Nav: Home

Crashes increase when speed limits dip far below engineering recommendation

December 12, 2018

Speed limits set only five miles per hour below engineering recommendations produce a statistically significant decrease in total, fatal and injury crashes, and property-damage-only crashes, according to a group of Penn State researchers.

"If (however) you lower the speed limit by 10, 15, 25 miles per hour, or more, drivers stop paying attention," said Vikash Gayah, assistant professor of civil engineering. "We found there was an increase in fatal and injury crashes at locations with posted speed limits set 10 miles per hour or more below engineering recommendations."

Speed limits are normally set based on results from engineering studies that collect free-flow traffic data and then select an appropriate speed using a statistical model. However, factors such as school zones, citizen or political pressure, and perceived safety issues contribute to the fairly common practice of lowering speed limits below engineering guidelines, the researchers report in Accident Analysis and Prevention.

"When doing speed research, we are looking at free-flow speeds -- the speed that drivers select based on geometric and prevailing weather conditions," said Gayah.

The team of researchers collected speed data on three different occasions from 12 roadway segments in Montana, a state that posts speed limits lower than engineers advise. Eight of the 12 sites had posted speed limits changed below engineering recommendations either by 5 mph, 10 mph, 15 mph, or 25 mph. The other four sites had posted speed limits set equal to engineering recommendations and served as comparison locations.

Each period of data collection considered either the presence of no law enforcement, light law enforcement, or heavy law enforcement.

Speed data was collected in daylight hours and in fair weather conditions using hidden pavement sensors. Large vehicles, such as trucks, and cars traveling too close together were excluded. Cars traveling less than 10 mph of the posted speed or greater than 20 mph of the posted speed limit, known as speed outliers, were also excluded. Crash history data from a four-year period before and after speed limits changed was also used in the analysis.

The researchers found that vehicles were two times more likely to obey the speed limit at locations with higher posted speed limits set at 50 mph or 55 mph compared to the base case of less than 50 mph, and four times more likely to obey when the posted speed limit was between 60 and 70 mph.

The presence of heavy law enforcement in zones with low posted speed limits showed an average reduced speed of 4 mph and greater speed-limit compliance. "The practice of setting speed limits lower than what would be recommended from an engineering study is okay if it's only by a little -- by five miles per hour," said Gayah.

Such a difference showed an increase in speed-limit compliance, a decrease in property damage only, and total, fatal and injury crashes. Larger differences between the posted and engineering-recommended speed-limit values were found to increase crash frequency and reduce speed limit compliance.
-end-
Other researchers included Eric T. Donnell, professor of civil engineering and director of the Larson Transportation Institute at Penn State; and Zhengyao Yu and Lingyu Li, graduate students in civil engineering.

The Montana Department of Transportation and the Federal Highway Administration supported this work.

Penn State

Related Engineering Articles:

Engineering a new cancer detection tool
E. coli may have potentially harmful effects but scientists in Australia have discovered this bacterium produces a toxin which binds to an unusual sugar that is part of carbohydrate structures present on cells not usually produced by healthy cells.
Engineering heart valves for the many
The Wyss Institute for Biologically Inspired Engineering and the University of Zurich announced today a cross-institutional team effort to generate a functional heart valve replacement with the capacity for repair, regeneration, and growth.
Geosciences-inspired engineering
The Mackenzie Dike Swarm and the roughly 120 other known giant dike swarms located across the planet may also provide useful information about efficient extraction of oil and natural gas in today's modern world.
Engineering success
Academically strong, low-income would-be engineers get the boost they need to complete their undergraduate degrees.
HKU Engineering Professor Ron Hui named a Fellow by the UK Royal Academy of Engineering
Professor Ron Hui, Chair Professor of Power Electronics and Philip Wong Wilson Wong Professor of Electrical Engineering at the University of Hong Kong, has been named a Fellow by the Royal Academy of Engineering, UK, one of the most prestigious national academies.
Engineering a better biofuel
The often-maligned E. coli bacteria has powerhouse potential: in the lab, it has the ability to crank out fuels, pharmaceuticals and other useful products at a rapid rate.
Pascali honored for contributions to engineering education
Raresh Pascali, instructional associate professor in the Mechanical Engineering Technology Program at the University of Houston, has been named the 2016 recipient of the Ross Kastor Educator Award.
Scaling up tissue engineering
A team at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A.
Engineering material magic
University of Utah engineers have discovered a new kind of 2-D semiconducting material for electronics that opens the door for much speedier computers and smartphones that also consume a lot less power.
Engineering academic elected a Fellow of the IEEE
A University of Bristol academic has been elected a Fellow of the world's largest and most prestigious professional association for the advancement of technology.

Related Engineering Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Changing The World
What does it take to change the world for the better? This hour, TED speakers explore ideas on activism—what motivates it, why it matters, and how each of us can make a difference. Guests include civil rights activist Ruby Sales, labor leader and civil rights activist Dolores Huerta, author Jeremy Heimans, "craftivist" Sarah Corbett, and designer and futurist Angela Oguntala.
Now Playing: Science for the People

#521 The Curious Life of Krill
Krill may be one of the most abundant forms of life on our planet... but it turns out we don't know that much about them. For a create that underpins a massive ocean ecosystem and lives in our oceans in massive numbers, they're surprisingly difficult to study. We sit down and shine some light on these underappreciated crustaceans with Stephen Nicol, Adjunct Professor at the University of Tasmania, Scientific Advisor to the Association of Responsible Krill Harvesting Companies, and author of the book "The Curious Life of Krill: A Conservation Story from the Bottom of the World".