February 2002
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APPLICATIONS

Highway Safety Barrier System 
Will Cut Accident Rates

In hopes of reducing the number of cross-over accidents on a busy segment of expressway in Oklahoma City, ODOT has installed a wire cable barrier system of a design that has never been used in the United States.

by Jeff Griffin, Contributing Editor

The four-strand Brifen Safety Fence is manufactured in the United Kingdom by South Yorkshire-based Bridon Ropes, Ltd.

The new safety barrier consists of 3,333 posts which support 140,424 feet of cable, says Carrie Clear, Oklahoma Department of Transportation spokesperson. Posts are 52-inches long with 16.5 inches buried in the ground. Ten and one-half feet separate the posts, which support the four strands of tensioned cable, each composed of 21 wires.

Construction began in April and was completed the last week of July. Total cost for system components and installation was approximately $1.3 million, $10 million less than the amount required to install concrete barriers. The contractor was Midstate Traffic Control, Inc., Oklahoma City.

The system is designed so that the energy of a vehicle striking the barrier is absorbed by posts and wire cables, with posts bending or collapsing on impact. After a vehicle strikes the barrier, it is guided along the fence as it slows to a stop. Damaged posts are pulled from their sockets and replaced with new ones.

The barrier was installed adjacent to the southbound lane of a seven-mile stretch of State Highway 74, also known as the Lake Hefner Parkway. The expressway connects an east-west turnpike and Interstate 44. North- and south-bound traffic lanes are separated by a grass median. Since 1999, four cross-over accidents have resulted in six fatalities and seven injuries.

Choosing the system

The Oklahoma DOT studied a variety of options, including concrete barriers and an American-made three-cable barrier.

The U.K. product is significantly different from the three-strand wire barrier systems used on some highways in the United States. Department research, Clear says, indicated that the Brifen system would be more effective than other designs.

“The Brifen system,” says Clear, “is installed with greater line tensions than other cable systems, which means that significantly fewer deflections occur on impact, reducing the seriousness of accidents. Compared to the three-cable system, vehicles that strike the barrier suffer less damage and their occupants have a lower risk of injury.”

Tests also indicated the system will perform better from a maintenance standpoint.

“While the Brifen barrier costs a little more to install,” says Clear, “it is less expensive to maintain.”

Clear says readjusting cables of other barrier systems is a time-consuming and costly part of their maintenance. Brifen cables are tensioned by the manufacturer before installation, eliminating stretching during installation and subsequent readjustments.

The Brifen safety fence, she continues, is easier to repair, and after minor hits, typically returns to its original alignment and retains the capacity to withstand additional strikes before repairs are made.

ODOT also found that the system is easier to dismantle to make emergency cross-over points.

“The highway where the cable has been installed is very accessible,” says Clear. “With entry and exit ramps every mile, we do not anticipate it will be necessary, there are joints every 500 feet with tensioning screws that can be easily removed. Emergency personnel then pull several posts from their sockets, and vehicles drive across the median.”

Before making the decision on which barrier to use, ODOT installed a 1,000-foot test section late last year so that researchers could observe construction, appearance, and maintenance of the system. In December, the test section prevented at least two cross-over collisions, Clear says.

Finally, ODOT liked the looks of the system.

“Brifen steel posts are smaller than the wooden poles used in the U.S. system,” says Clear, “allowing them to blend with the surrounding environment.”

Bridon Ropes, Ltd. says the four-cable barrier evolved from an original two-strand design based on cable systems used on aircraft carriers to restrain landing planes. The firm’s highway safety fences are used in 30 countries throughout the world.

Jeff Griffin is a freelance writer specializing in the construction industry.

Reprinted from Better Roads Magazine
February 2002

 

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Copyright © 2002 James Informational Media, Inc.
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