How does cruise control work? Is it safe?

Carlton Boyce / 02 February 2016

Could cruise control really improve your driving experience? Is using a device to limit your speed automatically safe?



Back when I could finally afford a car with electric windows, I was too arrogant to engage the cruise control that was also fitted. 

I, with the benefit of a whole two years’ driving experience on the roads, knew that I could do a better job of maintaining a constant speed.

Besides, I was a 'Driver', which meant driving, not handing control to a sissy system that deprived me of an opportunity to prove my prowess. (I, of course, felt the same about automatic gearboxes.) 

I was almost right about the cruise control. My arrogance had inadvertently stumbled upon a truth; cruise control was a bit rubbish, back in the day. It isn’t now, not by a long chalk.

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How cruise control works

My second job after leaving school was working as a sales assistant in Halfords. 

This gave me an opportunity to talk car-stuff with car-guys (girls that were into cars didn’t talk to a 17-year-old know-it-all like me), and one of the items we sold was a DIY cruise control kit. 

It worked, if memory serves me right, by using a cable to hold the throttle in a pre-set position. There was a crude automatic speed control built-in, so if you went up a hill for instance, it would try and maintain a constant velocity but it didn’t work very well. 

So, with my limited experience and frame-of-reference, not using my car’s cruise control in later years made some sort of sense.

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Cruise control has changed

This prejudice stayed with me for 20 years, despite cars becoming significantly more sophisticated; while the principle remains the same, the way a modern cruise control system achieves the same end is very different. 

While a constant speed can still be maintained, your car’s Electronic Control Unit (or ECU) will now do this regardless of the prevailing traffic conditions and geography. So, if you come up behind a slower car your Active Cruise Control-equipped car will brake to an appropriate speed and then maintain that distance. 

If the slower car moves out of the way, your car will then accelerate back up to your chosen cruising speed.

I tried it out on a 300-mile cross-country journey in a Subaru Outback and was stunned at how well it managed with some quite heavy traffic. 

It would leave a gap that was entirely consistent with the Highway Code’s recommendations, and when another driver cut into that space (a two-second gap is a much greater space than most drivers would normally leave), the Subaru would brake and slow down to maintain it.

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Its limitations

Active cruise control has limitations, of course. 

While I was merrily testing the Subaru’s ability to maintain a constant distance from the car in front, I failed to notice that the road curved sharply on the approach to a roundabout. 

The car in front braked, which meant that my car automatically did so too. However, the car in front then disappeared around the bend, which my Subaru took to mean it could accelerate smartly up to its previous cruising speed of 70mph, despite the looming presence of the roundabout.

Luckily, my foot was hovering over the brake pedal, so no harm was done but it was a timely reminder that software programming is a very literal science. 

(Joke: Wife to software programmer husband: “Could you bring a loaf of bread home, and if they’ve got eggs, buy half-a-dozen please?” Husband later brings home six loaves of bread and no eggs. When asked why, he said: “Well, you said to buy bread but if they’ve got eggs, to buy six…”)

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Tesla, and the self-driving car

If you know anything about self-parking cars, you’ll know that active cruise control is another significant step towards a fully autonomous car. 

After all, if you can detect the presence of another vehicle in front of you and can then adjust your speed accordingly, you have the beginnings of a car that will drive itself.

Well, such a car already exists and you can buy it in the UK today. The Tesla Model S has autopilot software installed that can do just that. By combining cruise control with its in-built camera system it not only detects other traffic, it also reads the road and keeps you within the white lines. 

If a slower car turns into your lane, it will scan the area and will, if it’s safe to do so, steer the car into another lane. This is close to autonomous and, while the software isn’t yet activated, it’s there and it’s been proven to work very well.

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The future of cruise control

A lot of modern cars are fitted with some sort of camera or radar system for use by the self-parking and cruise control systems and a significant minority also has Wi-Fi connectivity too. 

All cars will also have to have a ‘Black Box’ fitted that measures a variety of metrics, including speed, location and driving style by 2018, although many car manufacturers have already started to fit them. So it doesn’t take a genius to predict that cars of the future will link this technology to create a much safer road space.

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Life-saving cars

Imagine the scene: Two cars have an accident on the route you are driving. That information is instantaneously relayed to your car’s ECU ‘brain’ and the in-car sat-nav diverts you away from the scene, even before the first police car arrives.

Or, in a more dynamic scenario, the car in front of you crests a hill and is temporarily out of your line of sight. It has an accident that blocks your lane. That information is relayed to your car before you can even see the accident, much less had time to react to it. 

In that time, your car has already started to brake. It then calculates (in a thousandth of a second or so) that it won’t be able to stop in time but that’s OK, because it also knows that there is nothing coming in the opposite lane, so it steers you safely into that.

Your car has just saved your life. The other car is also busy saving the lives of its passengers as it has already phoned the emergency services, giving them the location of the accident, the severity of the impact, and the number of people on board. 

Science fiction? Not at all, every system that would be needed to activate this scenario is probably on your car already.

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Are we ready to hand over control to self-driving cars?

Cars of the future will, without the slightest doubt, use existing technology to drive themselves. The only reason they don’t do so already is because of consumer reluctance, alongside a few thorny little legal issues. That they could do so, and do so in a much safer way than even the best drivers, is beside the point.

We, as drivers, aren’t yet ready to hand the control of our cars over just yet. But we will, and I bet it’ll happen much more quickly than any of us expect.

For more tips and useful information, browse our motoring articles.


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