Car Advice

Myth Busting: Can you use cruise control in the wet?

The internet can be awesome, but it can also get things very, very wrong… and whether you should or shouldn’t use cruise control in the wet is one of those things.

TYPE INTO GOOGLE (or your search engine of choice), ‘should I use cruise control in the wet’, or rain, or any derivation of that statement and you’re likely to get some sensationalistic article written by someone from a lifestyle blog. They’ll trundle out that wonderful line about how you should never, ever, ever use cruise control when there’s water on the road, or it’s raining, or, whatever because your car will speed up. And they’re wrong.

And the amazing thing is that that opinion was never correct, not even in 1958 when cruise control arrived on the scene. Now, before we go too much deeper, I’d like to point out that this article isn’t my opinion, rather, I chatted with the technical boss of a major car company to get to grips with the subject and compiled it from that conversation.

Back to Google… the top entry I found was written by someone who clearly assumed that cruise control was some dim-witted force that could be tricked in wet weather. And, that if your vehicle began to aquaplane, which is the root cause of this myth, that cruise control will cause the vehicle to speed up out of control. The same writer then went onto argue with a reader in the comments section that he was correct because his car didn’t have traction control to stop his car from accelerating out of control in the wet.

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And no doubt you’ve all seen chain emails about some lady driving between somewhere and somewhere else in the rain, she aquaplanes and the vehicle begins speeding, she gets pulled over by a police officer who tells her you should never use cruise control in the wet. This email originated in the US, but has been Australian-ised. And it’s bollocks.

car aquaplaning

Let’s unpack some of this… What is cruise control and how does it work? According to The Macquarie Dictionary of Motoring, cruise control is: “[A] device which enables the driver to maintain a fixed speed. The vehicle will keep to the chosen speed regardless of road undulations unless the brake pedal or accelerator is touched. It is mainly used to reduce fatigue and comply with speed regulations on long trips.

Cruise control as we know it, was invented by a bloke called Ralph Teeter, who was blind. He came up with the idea after travelling with his lawyer who would slow down when listening to a conversation and speed up when talking. Teeter was one of the finest automotive engineers of his time and his patent for cruise control was filed in 1948 and launched on the Chrysler Imperial in 1958; and was standard fit on all Cadillacs of the time.

Cruise control controls your car’s speed via the throttle. In a car with a cable driven throttle there will usually be two cables side by side, one being the throttle cable proper and the other being the cable that the cruise control uses, which comes from an actuator via a vacuum instead of the throttle pedal. When cruise control is engaged the ‘second’ cable from the actuator moves, moving the throttle cable with it.

In a car with a fly-by-wire throttle then the system will be, obviously, an electronic arrangement. But, in both scenarios the system is designed to increase or decrease the engine’s power, which speeds up or slows the car down. The cruise control doesn’t usually touch the car’s brakes, it works on the throttle only. And if you’ve got a car with an automatic gearbox then the gears will change without affecting the cruise control.

The whole thing, no matter the mechanics of the system, is run through a cruise control computer which tells the cruise control when the car has reached the desired speed or when it’s falling away from the desired speed. It’s also able to tell the system when the brake pedal has been pressed so that it doesn’t try and fight against the driver, and instead deactivates the cruise system. Clever.

So, how does cruise control measure the speed of the vehicle? A vehicle’s speed is generally measured via either the driveshaft or transmission. And, so, cruise control is measuring the speed of the driven wheels. And here is the key, if your vehicle begins to aquaplane on a wet road and the wheels begin rotating faster, then cruise control will release the throttle to slow down the vehicle to the pre-set speed.

What does cruise control have to do with aquaplaning and travelling faster than the pre-set speed? Nothing whatsoever. Driving on a wet road is dangerous because grip is reduced. Add to that poorly maintained tyres (low grip, under or over-inflated) and you’ve got a recipe for disaster and cruise control has nothing to do with it.

