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Mobility & Transport - Road Safety

Designing to keep injury severity low

Designing to keep injury severity low


Designing to keep injury severity low

A large body of research shows the speeds and vehicle design criteria needed to keep injury severity within tolerable levels for car occupants in car to car collisions and for pedestrians impacted by cars (ref vehicle safety and speed chapters). Similar criteria could be used to align the protective design of a road to counter impacts between cars and roadside objects, or to limit the possibility of higher speed vehicle to vehicle impacts through median and junction design [85][52]. There is not yet sufficient knowledge to define speeds and infrastructure design that would result in tolerable injury severity for motorized two-wheelers or from impacts between cars and heavy good vehicles.

The Swedish concept of "safe speed"

Tingvall and Haworth (1999) proposed that the driver/vehicle/road system should operate in a way that, in the event of an impact, forces are not exerted on vehicle occupants or other road users which are likely to lead to a fatality. Thus, where pedestrians are present, vehicle speeds should be no higher than 30km/h. Where vehicle to vehicle impacts occur they should be at speeds below the impact speeds at which cars can be shown (through the European New Car Assessment Programme) to safeguard occupant life. These principles typically lead to the following "safe speeds".

Road type/traffic situation Safe speed (km/h)
Roads with potential conflicts between cars and unprotected road users 30
Junctions with potential side impacts between cars 50
Roads with potential head-on conflicts between cars 70
Roads where head-on and side impacts with other road users are impossible > 100

In Sweden, the concept of a safe speed has been adopted as a basis for considering appropriate speed limits. Ratings are being developed through the European Road Assessment Programme showing how well the road is designed to ensure forces involved in impact with road infrastructure also keep within the same thresholds, and these are being used in Sweden to indicate appropriate speed limits for roads with different ratings.

The Dutch vision of Sustainable safety.

This policy was launched at the beginning of the 1990s and accepted as a formal part of Dutch policies in the mid 1990s.

"The Sustainable Safe vision is based on two leading ideas: how to prevent human errors as far as possible, and how to ensure that the crash conditions are such that the human tolerance is not exceeded and severe injury is practically excluded. The starting point of 'sustainable safety' was to drastically reduce the probability of crashes in advance through safety conscious planning and design. Where traffic crashes still occur, the process that determines the severity of these crashes should be influenced, so that serious injury is virtually excluded. Within sustainable safety, man is the reference standard (human error and human tolerance). A sustainable safe traffic system has an infrastructure that is adapted to the capabilities and limitations of humans through proper planning and road design, has vehicles that are equipped to simplify the driving task and offer protection to the vulnerable human being (crash protection), and finally, has road users that are properly educated and informed, and whose driving behaviour is regularly controlled. The key-issue of 'sustainable safety' is that it has a preventative rather than a curative (reactive) nature."

The updated Dutch Sustainable Safety vision presents the requirements with regard to maximum speeds in different traffic situations that follow the safe speeds proposed by Tingvall and Haworth.

Source: Wegman et al (2005), Wegman & Aarts, 2005 (page 14; translated from Dutch)

Measures to minimize injury severity include creation of clear zones alongside roads (see section 2), use of passively safe materials or shielding of objects where impact would involve higher energy levels than those leading to tolerable injury levels [22].

Individual and collective risk

Design standards and remedial programmes need to consider both risk to the individual driver and collective or societal risk (i.e. total risk to all road users). The former is implicit in many design standards (eg by stating the width of roadside clear zone below which objects must be protected). The latter is reflected in cost benefit or cost effective assessments [70] which are used in deciding how to use the available budget to provide the highest safety return, in economic terms. An example of a process by which network wide accident costs can be used to identify roads on which potential improvements would be worthwhile is given in [8]. The EuroRAP programme provides a basis by which roads having high numbers of fatal and serious accidents compared with expected numbers for that road type can be identified within national rural road networks.