In-Service Performance Evaluation (ISPE) for G4 (1S) Type of Strong-Post W-Beam Guardrail System
This research is needed in response to new guidance for guardrail height issued by the Federal Highway Administration (FHWA) Office of Safety Design by memorandum dated May 17, 2010 to FHWA Division Administrators. The new guidance states that standard 27-inch guardrail does not satisfy crash test criteria (NCHRP Test Level 3) and calls for a guardrail height of at least 27-3/4 inches minimum to the top of the rail, including construction tolerance. The basis for increasing the guardrail height is to accommodate full size standard cab pickup trucks (representing the 2000P crash test vehicle for NCHRP Report 350 Test Level 3).
In a letter dated November 29, 2010 from the FHWA Division Administrator, FDOT was requested to provide a plan for implementing the May 17, 2010 memorandum. FDOT's current standard for Strong-Post WBeam Guardrail System is detailed in FDOT Design Standard Index 400 and is based on the G4 (1S) system. Florida has used a G4 (1S) type system with a standard guardrail height of 27-1/8 inches since the 1960's which is 5/8 inches less than FHWA's new revised minimum height guideline.
FDOT has requested the implementation of FHWA's May 17, 2010 memo be delayed until a thorough in service performance evaluation has been conducted on FDOT's existing 27-1/8 inch guardrail height to determine if there is any significant safety benefit in raising the guardrail height to 27-3/4 inches. This request was made because implementation of the FHWA May 17, 2010 memorandum will require substantial work and resources. A change in guardrail height will affect numerous FDOT standard index drawings and details (96 sheets). Additionally new vendor drawings for guardrail end treatments and crash cushions will need to be obtained from vendors and processed through FDOT's Qualified Products List approval process.
The requirements in the FHWA May 17, 2010 memorandum have not given consideration to InService Performance. Evaluation (ISPE). The lack of ISPE is of particular concern to FDOT as the importance of ISPEs has been well documented in NCHRP Report 350 and AASHTO MASH 2009. Both documents state that "the safety performance of a highway feature cannot be measured by a series of crash tests only." This is also recognized in the 2006 AASHTO Roadside Design Guide which states that "a barrier may be considered operational if it has been used for an extended period and has demonstrated satisfactory field performance in terms of construction, maintenance, and crash experience."
The proposed ISPE is intended to compare the collision performance of the Strong-Post W-Beam Guardrail design as it relates to passenger cars and pickup/light trucks. At this time, FDOT has no standard procedure or program for evaluating in-service collision performance of roadside safety hardware, which is especially critical when evaluating the implementation of FHWA guidelines that exceed FDOT's established design standards.
FDOT Roadway Design Office recently completed a preliminary analysis evaluating the crashworthiness (collision performance) of strong-post w-beam guardrail by comparing the five-year injury and fatality records of passenger car (automobile) versus pickup/light truck crash data in Florida. Percentage increases between automobiles and light trucks were compared for both limited access and non-limited access roadways. Initial comparisons indicate that there is "very little difference" between passenger cars and pickups/light trucks in the percentage of guardrail collisions resulting in injuries and fatalities. The preliminary analysis findings indicate a need to question the implementation of raising FDOT's minimum guardrail height requirement. To validate these findings and develop a department procedure or guideline, the team recommends further research.
The main objectives of this project are to:
- Identify appropriate methods of In-Service Performance Evaluation (ISPE) for the G4 (1S) Type of Strong-Post W-Beam Guardrail System,
- Perform an ISPE of both median and roadside G4 (1S) Type of Strong-Post W-Beam Guardrail System on both limited access and non-limited access roadways on the State Highway System, and
- Establish a procedure and develop a system for the collection and continued maintenance of guardrail inventory data.