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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

High Tension Cable Barrier in the Median of a Freeway in Alberta, Canada: A Case Study of Two Successful Projects

This paper presents the highlights of the design, construction, operation and maintenance, and safety and economic effectiveness of two high tension cable barrier (HTCB) projects in the median of Highway 2 in the province of Alberta, Canada. The projects were initiated by the provincial Department of Transportation (DOT), with a view to preventing cross-median crashes on this high traffic freeway. The design, construction-supervision, before-and-after safety evaluation, and benefit-cost analysis of the projects were undertaken by Tetra Tech. The first project consisted of an 11km 3-cable HTCB installation (in 2007) in the depressed median (varying in width from 16m to 31m) of the six-lane divided Highway 2 within the City of Calgary (locally known as the Deerfoot Trail), carrying 154,000 AADT. The second project consisted of a 135km 4-cable HTCB installation (in 2010) in the depressed median (varying in width from 14m to 18m, with varying sideslopes) of the mostly rural segment of the four-lane divided Highway 2 from Calgary northward, carrying 34,000 AADT. The main findings of the paper are presented below: 1) Design Aspects. Under a competitive bidding environment, the specifications should ensure that at least two suppliers are eligible to bid. Placing of the HTCB in the median, preferably near the shoulder, should ensure that the available deflection space exceeds the test deflection of the HTCB product installed; and if this condition cannot be met, the HTCB may need to be installed in the ditch or on both sides of the median; 2) Safety Effectiveness. After HTCB installation, the percentage of fatal median related collisions was reduced from 4.4% to 0% on Deerfoot Trail, and from 2.1% to 0.5% on Highway 2; 3) Cost Effectiveness. A rigorous benefit-cost analysis showed: for Deerfoot Trail, a benefit-cost ratio of 11.1 at the end of the design life of 20 years, with benefits exceeding costs in the first year after installation; and for Highway 2 a benefit-cost ratio of 4.58 at the end of the 20 year design life, with benefits exceeding costs in the second year after installation; 4) Innovative Construction Techniques. The innovative and cost-saving techniques that the HTCB contractor utilized for the Highway 2 installation included: a vibratory post driver instead of the usual drop hammer; straight line production utilizing two customized truck-trailers each carrying 20 spools of cable instead of the usual four spools; a custom-built guide post attachment mounted on a mini-excavator; a customized post developed to maintain some tension in the system during splicing; and tensioning of cables with an electric winch instead of the usual block and tackle arrangement. An innovation that had some challenges was the use of precast concrete for the end-anchor foundations, with many demonstrating movement beyond allowable tolerances, although not compromising the functionality of the HTCB. One of the lessons from this experience is that in the future Alberta Transportation will require that concrete foundations be cast-in-place, or if driven steel, to be specified at a greater driven depth. Precast concrete foundations are no longer permitted. 5) Operational and Maintenance Experience. The 11km Deerfoot Trail HTCB experiences an average of 3.2 hits per month (or 0.3 hits per km per month) requiring repair. The maintenance contractors find repairing HTCB not harder than other barrier types. Accumulated snow does cause occasional problems. Extra precautions are required to safeguard the safety of maintenance workers; 6) Training of First Responders. Familiarization sessions were held with first responders to disseminate knowledge about vehicle extraction and cable-cutting if necessary.; and 7) Public Acceptance. The reaction of the public and emergency responders to the HTCB installations has been consistently positive. The authors believe that the experience gained from the two Alberta Highway 2 projects would be of interest to highway jurisdictions around the world.

Language

  • English

Media Info

  • Media Type: Digital/other
  • Features: Maps; Photos; References; Tables;
  • Pagination: pp 362-377
  • Monograph Title: First International Roadside Safety Conference: Safer Roads, Saving Lives, Saving Money
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 01648408
  • Record Type: Publication
  • Files: TRIS, TRB, ATRI
  • Created Date: Oct 13 2017 8:51AM