SIOUX FALLS, S.D. (KELO) — Drivers may not realize it but there are sensors on a section of Interstate 29 that could tell them to slow down under certain road conditions.
The sensors are part of a Variable Speed Limit system installed on I-29 between Flandreau and Brookings. The sensors detect the weather, road conditions, traffic flow and other factors. Along with observations from state staff such as the South Dakota Department of Transportation, the speed limit can be adjusted in an effort to reduce crashes
“That segment has been a challenging one for us with crashes in the past,” said Craig Smith, the director of operations with the South Dakota Department of Transportation.
“We actually activated it this morning due to some icy conditions,” Smith said on Friday morning.
The speed is adjusted “…on that section of highway to what’s appropriate with the idea that what we want is a consistent speed of traffic so you don’t get a lot of differential speeds. You don’t want someone going 80 while another vehicle is more comfortable going 55 to 60,” Smith said.
When more vehicles are driving a similar speed, it reduces conflict, or the chances for a crash, Smith said.
Drivers may not notice the sensors. Some can be in the roadway while others may be off the roadway. But all the speed limit signs on that section of the interstate are electronic, so drivers should notice the change in the speed limit for that time period.
Friday’s activation is believed to be the fifth activation since the 2025-2026 winter season started.
“Some have been fairly short, it might be three or four hours until the road recovers and traffic can go back to the safe speed at the posted 80 mph,” Smith said. “I think we had one event that actually went overnight. We adjusted back it up in the morning.”
Reducing crashes, and any related injuries or fatalities is a goal for VSL but so is preventing road closures.
“The other benefit is just even keeping the interstate open. There is a cost to that if you have a semi that jack knifes…,” Smith said.
There is time lost and a cost for that with road blockage as well as the cost for clean-up, he said.
Success?
Field observations from law enforcement and DOT staff help determine the success of the VSL, Smith said.
“It is a new system for us so we’re kind of getting the feel for what those benefits are. We work closely with both law enforcement and our staff in the field,” Smith said.
“It’s been favorable so far. It’s met the intended goal (of slowing traffic down,” Smith said. It’s also helped to get vehicles to be traveling at similar speeds, he said.
Where next?
“We do use crash data to determine that,” Smith said of choosing a location for VSL. “The other one we do have identified is on Interstate 90 between Sturgis and Rapid City in that Tilford area we’ve got a segment in there with a high crash rate.”
Smith doesn’t expect the second VSL until at least the fall 2027.
“We want to get those two systems in place and then determine what the success is and then we will look to see if there is other locations that could possibly benefit from having this variable speed limits,” Smith said.