View Q&A

There has been a question raised about the concrete height transitions that are connected to the temporary type F barriers. MoDOT is using and has used a 20-foot long concrete height transition for at least 6-8 years. Located on page 3 of 6 at the attached email address http://www.modot.org/business/standards_and_specs/documents/61720b.pdf

We use the transition for road ways where the posted speeds are less than 35 mph. Other states use a 12.5-foot long concrete height transition. There are two questions:

1. Are there any concerns between the two different transition lengths of 12.5 and 20 feet?

2. Has the height transitions been crash tested at the lower speeds and how did they perform? If not, are there any approved crashed tested concrete height transitions? 

Question #1:

Although the full-scale vehicle crash testing program was conducted on the approximately 20-ft long sloped segments, HVOSM computer simulation modeling was also performed on the conventional sloped end treatment. Several small car simulation results were provided based on impact angle, impact speed, and impact location.

For end on impacts, the simulation results showed that vehicle overturn was likely at the 30, 37, and 45 mph speeds for taper lengths of 10 and 15 ft. For 20 and 25 ft length, vehicle rollover was only predicted at 45 mph and with the 20 ft long sloped end.

For oblique impacts (15 degrees), vehicle rollover was not predicted for the 20 and 25 ft section lengths and at any of the three speeds.

For oblique impacts (30 degrees), vehicle rollover was predicted for the 10, 15, 20, and 25 ft section lengths and at all three speeds.

The TTI researchers recommended the use of 20 ft sloped sections over 25 ft sloped sections due to the insignificant benefit observed with using 25% longer section. The longer lengths were also noted to provide measurable improvement over the shorter segment lengths.

Question #2:

Six full-scale vehicle crash tests were conducted to evaluate two sloped end treatment configurations, as report in NCHRP Report No. 358. A conventional sloped end treatment and the New York sloped end treatment were evaluated. The CSET was 20 ft long while the NYSET was 19-ft 11 1/2-in. long. The sloped end treatments were anchored along their length during the test program.

The CSET was evaluated with three tests:

    (a) small car impacting end at 45 mph and 0 degrees with left wheels aligned on barrier centerline - marginal stability observed - overturn possibly adverted by steel guide flag attached to front wheel assembly [test 8]

    (b) small car impacting 2 ft from end at 45 mph and 30 degrees with left wheel contacting side slope - vehicle overturn - failure [test 9]

    (c) small car impacting 2 ft from end at 30 mph and 30 degrees with left wheel contacting side slope - vehicle overturn - failure [test11]

The NYSET was evaluated with three tests:

    (a) small car impacting end at 45 mph and 0 degrees with left wheels aligned on barrier centerline - marginal stability observed - overturn possibly adverted by steel guide flag attached to front wheel assembly [test 5]

    (b) small car impacting 2 ft from end at 45 mph and 30 degrees with left wheel contacting side slope - vehicle overturn - failure [test 6]

    (c) small car impacting 2 ft from end at 30 mph and 30 degrees with left wheel contacting side slope - vehicle overturn - failure [test12]

As shown above, crash tests have been performed on 20-ft long, anchored sloped end treatments with small cars. At 45 mph. end-on impacts were mariginal. For oblique impacts at 30 and 45 mph, test results were unsuccessful with vehicle rollover observed.