Analysis of Accidents Involving Breakaway-Cable-Terminal End Treatments

Abstract

This paper includes an analysis of 50 accidents involving breakaway-cable-terminal (BCT) end treatments and 19 accidents involving median-breakaway-cable-terminal (MBCT) end treatments as used in Kentucky. The primary data base consisted of Kentucky accident records for the years 1980-1982; selected accidents were included that occurred before 1980 and after 1982. An attempt was made to document each accident with a police report, photographs, and a maintenance repair form. Results showed that the BCT end treatment performed properly in 60 percent of the accidents; that is, the end treatment performed as it was designed, with the wooden posts breaking away or the guardrail redirecting the vehicle. Only five impacts were known to involve small cars and the BCT performed improperly in four of those accidents. It should be noted that the BCT used in Kentucky is similar to the design tested and evaluated as part of the NCHRP studies and included in the AASHTO barrier guide. The primary difference was that before 1982, most BCTs in Kentucky were installed so that the last 125 ft of rail were placed on a simple curve (4.5 degrees) and there was a 6-ft offset rather than a parabolic flare with a 4-ft offset. However, Kentucky's MBCT design utilizes two BCTs joined together at the end section, and it varies considerably from the design tested as part of the NCHRP studies. The MBCT end treatment performed properly in 50 percent of the accidents. Problems related to stiffness of the end treatment are most apparent when impact angles are shallow. A recommendation was made to remove any existing MBCT designs from gore locations and replace them with crash cushions. A turned-down end treatment design was proposed for consideration at median installations.