NMA Email Newsletter: Issue #29

Common Sense Can Actually Dictate Driving Speeds

A study by Kansas State University researchers illustrates a surprising fact: that driving almost 20 mph under the posted speed limit can be consistent with a long-held position by the National Motorists Association.

Sunanda Dissanayake, associate professor of civil engineering at K-State, and Litao Liu, a civil engineering graduate student there, collected speed data on Kansas gravel roads. Almost 58% (78,000 miles) of that state’s roads have gravel surfaces. Dissanayake and Liu measured the speed of drivers at 41 locations, over the period of one week, without tipping off the drivers of the data collection. The goal was to determine whether there was a difference between actual driving speeds and the speed limit. Only straight sections of road were monitored, so that other factors that could affect vehicle speed, such as bends in the road and bridges, did not impact the results.

An interesting twist to the study is that some of the monitored sites were in Johnson County where the posted speed limit is 35 mph and others were in Miami County with unposted speed limits of 55 mph. What were the findings when drivers, unaware that their speed was being tracked, traveled similar roads under the same driving conditions?

Regardless of whether the gravel road speed limit was a posted 35 mph or an unposted 55 mph, the average actual speed on both sets of roads was 37.5 mph. Dissanayake noted that the study showed that drivers travel at a speed consistent with their comfort level. In the case of the Miami County gravel roads, where speeds up to 55 mph were allowed, people were routinely moving much slower than that. Drivers’ concerns of comfort and personal safety prevailed.

The NMA supports speed limits set at the 85th percentile, as established by engineering studies of actual driving patterns on the roads in question. Smoother, safer driving patterns emerge when speed limits are set at a level at or under which 85% of people are driving. The K-State study shows that responsible motorists will naturally find the safe speed to travel based on road conditions and other factors. For instance, the K-State researchers found that motorists drove at higher speeds when the gravel roads were sandier and/or wider.

Dissanayake and Liu recommend that it is not necessary for speed limits to be posted or reduced on Kansas gravel roads. The associate professor said, “I can’t say there is any harm in the posting. The only thing is that you might lose the respect of the drivers. The majority are driving under the speed limit, so why do you need to lower it?”

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