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Cable Barriers ’More Forgiving’ Than Concrete

Fri November 26, 2010 - Southeast Edition
Construction Equipment Guide

KNOXVILLE, Tenn. (AP) Transportation officials looking to prevent head-ons and other lane departure crashes in Tennessee are taking a close look at cable barriers.

After installing more than 1.4 million linear ft. of the multiple strands of thick wire in recent years, Tennessee Department of Transportation spokeswoman Julie Oaks said the department will evaluate how well the barriers are working.

Oaks said in an e-mail sent to The Knoxville News Sentinel that lane departure crashes in recent years have been a major cause of highway fatalities and the “cable barrier rail is one main strategy” to prevent them.

“What is so good about these barriers versus a concrete barrier or guardrail is that the cable is much more forgiving,” said Steve Hall, the department’s assistant chief engineer. “Concrete doesn’t give too much. The guardrail has a deflection of approximately 3 feet. These cables amount to about 8 feet and can deflect to about 10 or 12 feet. It stops the vehicle from crossing but doesn’t impact the occupant as with concrete.”

Oaks said that when a car strikes the cable the intent is to “catch it in that fence.”

Hall said the Tennessee Highway Patrol has said they believe the barriers are saving lives.

“We hit every place that we knew we had crossovers [wrecks],” Hall said in a telephone interview. “There could be a road safety report for some section of roadway that we may say, ’This is a good section for that.’”

The department has learned that the cable isn’t the answer everywhere.

“We try to keep them in a flatter area where the slopes are not more than 6-to-1 [ratio],” Hall said. “They [cars] will go under them [if the nose of the car is headed down a steep slope].”

Hall said the barriers are not put up in the middle of medians, which are usually at the bottom of slopes created by the roads built up on either side.

“Ours are roughly 8 feet off the shoulder,” he said. “If we have to replace a long run, we are looking at moving it a little farther to 10 feet from the travel lane.”

Hall said there have been some changes, including going from three strands of cable to four, although three strands meet Federal Highway Administration requirements.

“This will take care of a larger vehicle like a FedEx truck,” he said, adding that four strands of cable reduce the chance of vehicles going under the barrier.

The program to install cable barriers started in 2005 around Memphis, Chattanooga and the Tri-Cities and was expanded in 2008.

Most of the cable barrier has been installed in 2009-10, so far at a cost of $19 million. The cost to repair cable barrier rails is $5.50 per ft., compared to $16 per ft. for typical guardrails, Oaks said.

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