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Guard rail absorbs the impact of out-of-control vehicles while guiding the vehicle to safer stops. There are three types of guard rail in primary use today.
Also called W-beam, flex beam is the cheapest and easiest to install, and thus the most popular guard rail in use today. It consists of a w-shaped piece of steel with either wooden or steel posts. Significant research has gone into the placement of flex beam, and the following are the standard specifications:
End sections can be either turn down or a crash absorption barrier. Most state DOT’s have standards for both, and the manufacturers will build them all to the standard (i.e. you’re not likely to get anything else very cheaply).
The standard specification is AASHTO M180.
Thrie beam is similar to flex beam, but with three leading edges instead of two. It is used on bridges and other, generally specialty, applications. Here are the specifications for Thrie beam:
The standard specification AASHTO M180 applies to both W-beam and thrie-beam.
I’ve heard a few people rave about these. Although not all jurisdictions use them they have their die hard advocates. High tension cable barriers are proprietary products and therefore must be installed and maintained according to the manufacturers specifications. They vary substantially in details and methods of installation. Specifiers should review the FHWA’s acceptance letters.
The placement of the cable barrier should be either:
All three of these systems fall into the category of flexible guardrail. The other types are rigid and semi-rigid, such as concrete barriers. I will save those for another article.
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