This paper describes the preliminary engineering and design, by EBA Engineering Consultants Ltd. (EBA), of a National Cooperative Highway Program (NCHRP) 350: Test Level 4 (TL-4) high-tension (pre-stretched post-tensioned) cable barrier installed in May 2007 in the depressed median of an 11 km stretch of the Deerfoot Trail, an urban freeway in Calgary, Alberta, Canada. The paper reports the results of the research regarding the available TL-4 cable barriers approved by the US Federal Highway Administration (FHWA), including discussions with FHWA officials, review of the recent experience in several US states, published reports, and face-to-face discussions with each of the three manufacturers (Brifen, Gibraltar and Trinity) who supply FHWA-approved TL-4 cable barriers. A summary comparison of the three products is also presented. The paper discusses important design considerations, such as the post spacing and allowable maximum deflection of the cable barrier, the recent revisions to the AASHTO Roadside Design Guide with respect to placing of cable barriers on median slopes, and the pros and cons of placing the barrier at the edge of pavement, on the slope or in the median ditch. For the Deerfoot Trail project, a design deflection of 2.4 m (8 ft) and a maximum post spacing of 6.1 m (20.0 ft) was specified, and it was recommended that the supplier should be selected in a competitive bidding process. The median cable barrier was placed 4.0 m from the edge of the travel lane (the painted shoulder line). With the inside shoulder width being 2.43 m, this would place the cable barrier approximately 1.5 m from the edge of pavement, just below the top of the sideslope. This minimized the interaction with existing barrier systems, and allowed for emergency vehicles to park or pass on the inside shoulder. The authors believe that the research and design of the high tension cable barrier described in this paper should be of interest to future similar projects in Canada.
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