András Várhelyi (Lund University) explains the concept of safe speed limits, which are a crucial part of the Safe System approach to road safety. Safe speed limits are set based on well-established tolerance levels of the human body in the most prone crash type occurring at the location in question.
Since collision energy increases proportionally to the square of impact speed, impact speed is the most important factor in determining the severity of injuries in a crash. Impact speed originates from the general speed level at the accident site, hence the pivotal measure to regulate the speed level is to apply as well as enforce appropriate speed limits.
Differentiated speed limits at the heart of road safety
Based on research findings regarding the tolerance levels of the human body to endure a physical impact in a crash, the following differentiated speed limits constitute the basis of the Safe System approach in road safety management:
Lund University, Sweden
In areas where vulnerable road users and motorised vehicles share the same road infrastructure and collision between motorised vehicles and pedestrians/bicyclists might occur.
On urban roads where motorised traffic is separated form vulnerable road users and at intersections where side collision between motorised vehicles might occur.
On rural road sections with oncoming motorised traffic where frontal collisions might occur.
On rural road sections with middle barrier separating the opposing directions of motorised traffic, and thereby preventing frontal collisions. This type of road is called a “2+1 road” with an overtaking lane alternating between the two directions.
on motorways with full separation of oncoming, turning and crossing traffic.
Safe speed limits backed up by research
The Safe System speed limits for the various parts of the road infrastructure are well sustained by findings of numerous research activities comprising field observations, in-depth accident analyses and laboratory crash tests.
Spot speed measurements of free flow vehicles on accident sites, where a motorised vehicle hit a pedestrian/bicyclist showed that crashes with fatalities and severe injuries occurred on sites where the mean speed of vehicles passing in the travel direction of the accident-involved vehicle was above 30 km/h. On accident sites, where the mean speed was below 30 km/h the outcome of the crashes were slight injuries.
In-depth studies of accidents involving cars hitting pedestrians revealed that pedestrians’ chance to survive a collision with a car decreased significantly at impact speeds above 30 km/h.
Crash tests with dummies resembling the human body in passenger cars at various levels of impact speed revealed that a human has extremely small chances to survive in a frontal crash at impact speeds over 70 km/h – even in the latest brand of passenger car. Also, crash tests resembling side collisions revealed that the chance of a human to survive a side impact in a car at impact speeds over 50 km/h is even smaller, since crash protection in cars is mainly constructed to mitigate impact forces in frontal collisions. These findings are under the condition that the dummies were belted and protecting devices in the car, such as crash cushions were functioning during the crash tests.
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