After being overlooked for many years, secondary infrastructure safety is today, in many countries, receiving the research and investments it deserves, given the stakes involved.
A large percentage of accidents are caused or compounded by the presence of roadside obstacles or faulty road design. According to a report by the French Association Prévention Routière, the presence of a fixed obstacle is responsible (either partly or entirely) for 36.5% of the deaths in road accidents. This is higher than the percentage caused by alcohol (30.8% of road fatalities), failure to wear a seat belt (22%), or failure to maintain the safety distance (6.6%). Another study carried out by CERTU shows that fixed obstacles in urban areas are the cause of one out of every eight accidents, and one out of every three road deaths.
This is why many road operators, as well as the authorities, have made secondary infrastructure safety a priority, focusing on the notion of "forgiving road", i.e. a road that gives motorists enough space on the side to get back on course, and which eliminates dangerous obstacles close to the road.
Eliminate or neutralize obstacles
A collision at a speed of 50km/h is as if the vehicle fell from a height of 10 meters; at 70km/h, it is like a fall from a height of 20 meters. The deceleration force is such that it causes many internal parts of the human body to detach or tear away: the vehicle and the body's outer shell come to a sudden halt, whereas the organs continue their course inside the body and hit the body's walls or become detached from their supports. The chances of survival are slim in such a case. According to CERTU, a head-on collision with an obstacle can be fatal from a speed of 65km/h. And a side impact from a speed of 35km/h. Advances in passive infrastructure safety accordingly rely heavily on meticulous field work to deal with a maximum of potentially hazardous obstacles.
Identify and remove obstacles
The list of "aggressive" obstacles capable of hitting a vehicle that leaves the road is very long: electricity and telephone poles, lighting posts and masts, sign supports, constructions, trees, bus shelters, ditches and slopes, anti-noise screens, rocks, culvert ends, parked vehicles and so on.
According to Certu, the majority of fatal accidents involve collisions with three types of obstacles: buildings and walls (27% of those killed in car/obstacle collisions); sign supports, poles, light posts (19% of those killed); trees (17%).
One solution is to remove these obstacles, but, depending on the type of object concerned (tree, wall, pole, etc.) and the density of the surroundings (urban, periurban, open countryside, etc.), this may not be very easy. In the earlier phase of designing roads, it is also important to take a comprehensive approach, looking ahead and taking steps to keep potential obstacles at a distance from the road, in coordination with the urban planners and operators working near or within the road network (electricity, telecoms, public works, etc.).
In any case, these initiatives call for a detailed map of the environment to be made safe.
This consists in mitigating or preventing impact with potentially hazardous obstacles by surrounding them with a restraint system. Most of the time, this involves installing a guardrail, the effectiveness of which depends mainly on the position and the material.
Make obstacles passively safe
This innovative concept consists in designing obstacles to detach or crumple if they are hit by a vehicle, to reduce the deceleration force generated by the impact. Detachable sign posts are being developed in the United States and Europe, which come apart and are ejected in the event of a collision. The technology recently appeared in France and is being tested by the Allier département under the name of "Systeject".
Specialist manufacturers are also bringing out supports, masts and poles built in materials that fold and crumple, absorbing a maximum of kinetic energy in the event of a collision to limit its impact on vehicle occupants.
Codify passive infrastructure safety
Advances in passive infrastructure safety are also the result of tighter regulations and harmonized road standards. The countries with the most advanced passive infrastructure safety have adopted regulations that codify three vital secondary-safety measures for widespread use:
- a recovery zone (including the road shoulders) that is completely free of any obstacles, in which drivers can get back on course if they run off the road
- a safety zone, in which hazardous obstacles have been either removed or neutralized by restraint measures.
- a median area (for two-way roads), equipped with restraint systems to prevent cars from crossing it, and designed to cushion run-offs without making the vehicle bounce back onto the road.