Articles in Taxonomy

About Traffic and safety

Each road traffic “participant” is liable to be involved in an accident. Various factors play a role in this risk. Accidents are very rarely the result of a single unsafe action; they generally imply a chain of circumstances and events that may lead to an accident-prone situation. The following diagram (known as the Swiss Cheese Model) suggests that various types of error (“latent errors” and “unsafe actions”) must be present at the same time to cause an accident.

Diagram illustrating the causation of an accident by latent errors and unsafe actions
(Reason, J. (1990). Human Error. Cambridge, Cambridge University Press
Reproduced in CROW (2009). Road Safety Manual. CROW Record 26)

James Reason hypothesizes that most accidents can be traced to one or more of four levels of failure: organizational influences, unsafe supervision, preconditions for unsafe acts, and the unsafe acts themselves. In this model, an organization's defences against failure are modelled as a series of barriers, with individual weaknesses in individual parts of the system, and are continually varying in size and position. The system as a whole produces failures when all individual barrier weaknesses align, permitting "a trajectory of accident opportunity", so that a hazard passes through all of the holes in all of the defences, leading to a failure.

So the causes of road accidents are multiple and often find their source in a cocktail of events that can be related to one of the three components of the safety triangle – the user, his vehicle (or his travel mode), and the infrastructure he uses – or a combination thereof. According to several studies1 only 2 to 3 % of all accidents are caused by the road environment or the road infrastructure (alone). However, when combined with the other two factors (the user and his vehicle) the infrastructure plays a role in 18 to 34 % of accidents.

This finding obviously argues in favour of actively implicating the infrastructure in efforts to improve the safety of the road transport system, all the more as (intelligent) infrastructural measures often rapidly have a durable effect.

In concrete terms, the layout of a road can contribute to better safety in various ways:

  • as an active safety measure before any accident occurs, i.e. to prevent it from happening. This can be achieved:
    • by better road planning, among other things by using a functional classification approach that consists in recognizing the different road functions and making them compatible with the “shape” and “use” of the road;
    • by limiting conflicts at intersections (e.g. by ensuring good mutual visibility between road users at signal-controlled junctions and on roundabouts) but also on links (by observing the criteria for homogeneity in mass, speed, and direction of travel);
    • by improving road legibility, so that the road user cannot be surprised by an unexpected situation eliciting an inappropriate response;
    • by suggesting appropriate driving behaviour – either through adequate regulations and signing or through a geometry and environment that do not encourage speeding, or even through specific measures (e.g. rumble strips for path departure warning, dynamic signing);
  • as a passive safety measure after an accident has occurred, to limit the consequences. This can be achieved by safety rails or by treating roadside obstacles or the roadsides themselves – in short, by any measure which makes it possible to offer a road that “forgives” driving errors.

[1] Treat, J. & al (1979). Tri-Level Study of the Causes of Traffic Accidents, 1979, Washington DC – Cited in PIARC (2003). “Road safety manual”.
Hillier, P. 2002. “Highways liability and the investigation of road traffic accidents”. Paper at: IPWEA NSW Division Annual Conference 2002. TRL.