What does side underrun protection do on trucks and trailers?

If you see bars down the side of a truck or trailer between the axles, this is side underrun protection and it's designed to stop cyclists, motorcyclists low vehicles and other road users falling, sliding or driving underneath the truck or trailer and being run over by the rear wheels.

3.0 m 3000 mm Side Underrun Protection Foldable Holder Truck

Side Guards for Trucks - Truck Underride Guards - Windyne

Side Guards for Trucks - Truck Underride Guards - Windyne

Hino makes front under-run protection system standard equipment

What is a lift axle or retractable axle?

ATA releases side underrun protection program - Trailer Magazine

Design and Structural Analysis of Truck Rear under Run Protection

Underrun Side Guard Aluminium Rail - 6500mm

ATA releases side underrun protection program - Trailer Magazine

Semi-trailer truck - Wikipedia

ATA publish guidelines for heavy vehicle side underrun protection

DVSA publishes new guidance on fitting vehicle side guards – Moving On