely recognized. However, the environmental benefits of variable speed limits have been largely ignored. This paper presents a study of the potential benefits of variable speed limits in reducing mobile emissions. A Monte Carlo simulation approach is developed to evaluate the effectiveness of the idea of using variable speed limits to manage and reduce mobile emissions. A case study is performed on the IH-35 corridor in Austin, Texas. The numerical results indicate that on "Ozone Action" days, by managing the freeway/expressway traffic speeds at appropriate levels through VSL, the major pollutants, such as Nitrogen Oxides (NOx) emissions, could be significantly reduced. Considering the large contribution from freeway/expressway traffic to mobile emissions, a variable speed limit strategy could be an effective measure to balance travelers' need for mobility with conservation of the environment.

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.

An Investigation on the Environmental Benefits of a Variable Speed Control Strategy

An Investigation on the Environmental Benefits of a Variable Speed Control Strategy
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Zhong Wang and C. Michael Walton
University of Texas at Austin

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