Speed limit of 30 km/h: slow, but safe
21.03.2023 Implementing a speed limit of 30 km/h in residential neighbourhoods to make streets safer for pedestrians and cyclists has long been debated. Just how to go about doing it remains a divisive issue. An experiment at BFH demonstrates very clearly that 30 km/h zones improve pedestrian safety in instances of collision.
The Federal Roads Office (FEDRO) wrote in 2003: ‘Reducing speed limits to 30 km/h will make roads more attractive and safer for the most vulnerable road users, such as pedestrians and cyclists. This contributes to an improved quality of life.’ However obvious that might sound, the issue remains contentious. As per a 2022 motion in the Grand Council of Bern, 30 km/h zones have been decried as an environmental burden, a hindrance to traffic and insignificant in terms of improving safety.
Indeed, it is important to consider where 30 km/h zones truly make sense. And even with more and more EVs on the roads these days, it is worth asking whether these zones lead to increased greenhouse gas emissions. Nevertheless, the influence of lower speed limits on pedestrian safety is quite clear.
Keeping pedestrians safe
This is precisely what Raphael Murri and his team found in running their experiment. This dedicated BFH professor joined forces with the Dynamic Test Center and the Accident Mechanics work group to put it to the test.
In a series of crash tests, he and his team compared how much collisions at 30 km/h differ from those at 50 km/h. In this particular case, it was a small car colliding with a six-year-old child dummy. The differences were striking. After impact at 30 km/h, the vehicle showed hardly any traces of the collision. The measurement data on the dummy show that relatively little force was involved, and the risk of injury to a human body would have been similarly low.
Things looked different when the test was repeated at 50 km/h. The child dummy left clear marks on the vehicle, because at this speed, the body could no longer slide up onto the bonnet or windscreen, but clung to the front. The dummy’s head then whipped against the vehicle at up to 180 times the acceleration due to gravity.
High risk of injury at 50 km/h
The acceleration sensors attached to the dummy proved initial impressions: upon impact with the vehicle (primary impact), the load at 50 km/h was more than 11 times higher than at 30 km/h and exceeded the biomechanical threshold for severe head injury (Head Injury Criterion). At 50 km/h, the secondary impact (that is, when the dummy hit the street) was another three times higher.
Raphael Murri’s verdict is clear: ‘The two impact tests show that, for the child dummy, the chances of sustaining only slight injuries in collisions at 30 km/h are very good, whether that’s the primary impact with the front of the vehicle or the secondary impact with the road. In the case of impact at 50 km/h, on the other hand, the risk of fatal injury would have to be expected, especially in the case of a primary impact against the front of the vehicle.’
Speeding up the process?
The findings speak for themselves, especially since a vehicle’s stopping distance is half as long at 30 km/h. This reduces the risk of collisions with pedestrians in the first place. Scientific findings such as these provide a basis for social discourse on the topic of non-motorised traffic. These efforts will continue, in particular following the Federal Council’s work in 2022 to simplify the process of instituting 30 km/h zones.
Prof Raphael Murri: Lecturer in Automotive Engineering and Safety