Guard Rail - Automotive Safety

Automotive Safety

In traffic engineering, guardrails prevent vehicles from veering off the roadway into oncoming traffic, crashing against solid objects or falling into a ravine. A secondary objective is keeping the vehicle upright while deflected along the guardrail. The problem with this is that a guardrail of the optimum height for a car might not keep a truck from toppling over it, while a motorbike might slip under a higher rail.

In most cases guardrails would not be able to withstand the impact of a vehicle just by the strength of the individual posts in the area hit by the vehicle. Instead, the guardrail is effectively one strong band that transfers the force of the vehicle to multiple posts beyond the impact area or into a ground anchor at the end of the guardrail. Newer concrete barriers, while usually strong enough to withstand direct hits by cars, still work on a similar principle in deflecting heavier vehicles such as trucks.

Though they have usually prevented far more serious accidents, guardrails have frequently ranked as among the highest sources of injury and fatality in a fixed-object crash. Among the primary reasons for this is the type of treatment used at the end of the guardrail facing oncoming traffic. Most end designs will either deflect, absorb, or launch the vehicle.

Deflection causes the vehicle to be redirected back into traffic – particularly dangerous on undivided roadways, as the vehicle may travel into oncoming traffic. Deflection is prevented in modern installations by allowing the guiderail to deform under the load of the crash, giving a 1.5 metre capture zone behind the guiderail. It is important that the grades and fixed objects behind guiderails protect this clear zone, allowing safe capture of the vehicle by the guiderail. If the guiderail is too rigid to deform, deflection may be an issue.

Absorption is when the force of impact is directly transferred between the vehicle and guardrail, which may cause the end to puncture the vehicle. This is most common where a "whale tail" or blunt end treatment exists. To mitigate this a number of guiderail end treatments exist such as "Extruder end treatments", "eccentric loaders" and "Driveway wrap treatments" which result in blunt ends rarely being left exposed in modern installations.

Lastly, a vehicle can become airborne upon striking a guardrail with a buried end treatment, which may negate the purpose of the guardrail, if the vehicle continues beyond the guardrail and strikes the object the guardrail was protecting. Additionally, an airborne vehicle is likely to collide in a manner that the vehicle was not designed for, increasing the risk of failure in the vehicle's collision safety systems. Collapsible guard rails are safer than rigid ones, since a longer collision duration will result in a smaller average impact force.

Transportation engineers limit the guardrails as much as possible, as guardrails should only be placed when the roadside conditions pose a greater threat than the guardrail itself. In fact, in the hierarchy of five roadside safety treatments, shielding with guardrails ranks fourth. Therefore, while guardrails are often added as a retrofit to existing roads, newer roads are designed to minimize roadside threats, whether that may include aligning a road on a smoother curve or filling in a ravine which would eliminate the need for guardrail altogether. In addition to new research into end treatments, public awareness among both drivers and engineers has been gradually reducing injuries and fatalities due to guardrails.

There are four general types of guardrail, ranging from weakest and inexpensive to strongest and expensive; cable and wood posts, steel and wood/metal posts, steel box-beam, and concrete barriers. While cheaper guardrail is the weakest, often being destroyed from the impact of a light vehicle, it is inexpensive and quick to repair, so this is frequently used in low-traffic rural areas. On the other hand, concrete barriers can usually withstand direct hits from vehicles as heavy as trucks, making them well suited to high volume routes such as freeways. While rarely damaged, they would be considerably more expensive and time-consuming to repair. Concrete barriers are frequently installed in the median, being expected to withstand frequent impacts from both sides, while the shoulders of the road often have cheaper guardrail.

Guardrails are sometimes placed beneath the sides of high-sided heavy vehicles (such as lorries, semi-trucks, etc.). to prevent smaller vehicles from passing underneath the heavier vehicle during a collision and being crushed.