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Cruise Control and Aquaplaning: the facts

words - Joe Kenwright
Alarmist e-mails about the dangers of cruise control and various media attempts to clarify the issues have left some motorists more confused than ever.
NEWS FEATURE: Joe Kenwright looks at how cruise control works and takes a more realistic look at the situations that could land you in trouble

Cruise control is one of those 'magical' devices in the modern car that few people understand. It is the perfect candidate for anonymous emails that purport to present an assessment or conclusion by so-called experts.

In the case of cruise control, it's very easy to imagine a rogue computer taking over, speeding your car out of control and leaving you and your car in a smouldering mess. In reality, cruise control has been in everyday use for at least 40 years and existed on cars well before computers. It is one of the most simple, refined and reliable of car accessories and while modern computer power has smoothed out cruise control operation, the basic operating principles haven't changed over that period.

The following is typical of what is being circulated with lifesaving earnestness:

"A 36-year-old female was travelling between Wollongong and Sydney.
It was raining, though not excessively, when her car suddenly began to hydroplane and literally flew through the air. When she explained to the policeman what had happened, he told her something that every driver should know -- NEVER DRIVE IN THE RAIN WITH YOUR CRUISE CONTROL ON.
The policeman told her that if the cruise control is on and your car begins to hydroplane -- when your tyres lose contact with the pavement, your car will accelerate to a higher rate of speed and you take off like an airplane. She told the policeman that was exactly what had occurred. The policeman estimated her car was actually travelling through the air at 10 to 15km/h faster than the speed set on the cruise control.
The policeman said this warning should be listed, on the driver's seat sun-visor - NEVER USE THE CRUISE CONTROL WHEN THE PAVEMENT IS WET OR ICY."
So is this assertion true or false? The final conclusion and advice might be sound advice but not for the reasons given. Aquaplaning (sometimes referred to 'hydroplaning') can happen if there is standing water whether cruise control is in operation or not. (Ed: as far as accelerating when traction is lost -- that's a furphy!) Indeed, far from being the absolute cause, cruise control is almost a red herring in the above scenario.

How does cruise control work?
It was common for cars prior to 1950 to feature a hand throttle. Instead of having to keep your foot on the accelerator for hours on end, you simply pulled out a knob to match the amount of accelerator you needed and lifted your foot. Serious four-wheel drives still have them so your foot doesn't bounce off the accelerator in rough territory.

Using a hand throttle at speed ultimately proved too dangerous as traffic increased. The car could run away downhill, would slow going up hills and would overcome the brakes if you forgot to disengage it. If you forgot that you were using it and pressed the clutch pedal, you could damage the engine as it over-revved.

The hand throttle was best forgotten on road cars and disappeared entirely. The Alfa Romeo GTV6 was the last modern car to offer a hand throttle but it was more often used by owners to allow the engine to run smoothly during warm-up.

Then someone had the bright idea of powering a hand throttle with an electric solenoid or vacuum-powered mechanism which was linked to the speedo. If the car slowed below the pre-set speed going up a hill or into the wind, the mechanism would automatically pull on the throttle cable to access extra power then it would stop as soon as the set speed was reached again. If the car sped up over the set speed, it would lift off pressure on the accelerator until the car returned to the pre-set speed. It was that simple.

Computer controls for today's accelerators make this process even more accurate, smoother and responsive.

It was then a case of attaching a safety switch to the brakes so that as soon as you hit the brake pedal, it would cancel the cruise control. On manual cars, a similar switch was fitted to the clutch pedal so if you tried to change gear while the cruise control was on, it would cancel before the engine could over-rev.

Who invented cruise control?
It is widely acknowledged that Ralph Teetor, a blind and talented US engineer, was prompted by a ride with his lawyer to develop the cruise control. The lawyer was a serious chatterbox who sped up and slowed down according to the conversation, a tendency that disturbed the blind Teetor enough for him to pursue a patent on the device as early as 1945.

Chrysler was the first to fit it to certain upper level models in 1958 followed by Cadillac's across the range fitment in 1960.

Can a cruise control speed up a car beyond the pre-set speed?
No, but basic versions can't apply the brakes if a downhill grade causes the car to speed up. If the engine is not strong enough to hold the car downhill, most cars will go over the set speed even if the cruise control has shut down the throttle completely.

In this case, the car speeds up because a lower gear has not been selected, not because of the cruise control. New grade control logic in the latest automatic transmissions are now reading this situation and changing down automatically so even this scenario will soon belong to the past. Even more advanced systems will actually apply the brakes to maintain a speed downhill.

Equally, if the engine doesn't have enough power to climb the hill at the pre-set speed, the cruise control can’t maintain the speed unless the transmission changes to a lower gear.

Neither of these scenarios fit the situation described in the above email.

What stops it from speeding up a car over the pre-set speed?
This is the key that makes a nonsense of the email. Modern cars take their speedo reading from the driveshaft or transmission. This means the cruise control bases its responses on the speed of the driven wheels, not the car itself. This is an important distinction and fail safe position.

If the driven wheels skid because they lose grip, the spinning wheels will cause the speedo to show a higher reading which will force the cruise control to release the throttle faster than most drivers. Regardless of whether the car itself slows down or speeds up, the cruise control will always reduce the throttle no matter what until the driven wheels slow down back to the pre-set speed.

If the wheels continue to slip under this scenario, this will always leave the car travelling more slowly relative to the road, not faster as described in the email.

The only exception to this would be if the speedo was driven by a road wheel not connected to the engine and transmission. The only car in recent history where this could be an issue was the old Volkswagen Beetle which had its speedo cable connected to a front wheel when the rear wheels delivered the power.

