Publication

Criteria for roadside safety of motorways and express roads

A proposal for road authorities in the framework of the European research project SAFESTAR, Workpackage 1.2

Author(s)

Schoon, Ing. C.C.

Year

1999

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To protect occupants of vehicles that leave the road from serious injuries, safe roadsides and medians are important. This report describes the way to make safe roadsides by means of obstacle-free zones, slopes, frangible poles, crash cushions and safety barriers. The research performed here aims to define criteria for places where safety devices are necessary. The research was carried out by means of a literature study including the national European standards. Furthermore, results from a questionnaire which was send to European institutes and ministries are described. This questionnaire contained for instance questions about national standards and/or criteria for use of safety barriers, and about accident data on motorways and express roads where safety barriers were involved. Data from European countries, but also from the United States were analysed to prepare a proposal for standards and strategies for EU-countries. The first issue of the report deals with the desirable width for the obstacle-free zone. Figures are presented about the only European research carried out in the Netherlands in the 1980's. Figures for motorways, single-lane highways and local single-lanes are given. Based on the questionnaires, distances of obstacle-free zones from other European countries are mentioned. The second issue is a shoulder with safe slopes. Figures from the United States and European countries are discussed. The figures from the Netherlands are based on mathematical simulations and twelve full-scale tests on slopes with two gradients. If fixed objects are made to yield, the third issue, they can be placed in an obstacle-free zone without safety barriers. Different solutions are mentioned, such as slip base, plastic hinges, fracture elements or a combination of these. The fourth issue deals with crash cushions. If solitary rigid obstacles along a shoulder cannot be removed, they can be shielded with a crash cushion. Crash cushions are applied on motorways in mainly two different situations: in pointed areas at exits (often at the beginning of a safety barrier) and on shoulders to shield single objects. If crash cushions have been hit head-on, the vehicle usually remains within the shoulder so that it forms no danger for other traffic. In the case of a side impact, most types of crash cushions function like a safety barrier. Several European countries have their own, different types of crash cushions. In the concept of a safe road side (shoulders and medians), protection with safety barriers is the least safe solution (the last issue). An effectively functioning safety barrier prevents a vehicle from leaving the roadway and striking a fixed object or terrain feature that is considered more hazardous than the barrier itself. But a collision with a safety barrier is never free from the risk of injuries for the occupants of the colliding vehicle, nor is it for other road users. Requirements, CEN standards, containment levels, differences between steel and concrete barriers, and Dutch experiences with mathematical simulations are described. Proposals for motorway standards and strategies for EU countries are discussed. There are safety reasons for favouring wide obstacle-free zones. Based on information from many European countries a minimum width of 9metres is recommended. Also is recommended to carry out accident investigations in different European countries to collect more data in order to take a more well-founded decision for the European situation. Slopes may be a part of an obstacle-free zone if vehicular manoeuvres are possible. This is the case with a gradient of at least 1:5 for high slopes (>5m) and 1:6 for lower slopes (<2m). Only fixed roadside objects can be located within an obstacle-free zone, if their support poles are frangible. If solitary rigid obstacles can not be relocated, protecting them with a crash cushion is the solution. In the report a decision model is described for determining the choice for shoulders and the median: obstacle-free or safety barriers. If a decision is made for a low containment level, steel barriers are in favour if only the installation costs are calculated. Taking into account other aspects, it depends on the local circumstances which type of barrier is to be preferred. Differences between countries are too great for a general statement. Also for express roads and single carriageways recommendations are given for the width of obstacle-free zones and the necessity for safety barriers. The single carriageway roads are in fact at the heart of the problem of obstacle accidents in Europe. There are many of such accidents because there are so many old roads. Unfortunately, accidents with ‘natural' obstacles such as trees, are widely spread so that dealing with them cannot be targeted at concentrations of dangerous locations. Apart from the erection of safety barriers, the driving speeds will have to be drastically reduced to increase the safety of such roads. Subsequently, this means that the road's function will be changed. A procedure has been described for identifying the locations and establishing priorities for those most requiring the placing of safety barriers. As a cost-benefit analysis, the ‘one million ECU test' of the European Commission can be applied. A strategy developed in America to deal with these problems, appears to be applicable also in Europe. It concerns for instance better accident monitoring, research, more attention (education, spreading information, good management), and greater budgets

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