What is a Guardrail?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 06 October 2019
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A guardrail is a device which is designed to promote public safety by preventing people or vehicles from crossing from an established trail or road into a dangerous area. Many people are familiar with the positioning of guardrails along dangerous stretches of road, but any type of barrier intended to prevent falls or injuries could be considered a guardrail. For example, many hiking trails have guardrails in areas where steep falls are likely. In general, a set of conditions must be met before the installation of a guardrail will be considered.

The design of a guardrail can vary, depending on how it is intended to be used. As a general rule, the guardrail takes the form of a long strip of strong cast metal attached to posts. The metal is stamped so that it has multiple ridges. When something hits the guardrail, the ridges crumple, spreading the energy along the length of the guardrail. The multiple posts prevent a collapse, unless the impact is extremely heavy or at very high speed. The impacted section will usually need to be replaced after the accident has been cleared. Guardrails save lives by preventing people from falling and absorbing the impact of accidents.


Although a guardrail is intended to protect the public, a number of safety concerns are associated with guardrails, particularly those designed to work with automobiles. The first is the “end treatment,” or the finish on the end of the guardrail. Formerly, guardrails were not capped or finished in any way, leaving a wedge of sharp steel at either end of the stretch of guardrail. If someone collided with the end at a high rate of speed, the guardrail could potentially penetrate the car, causing injuries or death. Most modern end treatments are curved or flattened, and designed to resist the impact slightly.

If the guardrail or end treatment is too strong, it also has the potential of thrusting the car back into traffic. Therefore, most guardrails are designed to absorb energy, rather than deflecting it. In addition, traffic engineers have to think about the height of a guardrail. A tall vehicle may flip over a low guardrail, but a motorcycle could slide under a high one. Therefore, the height must be carefully considered before a guardrail is installed.

In order for a highway guardrail to be installed, traffic engineers must determine that there is a serious risk of severe off road collisions. In an area with a history of collisions, the risk is clearly illustrated, and the stretch of road will be added to the list of areas needing guardrails. In other situations, engineers have to evaluate the surroundings of the guardrail, looking for a cliff or other sharp change in elevation, and considering season conditions like ice and snow. If the risk is deemed serious enough, a guardrail will be installed.


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Post 2

@Brenda: Guard rails need to be weakly fastened to the posts. If not, the posts will pull the rail down under the car when the car pushes the rail. Depending on whether or not the ground is wet, muddy, frozen, or sandy, the ease with which posts can be extracted will vary greatly. If posts never came out, that would indicate that money was being wasted on their embedment.

Occasionally, a semi can be redirected, but not often. All accidents differ with vehicle weight, speed, and angle of impact. To a certain extent, the positioning of the impact point relative to the posts does matter. Guardrail types are basically designed by a trial and error process. A concept is developed

and then crash tested to a nationally specified set of criteria. If it passes the standard tests, that type can be used. If it does not pass, the design is tweaked until a variant can pass all of the tests. The suite of tests is intended to encompass what is taken to be a typical set of crash conditions.

For most highways, the test speed is 62 mph and the vehicle weights are (now) 5,000 pounds for a pickup and 2,420 pounds for a small car. The most severe angle is taken as 25 degrees. Be assured that MA has installation standards and that they reflect conditions that produced successful crash tests.

In general, a W-beam guardrail will still function satisfactorily after most bumps and mild impacts. Inspections do not need to be more than what an experienced engineer could see in driving by. If it's bad enough to require fixing, it will almost always be clearly visible. -- TH

Post 1

I would really like to know the MA laws of how a guardrail is supposed to be installed. Particulars, like how many feet down the rail has to be, if they are supposed to be cemented in, how often they are inspected, and if there are guidelines to how they are to be installed or re-installed after an accident. I've heard that a correctly installed guardrail can be hit by a tractor trailer and not break, but know of a case where 5 of the posts were pulled out of the ground and the rail bent in half and impaled a young girl.

Just wondering what the regulations are for the installation.

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