Thankfully, it didn’t have cruise control when in this case, the cruise control would increase power to a skidding rear wheel until the old Beetle's front wheel showed the correct speed. Whether the old Beetle had the power to break traction at highway speeds to create such a scenario is another issue!

What has aquaplaning got to do with it?
This is the key factor in the above email that has been lost in all the discussion about cruise control. Loss of control in adverse conditions can be lethal and probably accounts for more incidents because of the ignorance surrounding aquaplaning.

Modern cars, even small, lightweight models, have much wider tyres than ever before. Coupled with smoother roads that can hold an even film of water without high dry spots, the chances of water getting trapped between the tyre and road surface are higher.

It is similar to the difference between a stiletto heel and a wide flat heel on a shoe. Where a stiletto can punch through a film of water to contact the ground, a wide heel can slip. The tread pattern on a wider tyre assumes critical importance when a vehicle's weight alone is not enough to punch today's wider tyres through a film of water.

Tread patterns on modern tyres act as irrigation channels to sluice away the water under a tyre and allow the tyre tread to make contact with a dry road surface, no matter how wet the road. If these channels are not able to remove the water quickly enough, the tyre can actually loss contact with the road and skim across the film of water just like a skiffle board in the seaside shallows.

This creates the sensation of speeding up because you can no longer slow down, stop or steer. "Flying through the air" is not quite what happens but it certainly feels that way!

Braking and steering actions can be totally lost as the forces of nature dictate where you go next. Scared? You should be when in most cases it is caused by sloppy maintenance, not cruise control.

The mindset that doesn't replace tyres until they are bald is the real killer. Modern tyres have to be replaced long before the tread disappears. Drivers who drive with their tyres worn below the tread depth indicators are driving on tyres no longer capable of clearing the road of water. These drivers are setting themselves up for a serious aquaplaning crash.

Poor wheel alignment is a primary factor when it causes uneven tyre wear and robs the tyre of its grip in the main area where it contacts the road. Worn shock absorbers will also encourage aquaplaning if they cause the wheels to leave the ground after they hit road surface variation, and allow a film of water to collect between the tyre and road.

What has cruise control got to do with aquaplaning?
Not much, but it’s how most drivers react while the cruise control is on that will almost certainly decide whether a nasty aquaplaning situation turns into anything more than a scare.

Here are some examples you should consider...

  • After drivers engage the cruise control and no longer have contact with the accelerator pedal, they can no longer feel if the wheels are losing traction until it is too late.

  • When something does go wrong while the cruise control is engaged, most drivers stab the brake pedal to disengage the cruise control. If the car is starting to aquaplane, hitting the brakes could be enough to lock up the wheels completely. Even with the wheels locked-up under brakes and the cruise control disengaged, the car would continue on at speed. ABS-equipped cars could save most drivers in this situation.

  • As some drivers move their feet well away from the accelerator pedal when the cruise control is engaged, experts suggest that the above scenario is just as likely to be caused by the driver stabbing the accelerator instead of the brake as they tried to disengage the cruise control.

  • A car with cruise control engaged does not know to slow down before a corner nor does it know when to reapply the power as you leave the corner. A cruise control forces the car to rely totally on its front wheels to steer through a corner when good drivers would normally use a strategic sequence of brakes, accelerator and steering. If the front wheels start aquaplaning in the middle of a corner, the cruise control cannot register the loss of grip unless the car is front-drive. The cruise control would then keep pushing the car straight ahead off the road or across oncoming traffic. Applying the brakes to stop this could make an already dangerous situation worse.

  • Cruise control can make you inattentive. By the time you wake up to a problem situation and work out what to do, it can be too late.

So what's the bottom line?
The narrow margins applied in enforcing Australian speed limits dictate you should assess whether your use of cruise control is governed more by your need to protect your license than safe-driving practices. Some speed limits may be too high under certain weather conditions while in others, your direct communication with all the car’s controls may be more critical than sitting back with the cruise control on.

Consider some of the following issues and actions when you next engage cruise control...

  • If you feel yourself disengaging from driving as the cruise control engages, devise strategies to stay alert so you are ready to deal with whatever happens.

  • Minimise the chances of aquaplaning by keeping tyres fresh and backed by regular wheel alignments. Make sure that the shock absorbers/dampers are in serviceable condition so they can punch the tyres back onto the road as soon as they are disturbed by a bump.

  • Practise using the Cancel function or equivalent button to disengage cruise control so that you don’t initiate a violent loss of control by hitting the brakes.

  • Never engage cruise control on a road where you would normally have to slow down for a bend. If you can feel the car starting to get out of shape under cruise control, hit the Cancel button and have your foot hovering over the accelerator to smooth out any reduction in engine power. Most cars end up in trouble when braking or deceleration/acceleration is too sudden or too sharp.

  • If the roads are wet, don't use cruise control. This is a no-brainer when wet weather driving requires constant and gentle speed adjustments by carefully modulating the accelerator, not sudden brake applications.

  • While using cruise control, always have your feet in such a position so you know exactly where each pedal is relative to your right foot.

  • Anticipate downhill situations by disengaging the cruise control via the Cancel function not the brakes and taking over before it speeds up.

  • If you feel a lightness of steering when your car hits a puddle this is an advance warning of aquaplaning. Turn off the cruise control and back off on the accelerator immediately.

  • In winter in the colder parts of the country anticipate early morning or late evening situations where black ice may have formed. Again, hitting the brakes to disengage cruise control will be too late once you hit ice so don't engage cruise control at all in these conditions.

Prepared from discussions with Holden Special Vehicles Powertrain Engineer, Sam Davis.

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Published : Saturday, 15 April 2006

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