The Highways Department has been reviewing and improving road barriers and bridge parapets, taking recent international practices and local experience into account. The works are generally designed to prevailing standards at the time when new projects or major renovations are underway.

In Hong Kong, there are four groups of barriers designed to withstand vehicle impacts of different levels. They are P1 (normal), P2 (low), P4 (high), and an interim standard for holding heavy vehicles and buses for locations that have features similar to those at last year's Tuen Mun Road accident site.

Purpose built: A variety of bridge parapet designs have been adopted to suit different purposes, such as the second-generation P1 type for expressways, P4 type near railways, and a special P4 type for long-span bridges.

The P1 type, designed to restrain vehicles up to 1.5 tonnes traveling at 113 kilometres per hour, are mainly used on expressways and limited access roads. There are about 250km of P1 vehicular barriers in Hong Kong.

Roads with lower posted speeds are fitted with P2 type barriers that contain vehicles up to 1.5 tonnes traveling at 80km per hour.

About 1.2km of P4 barriers installed

P4 barriers can be found on bridges over railways and high-risk locations. They can  restrain vehicles up to 24 tonnes traveling at 50km per hour. There are about 1.2km of such parapets.

The first P1 parapet dates back to the 1970s, when a three-rail steel parapet was adopted to meet the standard set by the UK's Department of Transport. The parapet is light but when vehicles hit it, it absorbs and deflects part of the impact energy to hold the vehicles.

In 1975, an aluminium alternative was introduced. While it performs much the same as its steel counterpart, its cheaper to maintain since it does not rust. The material is also lighter and can be moulded into better-looking designs.

The department developed a concrete P1 parapet with a metal top rail in 1981. When struck by a vehicle, it will dissipate part of the impace energy and redirect it back to the highway.

P1 design refined four years ago

The P1 parapet design was refined in 2000 to enhance its containment capability, and a new generation of the three-rail parapet was developed with strengthened post-to-rail and post-to-base plate connections, bolted splicing between rails, and re-oriented top rails.

Computer simulation has proved the refined design can stopt a 1.5-tonne vehicle at 113km per hour but at a higher impact angle of 40 degrees, 20 degrees more than the first generation.

Apart from standard barrier designs, new designs have been developed to meet special needs for individual projects.

Special barriers tailor-made for long-span bridges

A special type of P4 parapet was introduced for the Tsing Ma Bridge, Kap Shui Mun Bridge and Ting Kau Bridge. It has five high-tension steel strands anchored on strong metal posts.

It is light, and particularly suitable for long-span bridges.

In view of the Tuen Mun Road incident in July last year (see Full Story), in consultation with the Independent Expert Panel the department immediately adopted an interim standard for stopping heavy vehicles and buses at locations that have features similar to those at the site of the crash.

The features are:

* high posted speed limit, traffic volume, bus and commercial vehicle usage;

* located near an expressway entrance with weaving traffic;

* located high above ground; and

* having residents or other special features such as the sea beneath the road structure.