Rather it’s how a driver reacts to aquaplaning that matters most, and quite often those who’ve engaged cruise control will nail the throttle to disconnect cruise control. And this can make a bad situation worse. Aquaplaning is the result of hitting a standing patch of water on the road and the tyre’s inability to shed the water quick enough, causing the vehicle to skim across the water without grip – travelling slower reduces the risk of this occurring. In most cases, you’ll skim only a short distance before the tyres regain their purchase on the road; your vehicle will aquaplane whether you’ve got cruise control on or not.

The next part of the argument is cornering and cruise control. Cruise control is simply a way of maintaining a set speed, it has no way of working out what’s on the road, what the corner’s like or whether you need to brake before the bend in the road. That’s the driver’s responsibility. And that’s the key here, when you get behind the wheel of a car, it’s the driver who needs to control the brakes, throttle and steering.

One example we were given by the engineer we spoke with was this: Say you’re on a road that’s signposted at 100km/h and you approach a corner where the suggested speed for that corner is 80km/h; the driver would slow down and drive around the corner. But, if you’ve got cruise control set, it doesn’t know the corner is there or how tight it is, it can’t read the sign, so it keeps the vehicle travelling at 100km/h. The perception is that the vehicle is taking the corner too fast and over-speeding and it is, only it isn’t. As mentioned, cruise control doesn’t know the corner is there and just maintains the speed that’s been set which is ultimately too fast, but it’s not cruise control’s fault, it’s the driver’s fault. And that’s because cruise control is not an auto-pilot.

Because the car has travelled around the corner too quickly and potentially lost grip and begun to either under- or oversteer, several vehicle safety systems will activate to keep the car pointing in the right direction. Traction and stability controls might activate, either killing engine power or braking individual wheels, or if fitted torque vectoring will activate and shuffle torque from the over-loaded wheels to those with grip. And, because all the systems are interlinked, cruise control will be killed.

But, in almost every scenario, it’s the driver’s responsibility to drive to the conditions, be they a corner, a dirt road, rain or heavy traffic. Blaming cruise control for your lack of attention, is like saying, the devil made me do it. So, let’s all take some responsibility for how we drive and call ‘time’ on the clickbait articles suggesting you can blame something else.

So, if you do want to use cruise control in the wet, go for it, but just like you would if you weren’t using it, set the speed a little lower than the signposted limit. And make sure your tyres are in good condition and inflated to the correct pressure.


1 Comment

  1. Doug
    January 21, 2018 at 10:10 pm — Reply

    Isaac, I believe this is only partly correct. The cruise control will not drive you past the pre-set speed irrespective of the road surface conditions unless you are going over a hill crest before the set speed is reached, in which case you will exceed your set speed, particularly if you are driving an auto trans, and particularly those where the top 2 or 3 gears are overdrives which they nearly all are now. However it WILL accelerate your wheel speed wildly if you have previously disengaged it and you resume it when your road speed is substantially below the pre-set and you are on a slippery surface like ice or sheet water and going up hill. The more powerful your car, the worse the resulting mayhem as the torque kicks down the transmission because you are going slower than the trans wants to operate. Let’s say you’re set for 100kph but doing 70 behind a slow coach in the rain and you arrive at the bottom of a long slow hill with a passing lane approaching so you pull to the right lane and engage your cruise. The cruise knows you are doing 70 but wants to get you to 100 as soon as it can by throttling up until the drivetrain reaches the calibrated wheel speed. If you have little or no traction or it is compromised by road surface conditions, it will go ballistic. It will go wheelspin, no wheelspin, wheelspin, no wheelspin until you either have a panic and the car flies off the scene, or you disengage it. I think this phenomenon is behind some excursions into the shrubs by other people in my part of the world (QLD) during wet weather. I would like to hear what others (I’m thinking automotive engineers here) think about this.

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Isaac Bober

Isaac Bober

Isaac Bober was born in the shadow of Mount Panorama in Bathurst and, so, it was inevitable he’d fall into work as a motoring writer. He began his motoring career in 2000 reviewing commercial vehicles, before becoming editor of Caravan & Motorhome magazine. He then moved to MOTOR Magazine before going freelance and contributing to Overlander 4WD, 4×4 Australia, TopGear Australia, Men’s Health, Men’s Fitness, The Australian, CARSguide, and many more.