Let’s do the math: Reasons to slow down traffic in 2014

By Chad Mullins

For the Deseret News

Published: Sunday, Jan. 26 2014 12:00 a.m. MST

Our communities’ vitality and quality of life is reflected in the number of people riding and walking on the streets and sidewalks, not by the speed of the motorists.

Michael Brandy, Deseret News

Enlarge photo»

Slowing down improves air quality and makes our cities and towns more livable. And yet, a new bill has been proposed to do the opposite, “Lawmaker to propose more 80 mph speed limits on Utah highways,” (Deseret News, Jan. 13).

Automobile gasoline engines are more efficient at lower speeds: most reach maximum efficiency at around 55 mph and are much less efficient at 70 mph, which is close to the average speed on Wasatch Front freeways. For most automobiles, emissions would be reduced 20 percent by simply slowing down. Newer, more efficient gasoline engines may reduce that number to 10 percent. However, since automobile engines are the largest source of our air pollution, 10 percent represents a huge reduction in emissions compared to other potential mitigation measures.

In Gov. Gary Herbert’s spirit of “doing everything we can to improve air quality,” we should reduce speed limits along the Wasatch Front during periods of imminent inversions and toxic air. The majority of Utah’s citizens who support action to improve air quality should be willing to spend a few more minutes on the freeway for cleaner air.

Increased road capacity is another major benefit of reduced highway speeds. At faster speeds drivers instinctively increase the distance between their car and the car ahead of them. This causes the capacity of the highway to decrease with increasing speed. Tests have shown that the maximum road capacity in terms of cars per hour is attained at approximately 45 mph (”The Capacity of the Guideways,” Aug. 1, 2006).

Studies show it is reasonable to assume an increased highway capacity of up to 25 percent can be achieved by reducing highway speeds. Hundreds of millions of dollars could be saved in avoided new highway construction in urban areas by merely reducing speed limits.

This does not mean our transportation system could not operate efficiently. Freeways and highways are only one component of our transportation system, and “automobile congestion, vehicle delay, and their proxy, level-of-service [traffic flow], are not measures of system efficiency. Nor are they measures of economic vitality. They are nothing more or less than measures of how convenient it is to drive an automobile” (“Rethinking the Economics of Traffic Congestion,” June 1, 2012).

Slowed traffic and longer commutes caused by traffic congestion can be a self-regulating transportation management tool. As cities grow, so too does the demand for travel. And when the roads and highways become congested and driving inconvenient, people adjust by moving to more accessible areas, living in higher density neighborhoods, traveling shorter distances and shifting travel modes by using public transit, walking and biking.

"Speed kills." The greatest danger to pedestrians and bicyclists is high-speed traffic. Lowering vehicle speeds vastly reduces serious accidents and fatalities. At 20 mph, the odds of a pedestrian death when struck by an automobile is only 5 percent; at 30 mph, the odds of a pedestrian death is 45 percent, and at 40 mph, the probability of death increases to 85 percent. Furthermore, reducing vehicle speeds reduces accidents because drivers have more time to react and stop before striking a pedestrian or bicyclist. (“Effect of impact speed on pedestrian fatality and injury,” 1999)

We must change the false premise of our fast-paced culture that getting somewhere faster is better. Our communities’ vitality and quality of life is reflected in the number of people riding and walking on the streets and sidewalks, not by the speed of the motorists. Reducing speed limits in our city centers and residential neighborhoods make our communities more desirable places for people to live, walk and ride bikes. By slowing down we make our streets safer and our communities healthier.

Chad Mullins was a volunteer for the Utah Division of Air Quality, the former chairman of the Salt Lake County Bicycle Advisory Committee, and currently serves on the Board of Bike Utah.

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