ROAD SAFETY BARRIER SYSTEM TYPES

A road safety barrier is used for shielding of vehicle and reducing the hazard of collision / crash.

A variety of road safety barrier systems can be used to meet specific site and project requirements.

The road safety barrier for highway shall meet standardised procedures for the crash testing and evaluation of both permanent and temporary road safety features, including barriers, sign supports, etc.

We have supplied Road Safety Barrier Systems for many countries, with excellent performance. While different types of barriers have road safety features that meet a given test level, they generally have different performance characteristics.

TYPES OF ROAD SAFETY BARRIER SYSTEMS

Flexible Roadside Barrier Systems

Flexible barrier systems, e.g. wire rope barriers, are generally more forgiving than other types because most of the impact energy is dissipated by deflection of the barrier and lower impact forces are imposed on the vehicle and its occupants.

All wire rope barriers approved for use on state highways have different deflection characteristics for a given NCHRP 350 test level.

Semi-rigid Roadside Barrier Systems

Semi-rigid barrier systems, e.g. W-Beam and Thrie-Beam barriers, are the most common type of road safety barrier used in New Zealand. This type of system redirects vehicles mainly by the beam action of the relatively strong steel rails transferring impact loads to closely spaced posts, which in turn transfer the loads to the ground.

Semi-rigid barrier systems have much less lateral deflection than flexible systems.

Examples of semi rigid road safety barrier systems available in New Zealand are:
• Strong Post W-Beam - TL-3
• Strong Post Modified Thrie-Beam - TL-4.

Rigid Roadside Systems

Rigid barrier systems, e.g. concrete barriers, work by transferring vehicle impact loads directly into the ground, resisting impacts through the inertial resistance of the barrier mass and its sliding resistance across the ground. The face of these barriers is designed to absorb impact energy by partly lifting the vehicle and controlling the behaviour of the vehicle after impact.

Rigid barriers have no lateral deflection.

Bridge Barriers

Although road safety barriers on bridges tend to appear similar in design to roadside barriers they are unique in their design.

The design of bridge barriers must comply with the requirements of the Bridge Manual.

Guidelines for the retrofitting of guardrails to existing bridges are in preparation.

End Terminals

(a) Redirective Gating Terminal

Redirective gating terminals provide:
• controlled penetration of the barrier when a vehicle impacts at, or near, the nose of the device; and
• smooth redirection of a vehicle when it impacts the barrier downstream of the nose section.

Part of the length of a redirective gating terminal is usually able to be included in the length of standard barrier needed to shield a hazard, but this varies from system to system.

(b) Redirective Non-gating Terminal

Redirective non-gating terminals will smoothly redirect a vehicle, without pocketing or penetration of the barrier, when a vehicle impacts the barrier:

• at, or near, the nose of the device; or • downstream of the nose.

Most of the length of a redirective terminal will usually be able to be included in the length of standard barrier needed to shield a hazard.

(c) Non-redirective Terminal

Non-redirective terminals absorb an impacting vehicle’s kinetic energy when they are hit head-on by an errant vehicle. However, they will not control vehicles that impact at an angle and pocketing, or penetration, of the terminal may result. All non-redirective terminals are gating.

Crash Cushions

Site conditions will usually dictate the type of crash cushion needed, e.g. fixed objects such as barrier ends which are less than one metre wide should be shielded by a narrow crash cushion. Wide obstacles, e.g. those greater than 5m, are best shielded by sand barrel arrays, custom designed attenuator systems or metal-beam “bullnose” attenuators.

Kerbs and slopes can cause an impacting vehicle to become airborne and reach undesirable roll and pitch angles before impacting the crash cushion. The surface on which a crash cushion is installed should be smooth, flat, and compacted. All of the energy absorbing systems must be placed on a hard, smooth pad or surface (usually concrete) to enable the unit to be compressed uniformly during an impact. In the case of inertial crash cushions, a paved surface provides uniform support for the sand barrels and, perhaps more importantly, provides a surface on which the pattern of the array and the required masses of the modules can be permanently marked for maintenance purposes.

If a crash cushion is installed on a structure, the location of expansion joints may dictate the type of device to use, or require some modifications to the standard design, e.g. non-anchored units such as sand barrels, may be susceptible to vibration-induced movement.

Climatic conditions in a particular area should also be considered because some crash cushions are affected by above or below average temperatures and may also be more susceptible to inadvertent damage caused by operations such as snow removal.

Transitions

A non-rigid barrier should be gradually stiffened on the approach to a connection with a more rigid object such as a bridge barrier, a retaining wall, an abutment wall, or another structural support. The transition design should result in barrier that will permit an impacting vehicle to be smoothly redirected without pocketing or snagging of the barrier.

Aesthetic and Environmental Considerations

While aesthetics are a consideration, they are not normally controlling factors in the selection of a side protection barrier, except in the environmentally sensitive locations such as recreational areas or parks. In such cases, it is important that the systems used be crashworthy as well as visually acceptable to the road controlling authority.

It is also important to consider environmental factors in the selection process.

For example:

• Barriers with considerable frontage area may contribute to a build up of drifting sand or snow in some areas.

• Metal railing barriers may deteriorate rapidly in highly corrosive urban/industrial environments.

In some cases, solid barriers may restrict sight distances of drivers entering from a side road or intersection, or may block a driver's view of a particularly scenic panorama.