The Idaho Transportation Department installed a cable guardrail Dec. 7 along a section of Interstate 15 median that had seen scores of crashes. Two days later a vehicle struck it, in an incident the ITD described as paying "immediate dividends."

Cable rail systems use steel wire cable strung along small metal posts. During a crash the posts are designed to bend or snap away, while the cable absorbs the impact energy but prevents a car or truck from getting through.

The ITD put the cable rail next to a busy northbound I-15 lane near Inkom in southeastern Idaho, in an area that has seen 58 crashes in the past five years. "It has a narrow, 25-foot median separating the northbound and southbound lanes of travel, and a history of errant vehicles crossing through the median and entering the opposing lanes of traffic," the agency said.

"ITD's southeast Idaho District 5 office strongly believed that this project could save many lives, and within the first 48 hours of completion, it had already proved its effectiveness," said District Operations Manager Steve Gertonson.

Workers installed the cable system where it could keep vehicles from either hitting the southbound lane's guardrail from behind or entering the oncoming traffic lanes at freeway speeds.

When a vehicle struck it Dec. 9, the posts collapsed while the cable remained intact as designed and "the motorist was able to drive away from this incident," the agency said.

This was District 5's first cable-rail project, but it had requested it for that location as a high-priority safety improvement.

"Cable rail was a good choice for our Inkom project," said District Engineer Ed Bala, "because that stretch features narrow median and sharper curves that lessen the recovery zone – meaning when vehicles depart they can cross through the median and end up in the opposite lane. Cable rail is designed to prevent that, and we feel like it did a great job the other day."

Because this was its first such project, the district did not have spare parts or tensioning tools on hand to repair the damage. However, the ITD's District 4 in south-central Idaho volunteered spare parts, tools and crew members, who helped complete the repairs.

The District 4 crews had already seen the value of cable barriers. "Earlier this year, District 4 installed the state's first median cable tension system on a stretch between Utah and the I-84/I-86 junction known as Sweetzer Summit," the agency said. About three weeks later, that barrier was also tested by a vehicle coming off the road, but it performed as designed