By EBA Tetra Tech
High Tension Median Cable BarriersTransportation Transportation Infrastructure
etween 1999 and 2005 there were seven fatal crossover collisions on an 11-kilometre segment of the Deerfoot Trail in Calgary. The six-lane divided urban freeway between 16th Avenue NE and Country Hills Boulevard carries up to 150,000 vehicles a...
etween 1999 and 2005 there were seven fatal crossover collisions on an 11-kilometre segment of the Deerfoot Trail in Calgary. The six-lane divided urban freeway between 16th Avenue NE and Country Hills Boulevard carries up to 150,000 vehicles a day.
Since a high-tension median cable barrier was installed in May 2007, however, there have been no crossover collisions on that section of roadway at all. This relatively new type of median barrier has proved so successful at preventing crossover collisions that Alberta Transportation has also installed it on 127 kilometres of Highway 2 between Calgary and Leduc, Alberta. EBA Engineering Consultants designed the median barrier for both projects.
Masood Hassan, P.Eng., one of the project leaders at EBA, explains that non-tensioned cable barriers have been around for decades. However, they fall to the ground after being hit by a vehicle.
The high-tension cable barriers, on the other hand, retain much of their tension after a hit and can take additional hits. These barriers are also more forgiving than the rigid concrete barriers or semi-rigid beam barriers. That is because the high tension cable barriers deflect and cushion the impact and guide a vehicle to a halt rather than bouncing it back to hit other vehicles in the same roadway.
There are five proprietary high-tension median cable systems approved by the U.S. Federal Highway Administration (FHWA), says Hassan. All the systems consist of three or four 3/4 thick 3/7 galvanized steel cables strung on “weak posts” designed to break at ground level when hit. The systems vary in the design of the posts and end terminals, and in the mechanisms used to attach the cables to the posts. The systems also vary in their post spacing and test deflections i.e. how far they bend or “give” when hit.
Usually only one median cable barrier is used to separate the two roadways. Where to place the single cable barrier in the median is a complicated process. Engineers must consider the FHWA’s placement-rules, the presence of bridge piers and light posts, the width of the median, and space for the barrier’s deflection plus a safety margin between it and the nearest traffic lane. To allow competitive bidding the variations of the different systems must also be accommodated. On the Deerfoot Trail, EBA located the cable barrier 4.0 m from the inside northbound traffic lane (the yellow shoulder line) by specifying a 2.4 m maximum deflection and a 1.6 m safety margin.
Over 5,000 kilometres of median cable barriers have been used successfully in the U.S., and they are also used in Europe and Australia. However, the Deerfoot Trail installation was one of the first installations in Canada. British Columbia has installed some sections, and Ontario is also testing the systems.
The cost of the Deerfoot Trail high-tension cable barrier was $92/m, which compares to the provincial average of $125/m for a W-Beam steel guardrail, and $308/m for a concrete barrier. The maintenance and repair costs for 2007-2008 were $4.14/m for the high-tension cable, compared to $6.74/m for W-beam guardrails and $1.72/m for concrete barriers.
While high-tension median cables are lifesaving, Hassan says that unfortunately they are not approved for medians with side slopes steeper than 1:4. They are also not suitable for narrow medians that lack the space for the cable barrier to deflect. cce
EBA Engineering Consultants, a Tetra Tech Company (Robyn McGregor, P.Eng., Masood Hassan, P.Eng.)