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Since MwRSF did the testing on flat ground with the low tension cable system, would you change your recommendation if the cable was placed on 1V:6H or 1V:8H slopes in front of and up to 4 ft. behind the cable before starting on a 1V:2H fill slope.  It would seem the vehicle would strike the barrier higher, so it may be necessary to constrain deflection even more.

I have reviewed you question regarding the use of the CASS adjacent to a steep slope with an 1:6H or 1:8H approach slope.

Previously we had given you guidance for using the CASS adjacent to a steep slope. We had suggested using a 4ft offset to the slope and reducing the post spacing of the CASS to 3 m. Your new question was whether or not these recommendations would hold true when the cable barrier was installed on a 1:6H or 1:8H approach slope. In order to address this issue, I looked into the performance of the CASS system, analysis of bumper trajectories for 2000P pickup trucks encroaching on approach slopes, and previous testing of cable barrier on approach slopes. Based on this analysis I have the following comments.

1. We have concerns with placement of the CASS on a 1:6H approach slope adjacent to a steep slope. This concern is based on effective capture of the vehicle by the CASS system. Previous testing was conducted on 30" high, low-tension cable barrier placed on a 1:6H slope with a 6 ft offset from the edge of shoulder. Two tests were conducted. The first was a test (3569-5) of a 1974 Plymouth sedan that weighed 4500 lbs and impacted the barrier at 59.6 mph and an angle of 24.75 degrees. The second test (3569-6) was a test of a 1974 Chevy Vega that weighed 2250 lbs and impacted the barrier at 58.4 mph and an angle of 17.25 degrees. Both of these tests showed safe redirection of the vehicle. However, the sedan and small car vehicles in this testing had bumper heights of approximately 18". Typical bumper heights for the 2000P and 2270P vehicles are around 26". Thus, there is concern that the capture and redirection of the vehicles observed in these tests would not be as likely with the higher bumper heights of the current test vehicles and vehicle fleet. The RDG recommendations for approach slopes are based on this testing, but do not take into account the higher bumper heights and CG heights of the current vehicle fleet. The reference for this testing is given below.

Ross, H.E., Smith, D.G., Sicking, D.L., and Hall, P.R., Development of Guidelines for Placement of Longitudinal Barriers on Slopes, Research Report 3659-2 (DOT-FH-11-9343), Texas Transportation Institute, May 1983.

In addition, I reviewed some analysis that we conducted on bumper trajectories of 2000P vehicles running off slopes and compared these trajectories with the cable heights of the CASS System. A chart is attached. In the chart, the green lines are the cable heights, the pink line is the slope, and the navy blue line is the truck bumper trajectory. You can see from the chart that vehicle encroaching on a 1:6H slope could have bumper heights higher than the top cable height of the CASS system which could lead to the potential for override of the system. Similar analysis performed by TTI on 1:6H slopes with the 2000P vehicle also indicated bumper heights that would exceed the top cable height of the CASS depending on the barrier offset ( http://tti.tamu.edu/documents/0-5210-3.pdf ).

Based on the existing test data and analysis of vehicles encroaching on 1:6H slopes, we are concerned about the use of cable barrier adjacent to steep slopes due to uncertainty about the effective capture and redirection of the vehicle.

2. No testing was available with cable barrier on 1:8H slopes. However, I did look at our bumper trajectory relative to the CASS system for an 1:8H slope. See the attached chart. In the case of the 1:8H slope, you can see that the bumper trajectory analysis indicated that there is an improved likelihood of vehicle capture as the bumper does not exceed the height of the top cable. This would indicate potential for capture and redirection. When using the CASS on an 1:8H slope adjacent to a steep slope, we would still recommend that you use the 4 ft offset from the cable barrier to the steep slope and also use the reduced CASS post spacing of 3 m.