Approach Slope for Midwest Guardrail System
REPORT NUMBER
TRP-03-188-08
AUTHORS
Erin Johnson, Karla Lechtenberg (Polivka), John Reid, Dean Sicking, Ronald Faller, Bob Bielenberg, John Rohde
PUBLICATION DATE
2008-12-04
ABSTRACT
AASHTO's Roadside Design Guide recommends that W-beam guardrails should not be placed on roadside slopes of 8:1 or steeper. This restriction often controls barrier placement decisions. Due to the slope limitations, designers are often faced with placing the guardrails near the edge of the shoulder. The long lengths of guardrail placed in close proximity to the edge of the shoulder greatly increases accident frequency with the guardrail. However, with the development of the Midwest Guardrail System, the mounting height and deeper blockout may provide improved performance to relax the recommendations provided in AASHTO's Roadside Design Guide.
An LS-DYNA simulation study was conducted in order to determine the critical slope and associated offset for placement of the Midwest Guardrail System. An 8:1 slope was identified as the critical slope for the pickup truck impact condition, and the front face of the Midwest Guardrail System (MGS) was placed 1.5 m (5 ft) down from the slope break point.
Two full-scale vehicle crash tests were performed on the system. The first was with a 3/4-ton pickup truck, impacting the system at a speed and angle of 100.4 km/h (62.4 mph) and 25.9 degrees, respectively. The second crash test was performed using a small car, impacting the system with a speed and angle of 99.6 km/h (61.9 mph) and 21.6 degrees, respectively. Both tests were conducted and reported in accordance with requirements specified in the National Cooperative Highway Research Program (NCHRP) Report No. 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features and were determined to be acceptable according to the Test Level 3 (TL-3) evaluation criteria.
KEYWORDS
Highway Safety, Roadside Appurtenances, Longitudinal Barrier, Guardrail, Approach Slope, Computer Simulation, MGS, Crash Test, Compliance Test.
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