It’s been about a year since electronic signs that display real-time information about individual lanes appeared on I-94 and I-35W, but drivers haven’t fully figured out the messages they’re getting.

Over the past few weeks, questions about the signs — and especially about the speed limits they display — have landed in the Drive’s mailbag. Today, I’ll answer a couple of the most asked.

The monitors, as some commuters call them, are Lane Control Signals and are part of what the Minnesota Department of Transportation calls “Smart Lane technology.” Their job is to display advisory information about looming congestion, blocked lanes and when carpool lanes are open or closed. The signs also indicate when advisory lanes can and can’t be used, as well as road conditions that motorists will encounter ahead.

Most drivers grasp simple messages such as the red X indicating that a lane is closed. But the meaning is not so obvious when lanes have messages that say “50 MPH.”

One reader asked if the speed shows how fast traffic is moving in each lane so drivers can pick the fastest lane? Or is that the desired speed for each lane? another wondered. A third asked whether that shows the top speed that a driver would attain if they stayed in that lane.

No, no and no.

The purpose behind the speed signs is to warn motorists of congestion or traffic issues ahead, and advise them how much to slow down, said Brian Kary, MnDOT’s freeway operations engineer.

The 180 signs on I-35W between Burnsville and downtown Minneapolis and the 110 on I-94 between downtown Minneapolis and St. Paul are spaced roughly a half-mile apart. Here’s how they work:

While driving at the normal speed limit, a motorist will pass a set of signs that say 50 mph, for example. At the next set, the advisory speed will say 40 mph, and then 30 mph at the next. The idea is that drivers will adjust their speed to match the signs and “ease into congestion as opposed to slamming on the brakes at the last minute,” Kary said.

A compelling video on the MnDOT website shows the type of crash that the technology is supposed to help prevent. In it, a vehicle is stalled in the center lane of I-94. After several vehicles swerve wildly and narrowly miss the disabled compact, a pickup truck traveling at freeway speed plows into the car just after its occupants escape.

In theory, the technology is supposed to reduce rear-end collisions when traffic queues up or when a hazard blocks the road. There are no stats to prove that here, yet, because it’s still too new. The Twin Cities and Seattle are the first U.S. metro areas to use the technology.

But Smart Lanes have been used in Europe for about 10 years and studies have shown a 30 percent decrease in collisions and a 22 percent increase in roadway capacity, Kary said.

“We hope to see similar results here,” he said.

So far, the lane control signals display green arrows (lane open), yellow arrows (use caution), red Xs (lane closed), diamonds to indicate use by carpools, and blinking chevrons to get drivers to merge.

“There are definitely questions and curiosity,” Kary said of the technology. “We will make different tweaks to better respond to traffic conditions and make it clear to the motoring public.”


Follow news about traffic and commuting at The Drive on Got traffic or transportation questions, or story ideas? E-mail, tweet @stribdrive or call Tim Harlow at 612-673-7768.