US drivers at risk from poorly designed HGV under-run bars

Offset crash testing of under-run guards on heavy vehicles has shown the dangers still faced by car drivers colliding with trailers.

The Insurance Institute for Highway Safety (IIHS) in America has conducted a series of comprehensive tests involving a vehicle colliding at 35mph (56kph) with the rear of a heavy goods vehicle (HGV) equipped with under-run bars which are designed to prevent the car from riding under the trailer body and causing catastrophic damage to the occupant capsule.

Rear under-run bars and side protection on heavy goods vehicles are now commonplace after they were introduced over 30 years ago with many countries having legislation that demands that HGV equipment is fitted with such safety devices to prevent cars from running under the vehicle during a collision. This legislation varies from country to country with some specifications more stringent than others.

In the USA, the IIHS decided to test whether under-run bars from certain manufacturers which met the legal specifications were providing the protection that was necessary. To understand this, the IIHS ran three collision types to reflect real-world conditions, all performed at 35mph (56khp).

The first test involved the car striking the rear of the trailer with no offset (or overlap) so that 100% of the front of the car made a perpendicular strike on the rear of the goods vehicle. The same test was carried out with a 50% offset and then with a 30% offset meaning that only 30% of the car was in collision with the goods vehicle simulating a swerve to avoid the HGV or an unsuccessful or misjudged overtaking manoeuvre.

In the 100% offset, the 2010 Chevrolet Malibu test vehicle was prevented from under-running the vehicle for all 8 of the manufacturers tested showing that they met the specification and that the specification was adequate for preventing such under-runs. Good results were also received for most manufacturers with the 50% offset. However, all the under-run bars failed in the 30% offset crash test except the one produced by Canadian manufacturer Manac. Specifications for under-run bars in Canada are roughly twice as stringent as they are in the USA.

Apart from the Manac guard, the failure points in all cases showed similar characteristics. The guard is usually attached to the trailer rails that run longitudinally along the vehicle body about 28 inches (71cm) from the edge of the body. This “overhang” presented a significant weakness in the 30% offset with the guard failing in the outer reach and enabling the car to under-run the trailer. The Manac guard was anchored to the vehicle differently 18 inches (46cm) from the trailer edge. It was able to withstand the 30% offset test.

The Chevrolet Malibu cars used in the crash tests were occupied by dummies on which measurements were taken. The occupants were adequately protected by in-vehicle safety systems such as air bags and seat belts when the under-run bar withstood the impact. However, when the under-run bar failed, head and neck accelerometer readings from the dummy indicated that real occupants would have been killed in all cases.

Modern vehicle safety systems are designed to protect occupants in a range of different impacts and are most effective in frontal impacts when energy absorbing crumple zones, air bag deployments and seat belt tensioning all serve to provide protection to the driver and passengers. In an under-run accident, none of these mechanisms are effective and the full force of the impact is taken by the vehicle roof, side pillars and the occupants.

The IIHS has consulted with the National Highway Transportation Safety Administration (NHTSA) on the subject of improving under-run bar specifications to offer better protection in offset collisions.

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