Category:606 Guardrail and Guard Cable: Difference between revisions

Historically, about one in every three fatal accidents is a result of a single vehicle leaving the roadway and striking a fixed object or overturning. Guardrail and guard cable play a major role in saving lives in vehicle crashes by preventing vehicles from leaving the roadway and striking a hazard. Guardrail and guard cable work by containing the errant vehicle and redirecting the vehicle away from hazards. Guardrail and guard cable can shield fixed objects and steep slopes, can be used in tangent or curved sections of roadway and can be used to shield bridge ends and piers from vehicle impacts.

Multiple roadway safety studies have shown that guardrail along the edge of the road can reduce the number of crashes and their severity. A 1995 study cited in an ATSSA roadside safety report indicates that the presence of guardrail reduced the overall crash rate by approximately 30%, reduced fatality crashes by 45% and reduced injury crashes by 50 percent. NCHRP Report 490 (2003) presents in-service performance evaluation findings from studies in Connecticut, Iowa and North Carolina that found only a 3% severe injury rate for crashes involving w-beam guardrails.

When properly located, guardrail and guard cable can greatly improve roadside safety.

Installation of a roadside barrier is considered when a vehicle striking the obstacle being shielded would find impact with the obstacle less forgiving that striking the roadside barrier itself.

When a roadside obstacle exists, treatment should be considered in the following priority:

1. Eliminate the obstacle.

2. Redesign the obstacle so it can be safely traversed.

3. Relocate the obstacle to a point where it is less likely to be struck.

4. Treat the obstacle to reduce accident severity with a breakaway base or appropriate grading.

5. Shield the obstacle with a median barrier, roadside barrier or crash cushion.

6. Delineate the obstacle if the above alternatives are not appropriate.

In many cases, it is either impractical or impossible to eliminate a roadside obstacle, redesign the obstacle so it can be safely traversed, relocate the obstacle to a point where it is less likely to be struck or provide an appropriate breakaway device that will reduce impact severity. In these cases, shielding of the obstacle will be needed to protect the errant vehicle from the obstacle and to provide a more forgiving roadside.

Roadside barriers, whether they be concrete, guardrail or guard cable, are themselves obstacles. Even though they are engineered and rigorously tested to preserve the safety of vehicle occupants, all shielding systems cause damage to the vehicle and/or sustain damage themselves when impacted.

Three basic types of obstacles commonly shielded using roadside barriers:

• steep slopes, lateral drop-offs, or terrain features
• bridge ends or bridge piers
• fixed objects and other roadside obstacles that cannot be eliminated, made breakaway, or otherwise traversable, or relocated.

Guardrail

Guardrail is the most common roadside traffic barrier used to shield traffic from obstacles. Simply, guardrail consists of two key functional components: the guardrail face and the end terminal. Guardrail also includes the posts, the connection of the guardrail face to the posts, the soil that the posts are driven into and the anchoring system at the end terminal. All of these elements have a bearing in how the guardrail will perform if impacted.

Guardrail Face. The guardrail face is the length of w-beam or thrie beam guardrail extending from the end terminal alongside the roadway. Its primary function is to direct impacting vehicles away from the obstacle the guardrail is shielding and redirect them back onto the roadway.

End Terminal. The end of a guardrail run facing approaching traffic needs to be protected by a crashworthy end terminal. Crashworthy end terminals are energy-absorbing end treatments that are designed to absorb the energy of an impact by having an impact head slide down the length of the guardrail end terminal. These end terminals function in two ways. When hit head-on, the impact head slides down the guardrail flattening, or extruding, the guardrail and redirecting the guardrail away from the vehicle until the impacting vehicle’s energy is dissipated and the vehicle has decelerated to a stop. When hit at an angle, the impact head may partially extrude the guardrail and then "gate" out of the way allowing the vehicle to pass behind the guardrail. This means the end terminal and guardrail are pushed aside, as if opening a gate.

Guard Cable

Guard cable contains and redirects vehicles away from impacting an object or terrain that is less forgiving than the guard cable. It consists of a series of steel cables mounted on weak posts placed to capture and redirect errant vehicles.

Guard cable is also commonly used in interstate medians to help prevent vehicles from crossing the median and entering into opposing traffic lanes. Guard cable may be used instead of guardrail on new construction projects in locations outside of the clear zone where the designer wants to protect an errant vehicle from leaving the roadway and entering areas with steep slopes and high fills. Substitution of guard cable for guardrail is not recommended on sharp curves.

Testing Criteria. Current roadside safety hardware testing standards are published in the AASHTO Manual for Assessing Safety Hardware (MASH) 2016. It updates and replaces the previous standard, the National Cooperative Highway Research Program Report No. 350 (NCHRP 350). MASH 2016 retains and modifies the six separate test levels (TL) representing different vehicles, impact angles and speeds.

Test level three (TL-3) is probably the most common as it establishes safety criteria for both small cars and pickups at 62 mph. This category of traffic accounts for nearly 90% of all vehicle traffic in Missouri.

The table entitled "What is TL-3?" summarizes data for the six test levels.

A roadside safety hardware feature must undergo rigorous safety testing before it can be used on the National Highway System (NHS). Most states have adopted the same testing criteria for highways that are not on the NHS. The standard by which all roadside safety features are measured is contained within the MASH.

MASH evaluates safety hardware according to three general factors:

1) Structural Adequacy: the system must contain and redirect the vehicle with no under-riding, overriding or penetration.

2) Occupant Risk: fragments of the system cannot penetrate the passenger compartment, the vehicle must remain upright during and after the collision, and the passenger must not undergo excessive impact or deceleration.

3) Vehicle Trajectory: after the impact, the vehicle should not intrude into adjacent traffic lanes nor should it exit the system at an angle greater than 60% of the entry angle.

Construction Inspection Guidelines for Sec 606

Construction Requirements (for Sec 606.3) Inspection of guardrail, crashworthy end terminals, three-strand guard cable and one-strand access restraint cable is usually done on a spot basis. The contractor is to be given enough reference points from which to establish line and grade. Points to check during installation are alignment of posts and possible damage from driving. Completed rail or cable should be checked for alignment, position with respect to the travelway, elevation, soil backfill and proper direction of lap for beams. Bolts are to be tightened to manufacturer specifications. All elements of construction are to be checked for installation damage.

Post driving frequently causes damage to stabilized shoulders. No installation is to be accepted until such damage has been satisfactorily repaired in a manner preventing ponding of water around the posts.

Delineators (for Sec 606.10.2.3). Delineators should be placed on all guardrail located 2 ft. or less from the edge of the shoulder. The delineators are to be spaced at 50 ft. intervals. Refer to this specification for more detail on delineator body, retroreflective sheeting and delineator color requirements.

Refer to EPG 1040 Guardrail, End Terminals, One-Strand Access Restraint Cable and Three-Strand Guard Cable Material for procedures establishing inspecting sampling, accepting and reporting of guardrail and guard cable material specified in Sec 1040.

Crashworthy end terminals (CET) may not be substituted for a lower type CET without the districts’s core team approval. For example, a Type D CET cannot be replaced by a Type C device.