Speed limit enforcement is a major focus by police and traffic agencies globally. There are a number of reasons for this: faster speeds mean you are less likely to survive an accident and (in general) more likely to have one, and speed is an easy metric to detect and prosecute against.
Keeping under the limit
Knowing your car’s limitations
Many car’s odometers/speedometers will over-read by up to 5mph. That means that if it’s indicated you are doing 60mph on your speedo, you might only be doing 57mph. Add to this the effect of your tyres being worn. As your tyres wear, the circumference diminishes and therefore the wheels have to turn more times to travel the same distance. Your speedo does not factor this in. Both of these work in your favour, meaning you will actually be travelling at a slower speed than is indicated.
Vehicle manufacturers do this deliberately because they don’t want thousands of lawsuits from drivers who thought they were travelling at or under the speed limit, but subsequently get a speeding ticket.
Where you can strike problems is if you put on a set of tyres that has a larger circumference than the original specification. This will mean that your actual speed might be more than the indicated speed.
Almost all new cars come with cruise control nowadays. Cruise control allows you to set your car at a certain speed and the vehicle will do its best to maintain that speed, applying more power on uphill stretches and reducing power on downhill sections.
You do need to be cautious with cruise control, though. It will never maintain an absolutely consistent speed because it doesn’t react fast enough. On uphill sections you might drop 5mph before it kicks in and adds power to bring you back up to speed. On downhill sections most cars will gain speed if the gradient is sufficiently steep. A few modern cars have braking systems built into their cruise control either by applying the brakes or changing down one or more gears, but almost all older vehicles don’t have this.
Once you brake to slow your car on a downhill section you will cancel the cruise control. You can use the RES (resume) function to reactivate it at the previously set speed.
Advanced cruise control systems come with a speed limiter function. Setting the speed limiter means that the maximum speed the vehicle will go is the limit you set it at. Obviously, the same issues can apply as with cruise control, but mainly on downhill roads.
Speed limiters almost always have a ‘switch’ at the bottom of the accelerator pedal travel which overrides the limiter. This is for safety (sometimes you need to accelerate to avoid an object, and when you press the accelerator to the floor it will override the limiter).
Some vehicles (particularly heavy vehicles) are fitted as standard with a speed limiter. You can find Highway Code questions on speeds limits and other road rules here.
Some cars allow you to set a speed limit warning or an ‘overspeed’ warning. This is usually an audible warning that sounds when you exceed a certain speed limit. Japanese import cars from the 1990s often have one built in that is a continual bell or chime that is set to around 115kph (around 70mph); modern cars will just give you one or two warnings.
Many satellite navigation systems will give you a much more accurate speed reading than your vehicle’s speedometer. Rather than using the wheels to determine the speed (which we’ve already discovered can be flawed because of tyre wear), it uses the GPS signal to detect how fast you are going. They are often accurate to within 1mph.
Newer satellite navigation systems can be set to warn you when you are entering a 30mph zone from a 60mph zone, or to change colour or provide an audible warning if it thinks you are exceeding the local speed limit.
Knowing your speed by your engine’s pitch is something you get used to. This technique works only on vehicles where you can hear the engine’s tone, and only with automatic or manual gearboxes, not CVT. With CVT, the revs rise and fall as the engine needs more torque, not necessarily in relation to speed.
The more experience you have in your car, the more likely you are to be able to detect your engine’s pitch and know roughly how fast you are going. Here’s a video about how CVT works for all your tech-heads out there. If you don’t reckon you’ll be awake at the end of the 5 minutes, scroll down a bit further to finish reading the article.
Glancing at the speedometer
Getting in the habit of glancing at your speedometer every minute helps you stay under the limit.
To be safe, it’s best to use systems that don’t require you to take your eyes off the road. Cruise control and speed limiters are excellent like this. If you don’t have those systems, aim to travel at just under the speed limit so that you have a buffer; if you accidentally speed up a few miles per hour, you are not going to be ticketed for speeding (unless the prevailing conditions mean that you are driving dangerously).
Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.