Driver Knowledge Tests

Excessive speed vs inappropriate speed vs safe speed

When it comes to choosing your speed on the road, there are three definitions that are useful to know:

Excessive speed: any speed above the speed limit, e.g. 120km/h on a road with a limit of 100km/h

Inappropriate speed: any speed under the speed limit, but too fast for the conditions or the driver’s skill. For example, driving at 95km/h on a motorway in torrential rain when visibility is less than 50m and there are large patches of standing water

Safe speed: a speed at which both the vehicle and driver are capable of handling typical scenarios that would happen on that stretch of road without posing a danger to other road users.

It’s best summed up in this diagram:

Safe speed isn’t one fixed speed. The inappropriate speed band can grow as conditions deteriorate

How do you know what is a ‘safe speed’?

The safe speed is determined by the characteristics of the road, the capabilities of your vehicle, the weather, how far you can see ahead and whether there are any other road users.

narrow rural road
While the conditions are perfect, this road is narrow, there’s a corner ahead and not much in the way of run-off areas. The safe speed decreases as you approach the corner. Once the corner opens up again and you can see further ahead, the safe speed will increase again. Ask yourself if you could stop if a vehicle came around that corner towards you at an equal speed.
Good visibility with light traffic. The safe speed is likely to be the speed limit, if your vehicle is capable of it
A horse on a narrow road. The safe speed will be very low until you pass them
Unsealed road leading up to a blind crest – the safe speed will be low
foggy road
There’s a car visible in this image – look closely – it’s less than 150m away and the speed limit is 100km/h. The safe speed will be reduced so that you have time to stop if an obstacle appears out of the gloom
Parked cars in an urban area. Dappled light. Is 50km/h an appropriate speed? You’ll need to make that call

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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