In traffic engineering, highway guardrail may prevent an errant vehicle from impacting roadside obstacles which may be either man-made (sign structures, culvert inlets, utility poles) or natural (trees, rock croppings), running off the road and going down a steep embankment, or veering off the roadway into oncoming traffic (commonly referred to as a median barrier).

A secondary objective is keeping the vehicle upright while deflected along the guardrail.

What is the purpose of a guardrail?

Purpose of a GuardrailA guardrail is, first and foremost, a safety barrier intended to shield a motorist who has left the roadway. The best case scenario, if a car is careening off the road, would be for that car to come to rest unhindered. In some cases and places, however, that is not possible. The roadway may be abutted by steep embankments or side slopes, or it may be lined with trees, bridge piers, retaining walls, or utility poles. Sometimes it is not feasible to remove those things. In those cases – when the consequences of striking a guardrail would be less severe than striking the other objects next to the roadway – guardrails should be installed. They can make roads safer and lessen the severity of crashes. The guardrail can operate to deflect a vehicle back to the roadway, slow the vehicle down to a complete stop, or, in certain circumstances, slow the vehicle down and then let it proceed past the guardrail.This is not to say that guardrails can completely protect against the countless situations drivers may find themselves in. The size and speed of the vehicle can affect guardrail performance. So can the vehicle’s orientation when it strikes the guardrail. There are many other factors.Transportation engineers, however, carefully weigh the placement of guardrails so that for most drivers in most conditions the barriers work – and work well.