Yes, you can use cruise control in the wet

There’s a persistent myth about not using cruise control in the wet. It is perpetutated by posts like this:

I had a wreck a couple of weeks ago and totaled our Lincoln Town Car. I hydroplaned on Hwy 135 between Gladewater & Kilgore, Texas. I was not hurt, just emotionally rattled! I know the Lord was with me.

I learned a lesson I’d like to pass on to you. You may know this already — but the highway patrolman told me that you should NEVER drive in the rain with your cruise control on. He said if you did and hydroplaned (which I did) that when your tires were off the road your car would accelerate to a high rate of speed (which it did). You don’t have much, if any control when you hydroplane, but you are totally in the hands of God when the car accelerates. I took off like I was in an airplane. I’m so thankful I made it through that ordeal. Please pass the word around about not using cruise control when the pavement is wet or icy. The highway patrolman said this should be on the sun-visor with the warning about air-bags.

The only person I’ve found out who knew this (besides the patrolman) was a man who had a similar accident and totaled his car. This has made me wonder if this is not why so many of our young people are dying in accidents.

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This is why the advice above is wrong.

How does cruise control work?

The car’s cruise control is run by a computer. The computer knows how fast the car is travelling, and it knows the target speed the cruise control is set at. If the car’s speed drops below the target speed then the computer increases the throttle, if it goes above then it reduces the throttle. It’s that simple.

Newer systems may also apply the brakes if reducing throttle doesn’t bring the speed back to the target, and if that doesn’t happen then you may find the car accelerating downhill above its target speed – this was a factor in this caravan crash I analysed.

Back to the scenario in the viral post above. According to the post, the car’s drive wheels lose traction, and then spin faster leading to a loss of control.

That doesn’t happen.

What really happens with cruise control in the wet?

So let’s say you’re travelling at speed, cruise control on, in the wet. The tyres are doing their job, which is moving water out of the way so the tyres can touch the road and therefore grip. If you look at cars on a wet freeway you’ll see this effect; there’s huge spray from each tyre.

Now the drive wheels hit a puddle. There’s so much water the tyres cannot move the water out of the way, and the tyre skims over the puddle, not touching the road. This is called aquaplaning or hydroplaning, and there is of course no more grip which can lead to a loss of control. It’s not good regardless of cruise control or not, and whether you aquaplane or not is determine by only two things; the speed you’re travelling, and the depth of the water. High speed, deep puddles mean aquaplaning – cruise has nothing to do with it, just speed and water.

So we’ve got cruise engaged and we’re aquaplaning. There’s very little grip. This means the drive wheels may spin slightly, so they go faster. This again has nothing to do with cruise – it’s what would happen anyway.

So what does the cruise control computer do when car aquaplanes? It detects the fact that the wheels are now going faster than the target speed…and reduces the throttle to suit. And it probably does that faster than a human could. That means that this idea that cruise control somehow spins the wheels out of control…is fundamentally wrong.

However, that doesn’t mean to say cruise control is entirely safe in the case of aquaplaning. Let’s say you’re at 100km/h prior to the aquaplane and somehow the wheels spin briefly to 103km/h, then the cruise brings it back to 100km/h. But the right thing to do might, and probably is, continue to reduce speed and power smoothly, and cruise won’t do that, it’ll try to maintain 100km/h.

So my advice is this. Driving on wet roads at speed is risky, regardless of cruise, as your traction is decreased by a huge margin, and there is a risk of aquaplaning. You may well need to react to changing conditions…and by ‘react’, I mean anticipate and mitigate, for example spotting a puddle ahead of time and reducing speed. And if you do hit a puddle at speed, gently easing off the accelerator and ensuring you look well ahead, steering to where you want to go. Cruise control doesn’t make either of those jobs easier, so I’d consider not using it in very wet conditions at speed.

If you do aquaplane, what does it feel like, and what do you do?

Well, the steering will feel light, as there’s now very little grip. The car may slightly slew left or right. And you may remember you’ve just hit a puddle.

What you do depends on the car you’re driving – one with electronic stability control and anti-lock braking (ABS), or not. In Australia, almost every car since around 2007 has stability control/ABS so the skills required to keep a car under control in the event of an aquaplane are different to say a 1970s car with no electronic driving aids.

In the ’70s car, you’d need to be a very skilled driver; slow down but not brake so hard the wheels lock and you skid, probably put in steering inputs to stop the car rotating…it’s advanced race-driver skills here.

In a modern car, a lot of that is done for you. You can slam the brakes on and the car will, probably, manage to use its electronic driving brain to keep you on the road. But you can still help the car help you – ease off the accelerator gently to slow down rather than slam the brakes on, look where you want to go and steer that way, and generally don’t panic and be smooth. Aquaplaning usually means you’re only just out of grip, and it won’t take much to get you back into grip, so don’t overreact. It is however very hard not to unless you are a trained driver used to ‘interesting’ situations in cars!

Some modern cars are known to cancel cruise control if the computers can’t handle a situation, for example very wet conditions on dirt, and cancelling it soon as you feel the car is not behaving how it should is a good idea. You can simply touch the cruise control to turn it off, or slightly brake. Changing gears in a manual (not auto) also cancels cruise, but don’t do that as a first resort in a car skid situation.

Do tyres make a difference?

Definitely. You want new (less than 5 year old) tyres, correctly inflated, and not worn. That makes a huge difference to car safety.

By the way, narrow tyres are better than wide in the wet as the narrow tyres have less frontal area to dissipate water, and a longer contact path to get rid of the water. Running slightly higher tyre pressures is a good idea too; the reason is not ‘cut through’ to the water, but because water has a cooling effect and therefore there’s not as great a rise from cold to hot pressures, so the tyres run under-inflated a little.