Idaho’s new 80 mph speed limit scrutinized

BOISE – When Idaho lawmakers voted to boost the state’s top speed limit to 80 mph, the focus fell on Southern Idaho where interstate highways link with Utah’s similarly wide, smooth freeways where the speed limit has been 80 mph since last year.

Yet the Idaho Transportation Department went further and announced that it’s also studying all rural stretches of interstate highways in the state – including I-90 in North Idaho. And that is raising some eyebrows in North Idaho.

“The roads are not as straight and flat as down there, and it just doesn’t work,” said former state Sen. Jim Hammond, R-Post Falls, who chaired the Senate Transportation Committee until 2012. “In fact, I’m surprised that there would be any recommendations for higher speed limits up here.”

The new law took effect July 1, but speeds weren’t expected to increase right away, because it simply authorized transportation officials to designate routes where an 80 mph limit would be appropriate. Then in late June the transportation department issued a surprise: Starting July 1, top speeds on all interstates through rural stretches of southern Idaho that now have 75 mph limits would rise to 80 mph.

Before new signs could be erected, concerns sped into Boise.

AAA had safety questions. The Idaho Trucking Association asked about differing speed limits for trucks.

Within four days transportation officials slowed down and said they would wait to raise speed limits until studies could be reviewed later this month.

“The department continues to study I-90 for potential increases,” ITD said in a news release.

Some of the 80 mph speed limit signs are ready to go.

“The speed studies are done, the geometric analyses are done” for Southern Idaho freeways, said Blake Rindlisbacher, who leads engineering services for ITD. “But I-90’s a little bit different than Southern Idaho interstates, so they’re going to study that. The topography’s a little bit different – they’re traveling in river corridors and have mountain passes.”

“We didn’t have necessarily any 80 mph candidates, but we did have a couple of segments of I-90 that might bump up 5 mph, maybe to 75. So we’re going to do those studies this summer,” said Damon Allen, ITD’s district engineer for North Idaho.

Allen said the stretch of I-90 from Stateline to Coeur d’Alene could rise from 70 mph to 75, and the stretch roughly from Kellogg to Wallace could go up from 65 to 70 mph.

Locals haven’t been requesting speed limit boosts. “Nah, it’s been really quiet about the speeds up here,” he said.

ITD’s speed studies determine the “85th percentile” speed – the rate that 85 percent of drivers are traveling at or below.

“National studies tell us if we set our speed limits as close to that 85 percent as we can, that’s the safest speed,” Rindlisbacher explained.

Then, ITD examines crash data and the layout of the road, including curves and banking, to determine whether higher speed limits are safe or not.

“Interestingly, when we did the speed studies, drivers are going 80 mph on average on the interstates in rural Idaho already,” Rindlisbacher said about Southern Idaho freeways. “So they’re comfortable. So if we move the posted speed to 80 mph, that’s a prudent, safe thing to do, even though it may seem counterintuitive.”

Allen said North Idaho may or may not see any speed limit changes.

“The 80 mph legislation kind of prompted us to take a look,” he said. “We’ll be studying these this summer.”