The things preventing you from falling off a bridge on either side. The fences of bridges or balconies. The construction preventing millions of deaths each year simply by preventing so many falls down lethal heights.
What are they called?
Sherlock Holmes said
Railings.
Halo said
Ah, but what type of rail? Wikipedia has once again astounded me by informing me that there are different classifications of railings - guardrails and handrails.