Speed limits based on safety

The faster you drive, the more difficult it is to avoid a serious collision. Driving faster means less time available to slow down or stop. In my all-time favourite road safety video, Professor Ian Johnston from Monash University shows how a little more speed can have a big impact. It’s a must-watch.

The faster you drive, the more likely you are to be killed or seriously injured in a collision. A study by Nilsson (2004) demonstrated that a 5% increase in average speed typically leads to a 10% increase in the number of injury crashes and, worse still, to a 20% increase in the number of fatal crashes.

Research by Wramborg (2005) found that, as speed increases above 30km/h (19mph), the risk of a pedestrian being killed in a collision increases sharply. Similarly, as speed rises above 70km/h (43mph), the risk of a vehicle occupant being killed in a head-on crash increases strongly. And, as speed rises above 50km/h (31mph), the risk of a vehicle occupant being killed in a side impact crash (at T-junctions, for example) increases dramatically.

Because of this, setting speed limits should be based on safety and this requires a change to traditional methods. It is a crucial part of achieving our long-term goal of zero road deaths.

Setting a speed limit based on safety means making sure that road users can survive the types of crashes which could occur on a route. So, where there are pedestrians and cyclists present, speeds need to be below 20mph. Where there are T-junctions, speeds need to be below 30mph. And, where there are no physical barriers between vehicles moving in opposite directions, speeds need to be below 40mph (fortunately, vehicle design has improved since 2005 so some ‘safe speeds’ might be a little higher – updating this research is necessary).

We also need to make sure that speed limits are credible among drivers, because we want drivers to willingly comply with them. We can design intuitive road environments so that speed limits appear reasonable to drivers, making it more likely that they will be observed. Intelligent Speed Assistance (ISA) should also help achieve even higher levels of compliance.

There is no doubt that this is a tremendous challenge which would radically change how we approach speed, but it is the only way we will reach zero road deaths.