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Study: Reducing Speed Limits Does Not Improve Air Quality
Air quality measurements in Switzerland show that a 25 MPH reduction in freeway speed limits during the summer did not reduce ozone levels.

Predicted ozone
A study of the effect of environmental speed limits in Switzerland showed that reductions of as much as 25 MPH failed to yield measurable improvements in air quality. Researchers at the Paul Scherrer Institute released a study in May that focused on the change in air quality that followed the Swiss government's August 2003 order imposing an emergency reduction in the highway speed limit from 120km/h (75 MPH) to 80 km/h (50 MPH) to address heavy pollution during a particularly hot summer. The report suggests the government's "environmentally friendly" speed limits did not achieve their goal of reducing ground-level ozone, a pollutant harmful to respiratory systems.

"Traffic speed reductions in Switzerland to a maximum 80km/h decrease ozone concentrations by less than 1 percent, which corresponds to less than 1 part per billion," the report concluded. "Overall, the traffic speed reduction alone is not sufficient to significantly reduce the ozone levels."

The Paul Scherrer Institute undertook the study because, according to the report, "There are only a few investigations dealing with the impact of vehicle speed variations on emissions and air quality." The new study considered measurements of emissions including nitrogen oxide, carbon monoxide, carbon dioxide, non-methane volatile organic compounds, and particulate matter in Ticino and Graubuenden. Software modeling was used to take account of weather patterns, industrial and residential pollutant output in the test areas.

After analyzing all of these factors, the data showed a four percent reduction in nitrogen oxide following the speed limit change. Nitrogen oxide is a key component of ozone, which forms from the reaction of sunlight on air containing nitrogen oxide and hydrocarbons. However, because there was no significant change in the other factors that create ozone, there was no meaningful decrease in ozone levels. The researchers believe that fifty percent reductions in nitrogen oxide and volatile organic compounds are necessary to reduce ozone by a meaningful amount.

The new study's findings match those of a 2004 German study which concluded: "Temporary and locally restrictive measures, such as a speed limit, a ban on non-cat[alytic converter equipped] motor vehicles, or the burning of reformulated fuels, are not very effective in decreasing high ozone levels, because the attainable emission reduction is small. For each single measure, the decrease of peak ozone concentrations would be at most 5 percent." (Umweltbudesamt, Programme of Control Concepts and Measures for Ozone, 2004)

Source: The impact of reducing the maximum speed limit (Environmental Modelling and Software, 5/30/2007)

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