Most public spaces are fitted with guard rails as a means of protection against accidental falls. Any abrupt change in elevation where the higher portion is accessible makes a fall possible. Due to this responsibility and liability, rails are placed to protect people using the premises. Guardrails are generally required by code where there is a drop of 30" or more.
Building codes also require that no opening in a guard be of a size such that a 4" sphere may pass. There are three exceptions according to the 2003 International Building Code Section 1012.3 which allow openings to not exceed 8" or 21" depending on occupancy groups or special areas.
An architect who was famous for creative use of handrails for social stability was Alvar Aalto. The guard rails of an observation tower such as the Space Needle or Eiffel Tower become exaggerated to the point of becoming a fence or cage. This is also done on bridges and overpasses to prevent accidents and suicides.
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 (1) 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. Absorption is when the force of impact is directly transferred between the vehicle and guardrail, which may cause the end to puncture the vehicle. Lastly, a vehicle can become airborne upon striking a guardrail's end, 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 has cheaper guardrail.
Guardrails are also frequently placed beneath the sides of high-sided heavy vehicles (such as lorries, semi-trucks, etc). This is in order to prevent smaller vehicles (e.g. cars) from passing underneath the heavier vehicle during a collision and being crushed by its rear wheels.