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What are lateral protection devices and side underrun protection?

Lorry bodies sit high off the ground and this means that vulnerable road users such as pedestrians, motorcyclists and cyclists can more easily fall under the rear wheels of a trailer or lorry if they hit or are hit by the vehicle.

These kinds of accidents are frequently fatal as the rear wheels run over the person. Side guards, also called lateral protection devices and side underrun protection, are designed to deflect these road users away from the lorry or trailer.

There is a standard that must be adhered to (UN ECE R73) which defines dimensions, strength, types of materials and more. Toolboxes and fuel tanks can form part of this protection for ‘unprotected road users’ with the remaining space between axles being filled by guardrails.

Two sidebars are shown between the front and rear axles on this rigid truck
EU truck parked up in South London – the space between the axles is occupied by a tank or toolkit that meets the standard, therefore no side guards are required
Side and rear underrun protection is shown on this artic
Underrun on a pull trailer; the guards don’t cover the marker lights down the side and they are finished in partially reflective tape to give better visibility to the trailer side-on at night

Front and rear underrun helps prevent more serious injuries in nose-to-tail incidents. A typical trailer or truck has its structure higher off the ground than is optimal for a car’s crash protection systems (i.e. crumple zones); a car will hit it with the bonnet, the suspension is compressed and the rear of the trailer slices through the windscreen and A pillars.

The same applies for a lorry running into the back of a vehicle in front: front underrun ensures that the force is transmitted through the vehicle’s structure rather than the cab riding up over the boot and through the rear window.

Side guards can provide some aerodynamic assistance when they are solid rather than bars.

Aerodynamic fairing shown on an American trailer; it serves a dual purpose of improving fuel economy and acting as side underrun protection

Another benefit of side guards is their ability to provide early warning when turning of an obstacle that might ordinarily go under the rear wheels and damage tyres or suspension, or for the guards to push that object out of the way.

DVSA has guidance of where side guards may need to be fitted to your lorry or trailer.

Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.

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