No two crashes are the same. So how do you make roadside hardware such as barriers, bridge rails, and sign supports as safe as possible knowing that they will be impacted differently when crashed into by cars, pickups, SUVs, or semis?
Before roadside hardware can be placed on our highways, it must undergo full-scale crash testing using a variety of vehicles impacting the equipment at different angles. Since 1993, guidance for conducting these crash tests was found in the National Cooperative Highway Research Program Report 350 (NCHRP 350). Chris Poole, with the Iowa Department of Transportation’s Office of Traffic and Safety, currently serves on the American Association of State Highway and Transportation Officials committee tasked with providing guidance on standardizing these crash tests.
In general, three factors determine whether a crash test with a barrier is considered a “pass” or a “fail.”
“NCHRP 350 was a huge leap forward in crash testing guidance,” said Poole. “It was the first time the states, working through AASHTO, had a formal agreement with the Federal Highway Administration related to crash testing. It meant all states were operating under the same set of standards.”
But since that time, vehicle sizes and weights have changed and the testing standards need to adapt to those changes. AASHTO’s Technical Committee on Roadside Safety, of which Poole is vice chair, began updating the test standards in the mid-2000s. By 2009, the Manual for Assessing Safety Hardware (MASH) was released, replacing NCHRP 350.
“As with most everything, you learn from your failures. In developing MASH, we considered what we had learned from all the NCHRP 350 tests. We addressed changes in the nation’s vehicle fleet, always keeping in mind that having the safest barriers possible is the goal. The science of crash testing has also evolved.”
But crashing a Nissan Sentra and a Mack truck into the same barrier will provide very different results. So how does the committee address the differences? Poole said, “There is a fine line when you design barriers to contain heavy vehicles. They need to be able to withstand the impact from a large truck, but they have to be forgiving enough so that they don’t stop a smaller vehicle too abruptly, resulting in potential injuries to the driver and passengers.”
Cable barriers seem to balance that trade-off of strength and flexibility. The determination of new crash tests for cable barriers installed within medians is the main reason MASH was recently updated. Poole said, “When states started using cable barriers, there weren’t good crash tests. Now that we have several years’ worth of data on these devices, we have come up with tests that prove the barriers are both strong enough to stop a large truck and forgiving enough to limit the chance of injuries to occupants of smaller vehicles.”
Although MASH has been in existence since 2009, states and safety device manufacturers were slow to make the change from NCHRP 350 testing standards. Now that an updated version of MASH was just approved, AASHTO and the FHWA have signed an agreement that provides specific dates for states to implement MASH-compliant safety devices. For example, longitudinal barriers such as guardrail and cast-in-place concrete barriers included in projects after Dec. 31, 2017, will need to meet MASH testing requirements. Implementation dates for other categories of roadside hardware will be phased in over the following two years.
Poole cautions that only new or replaced barriers will be required to meet MASH. He said, “It wouldn’t be cost-effective to go out and replace all barriers at once. However, states will be required to have policies in place that address how they will manage the maintenance of these devices, and what happens when they become damaged beyond repair.”
In the end, we would all like to keep our vehicles safely on the road all the time. But if you do happen to crash, the work done by Poole and the others on the technical committee can help lessen the impact.
All photos courtesy Midwest Roadside Safety Facility (MwRSF) at University of Nebraska - Lincoln (UN-L)