The science of low-level speeding

Many drivers think that the speed they travel isn’t important, and travelling just above the speed limit isn’t dangerous or risky. However, even small increases in speed are an important factor in both the occurrence of road crashes and the severity of crashes.

While most drivers know the dangers of high-level speeding, many of us don’t understand that a few kilometres over the limit can also be dangerous. After all, many people have travelled a little over the speed level without crashing. So it’s easy to think that it’s safe. But it’s not.

Road authorities set speed limits at a level to limit the risk of a crash occurring. However travelling at any speed involves some risk. Speeding above that limit increases the risk further, even if only travelling slightly above. This is referred to as low-level speeding. 

But a small increase in speed doesn’t affect my chance of a crash … does it?

Because a lot of people speed a little bit and don’t have a crash, they may think that the chance of having a crash isn’t increased. But road safety researchers have found that travelling as little as 5 km/h faster will in fact double your chance of a casualty crash. So travelling just a little faster can have disastrous consequences on the road for everyone, especially pedestrians and cyclists. Studies have also shown that the increase in the chances of having a crash by travelling as little as 5 km/h faster is the equivalent of having a blood alcohol level of 0.05. So if you look at the whole population, if everyone doubles their chance of a casualty crash by low-level speeding, it would double the road toll.

Of course, people who drive at dangerously high speeds have a much higher chance of a serious crash, and obviously pose a much bigger risk for everyone. 

But why does a small increase in speed increase my chances of a crash?

Small increases in speed have a surprisingly large effect on two main areas of crash avoidance: the reaction time of a driver, and the braking distance to stop. Even with small increases in speed, when driving slightly faster than the speed limit and encountering an emergency, before the driver has even reacted and started to press the brake pedal, the car will be closer to an impact. And obviously, the faster a car is travelling, the longer it will take to stop once the brakes have been applied, even with small increases in speed. If you combine reaction time and braking distance together, the effect can be surprisingly large.

Let’s look at two cars travelling next to each other. One at 60 km/h and the other at 65 km/h. Both see and react to a hazard on the road at the same time. At the point where the car travelling 60 km/h has stopped, the car travelling 65 km/h is still travelling at over 30 km/h. If this incident involved a pedestrian, an impact of over 30 km/h has a very high risk of serious injuries. Or even death.

So how does a small increase in speed affect the severity of a crash?

Sometimes the laws of physics act in ways you don’t expect. The energy of a crash, and hence the energy experienced by someone involved in a crash, is calculated by this equation: E = 1/2m². In the equation, m equals the mass of the car, and v is the speed of the car. However the speed of the car is squared. This means that the energy increases dramatically for every increase in speed. For example, by just increasing the speed by 5 km/h, the amount of energy in the crash increases by 18 per cent. 

So what happens if we all slow down?

Road safety researchers have found that despite lower speed limits, the average journey time barely changes at all. After reducing South Australia’s urban speed limits by 10 km/h there were over 20 per cent fewer casualty crashes. So if we all do our bit and slow down, we can make the roads safer for everyone.

Well, that’s a saving worth making.