Many drivers think that the speed they travel is an 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 limit without crashing, so
it's easy to think that it's safe, but it's not.
Road authority sets 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 kilometres per hour 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 kilometres an hour faster is 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 will 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 breaks
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 kilometres per hour
and the other at 65 kilometres per hour.
Both see and react to a hazard on the road at the same time.
At the point where the car travelling 60 kilometres an hour has stopped, the car travelling 65
kilometres per hour is still travelling at over 30 kilometres per hour.
If this incident involved a pedestrian, an impact at over 30 kilometres per hour 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.
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 kilometres per hour, the amount of energy
in the crash increases by 18%.
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 kilometres per hour, there were
over 20% 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.
