Driver Knowledge Tests

Tips for Keeping Under the Speed Limit

It wasn’t that long ago that a human’s maximum speed was how fast he could run. Then he tamed horses, then the wind in sailboats and then horsepower in engines. Now, we can travel at speeds well in excess of our abilities to control the results, so until self-driving autonomous cars are readily available and proven safe, road authorities have to restrict our speed to what is both sensible and practical for the condition of our roads. If we exceed those limits, we get fined (if caught).

Here in Australia we have posted speed limits usually rounded to 10kph apart from in South Australia where they have a 25kph limit. The maximum speed limit you will encounter in Australia is 130kph in Northern Territory, but the majority of highways are 100 or 110kph, depending on which state you are in. In urban areas (usually defined by the presence of street lights), the maximum limit is 50kph everywhere except Northern Territory which is 60kph. In school zones the maximum is 40kph where the limit would usually be 50kph, however Queensland, Tasmania, Victoria and Western Australia have different limits where the usual speed limit for the road might usually be 60 or 80kph.

How do you keep under the speed limit?

Habitual glancing at the speedo

You’ll be scanning the road ahead all the time, but periodically you need to glance down at your speedometer to check you are not drifting from the speed. Around once a minute is fine. This doesn’t only apply to going over the limit, it actually applies to maintaining a consistent speed so that you don’t aggravate other drivers. If you keep slowing down and speeding up you make it difficult for following drivers to predict what you are doing to do, or pass you.

Bear in mind that your speedometer will over-read, sometimes by up to 10kph. That is, your speedo may show 53kph, but your car might be doing 53, or 49 or 46 – it depends on several factors including how accurate the speedometer is and how worn your tyres are. If you want to know your speed accurately you will need to use a GPS (see Satellite Navigation, below).

The pitch (note) of your engine

If you drive a manual or regular automatic vehicle then a certain speed in a specific gear will produce a pitch that will not vary that much. It’s related to the number of revolutions per minute that the engine is doing and is mixed with road and tyre noise. The more time you spend in your car the more attuned you get to this pitch. Our ears are extremely sensitive to changes in pitch.

If you’re driving a very new and very quiet car, this technique will be difficult to employ. The other factor that renders it useless is a CVT gearbox. These gearboxes continuously vary the engine revs to maintain the required amount of torque, therefore one particular rev range does not indicate a specific speed.

Cruise control

Most new cars come with cruise control. It allows you to set a certain speed and have the car (attempt to) maintain that speed. If you reach an uphill section the car will apply more power and if you are going downhill it will apply less or no power. Some systems brake the car as well either by using the brakes or changing down a gear or two if it gains speed over a predefined margin above your set speed.

If you brake to slow the car down it cancels the cruise control. You can always press RES to resume at the speed you had previously set. Bear in mind that cruise control doesn’t know that corners are coming up and it will try to maintain the same speed through corners. Because it is our natural reaction to slow slightly around corners (and the car does this automatically if no extra throttle is applied due to energy lost changing direction), it can seem like you are accelerating around the corner.

Speed limiters

Less common are speed limiters. They are usually a mode in the cruise control system. When you set the limiter you will not be able to exceed the speed regardless of how far you push the throttle down, unless you trigger an override. This is usually the final inch or so travel of the throttle and it requires more pressure to push the throttle through this. It means that you do have acceleration available in an emergency without first having to cancel the limiter.

Many heavy vehicles and passenger vehicles are fitted with speed limiters.

Overspeed warnings

Some cars let you set and audible and/or visual warning if you exceed a certain speed.

Satellite navigation

Some satellite navigation systems will give you a speed warning if you exceed a certain amount above the limit for that area. As sat nav systems know the local speed limits, and can very accurately calculate your speed using GPS, they can warn you effectively. GPS speed calculations are usually within 1kph.

The safest ways to keep under the speed limit are ones that don’t require you to take your eyes off the road. If you’re driving an unfamiliar car or you don’t have any technology to help you, aim to drive just below the limit and that’ll give you a little buffer if your speed creeps up slightly.

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

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Posted in Advice, Car, Heavy Vehicle, Motorbike