My view: Slow down to 55 to ease inversions

By Mark Lehmkuhle

Published: Wednesday, Feb. 2 2011 12:00 a.m. MST

An easy way for individual Utahns to put some extra cash in their pockets while helping to clear the winter smog and improve air quality over the Salt Lake Valley would be to simply slow down.

Let's imagine that over the four winter months, when the air quality is the worst, we reduce highway speed limits to 55 mph. Not only would this reduce the amount of emissions that build up in the northern valleys during inversions, but slowing down would also result in fewer and less severe traffic accidents, fewer related deaths and severe trauma, and people would feel less vulnerable in smaller cars.

Furthermore, it would save Utahns money immediately. How much money? Here's an example. I own a 2002 Subaru that gets a consistent 25.4 miles per gallon when I drive between downtown Salt Lake City and downtown Ogden. This is with driving about 72 mph on I-15. Total distance for my trip is 40.5 miles one-way. However, when I slow down to 55 mph on the highway, I have been getting 26.2 mpg, a difference of what seems to be a measly 0.8 mpg. My drive takes only an extra six minutes, one way. If the cost of gas stays the same, I would save about $25 over the course of the four winter months during the daily commute.

That doesn't seem like much. But, on Utah highways and interstates through Salt Lake County alone there are, on average, 8 million vehicle miles traveled per day. The U.S. Department of Transportation says that the average fuel economy in the United States is 17.1 mpg. If everyone got 0.8 mpg better by driving a little slower, that means Utahns could save, on average about 21,000 gallons of gas per day, or the equivalent of two gasoline tanker trucks. At $3 per gallon, that's $63,000 a day. Over the four winter months it would be about $7,560,000. And this figure is conservative because other studies show an even greater fuel savings from driving 55 mph.

What could this mean for our air quality? During a typical winter the concentration of particulates smaller than 2.5 microns (PM2.5) in the Salt Lake Valley is above and beyond what the EPA says is a safe level for 30-45 days, resulting in an unhealthy, glowing haze over the Valley. According to the Utah Department of Environmental Quality, about 38 percent of PM2.5 and 65 percent of oxides of nitrogen (NOx) emissions in the valley comes from motor vehicles. NOx is a precursor to PM2.5 particulates. During an inversion, NOx and other molecules will react in the air to form some of the particulates that collect over time until it gets so bad that the highway signs are telling us to drive less.

Information from the U.S. Department of Transportation shows that reducing highway speeds from 75 to 55 mph could result in a 50-60 percent reduction in the amount of NOx emissions for the average vehicle fleet. In the Salt Lake Valley, this would mean a reduction of at least 18 tons of NOx per day, out of the total 50 tons of NOx emitted by motor vehicles each day.

In order to be effective, reduction strategies would have to precede the onset of our inversions by several days. Essentially, a "seasonal" reduction in speed limit from Nov. 15 to March 15 would be required in order to help, but it would be a negligible sacrifice for all of us. The medical science says that even small reductions in air pollution offer significant improvements in health, and our children would benefit the most.

Although slowing down is no magic bullet for reducing the number of red air alert days, a winter season speed limit reduction would make a significant improvement in valley air quality with the added benefit of saving us all a little extra money. Slowing down could be a win for the environment, a win for our health, a win for our economy and a brighter future for our state.

Mark Lehmkuhle, PhD received his doctorate in Bioengineering at the University of Utah where he is a research scientist.

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