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.
SIDE ISSUE
Individual Vehicle Approval (IVA): vehicle side guards (lateral
Design and Structural Analysis of Truck Rear under Run Protection
Under-Run Bars on Trucks : Why they are important for you! - Team-BHP
Good driving etiquette: being polite on the road
Side Guards for Trucks - Truck Underride Guards - Windyne
Strenx® 960 for stronger and lighter RUPD (Rear Underrun
What is side underrun protection on a truck or trailer?
PDF) Crashworthiness Simulations of Side Underrun Protection Devices
Side underride guards: Life savers, or misdirected resources
Under-Run Bars on Trucks : Why they are important for you! - Team-BHP
Under-Run Bars on Trucks : Why they are important for you! - Team-BHP
What does side underrun protection do on trucks and trailers?
Strenx® for curtain sider trailers - SSAB
Truck Side Guard Rails, HGV Parts, HGV Trailer Parts – Nationwide