Variable speed limit signs help reduce accidents on I-80
by Wyoming Department of Transportation
October 14, 2010
Crashes and road closures were down each of the two years a seasonal speed limit of 65 mph was enforced on a 52-mile section of I-80 between Laramie and Rawlins, but with variable speed limit signs now installed the full length of the section, the seasonal limit won’t resume this year.
The variable speed limit signs allow WYDOT to reduce the limit in 5 mph increments to as low as 35 mph as weather and road conditions warrant between the Quealy Dome Interchange 20 miles west of Laramie and the Peterson Interchange 22 miles east of Rawlins.
"The variable speed limit signs allow us to set the speed as the actual situation and conditions on the road dictate," WYDOT District Engineer Jay Gould said. "Since we have complete variable speed limit coverage now in the area that was previously covered by the seasonal speed limit, we feel we can better respond to the conditions, creating a safer environment for motorists."
Data from speed sensors in the highway also show drivers complied at a higher rate with speed limits set on the variable signs than with the 65 mph seasonal limit.
WYDOT first implemented the seasonal speed limit for six months beginning on Oct. 15, 2008 to give drivers a better chance to safely react to the rapidly changing and extreme weather conditions that occur on the highway section. The first of the variable speed limit signs began operating on a portion of the seasonal speed limit section in February 2009.
The seasonal limit resumed for another six-month period beginning on Oct. 15, 2009 and by the end of that year additional variable speed limit signs had been brought online to cover the remainder of the 52-mile section.
Crash numbers for the section had peaked during the year prior to the first seasonal speed limit enforcement, with 402 crashes, 91 of them resulting in injuries and three resulting in fatalities. The following year, with the seasonal speed limit and later variable speed limits in effect, total crashes dropped to 248, with 59 resulting in injuries and no fatalities. During the 12-month period that included last winter, when the seasonal limit was again in effect and the variable speed limit section expanded, total crashes dropped to 181, 39 causing injuries and four resulting in fatalities.
When the data is narrowed to only crashes involving commercial vehicles, the number dropped from 183 during the severe weather season of 2007-2008, to 91 the following season and 51 last season.
Highway closures on the section also dropped. During the two severe weather seasons before the seasonal and variable speed limits were in effect, there were 39 closures for a cumulative total of 369 hours. During the two seasons the seasonal and variable limits were in effect there were 25 closures for a cumulative total of 165 hours.
Because of variations in the length, frequency and severity of severe weather conditions and in traffic volumes on the section, the drop in crashes and closures can’t be attributed solely to the ability to lower the speed limit, but WYDOT officials believe it has contributed.
"We don’t have all the data we want yet, and we will continue to look at it, but we do think the variable speed limit is improving safety," Gould said. "If you talk to our Maintenance and Highway Patrol people who work on that highway section, they think the ability to match the speed limit to conditions has made a positive difference."
"Now we have a baseline of crash statistics to compare whether putting a blanket speed limit across there is beneficial," Gould said. "There are many days there when the sky is blue, the wind is light and the road conditions are good, so there is no need to slow the motorists down."
WYDOT is confident enough in the benefits of the variable speed limit system that a new section is scheduled to come online between Rock Springs and Green River by the end of the year. That section is the busiest on I-80 in Wyoming, carrying an average of more than 20,000 vehicles a day.
Modern Electric Co. of Casper is installing 32 variable signs on the section, one in the median and one along the shoulder at each access ramp to the highway. The $917,000 contract for the project was awarded in May.
Like the signs between Laramie and Rawlins, the new signs will allow operators in WYDOT’s Transportation Management Center in Cheyenne to lower the speed limit when weather or traffic conditions on the section make it necessary for safety.
A project also is planned to install variable speed limit signs between Cheyenne and Laramie in time for the severe weather season of 2011-2012.
"This is all part of our continuing effort to use new technology to help keep traffic moving safely on our highways," said Vince Garcia, manager of WYDOT’s Intelligent Transportation Systems program. "We’ll continue moving ahead with these projects as funding allows."