Can you use cruise control when driving in the rain?

By Martin Pretorius 

Cruise control is a useful feature when driving long distances. It allows you to avoid speeding fines (if you set it to a legal speed, of course), relieves stress on the driver, and may even help to save some fuel. But it may not be safe to use it by default, because some road conditions are not really compatible with handing control of your speed to the computer...

 How does cruise control work?

Most cruise control systems operate on a simple input-output basis: it obtains speed readings from the wheels (as a measure of road speed), compares these readings to the pre-set target speed, and adjusts the accelerator accordingly to either accelerate gently or slow down a little, matching the driver's desired speed setting as closely as possible. We'll leave adaptive (radar-guided) cruise control systems out of the discussion for the sake of simplicity, but the same principles apply with a few extra caveats.

If the speed drops below the desired value, the system instructs the engine to produce extra power, thus speeding up the car. Conversely, if the speed overshoots the desired value, it will request a power reduction from the engine in an effort to shed the excess speed. It is important to remember that the engine is always producing power while cruising, except on steep downhill sections, meaning that there's always power going to the driven wheels.

 What happens when it rains or on a wet road?

That largely depends on the amount of water on the road. As long as the tyres can maintain a good grip on the road surface (with no slip between the tyres and the road), the cruise control will function as it's supposed to.

The catch comes when the driven wheels lose contact with the road surface, such as when the car hits a deep puddle which causes the tyres to lift clear of the roadway (aquaplaning). When this happens, the friction between the driven tyres and the road will be momentarily reduced, and because the engine is always producing power, that will result in the driven wheels starting to spin up.

The cruise control will interpret this sudden perceived speed increase as a prompt to reduce the engine's power in an attempt to bring the road speed back under control. That's what it intends to do, at least, but what really happens is that the sudden torque reversal on the driven wheels could unsettle the car, possibly leading to a loss of control.

 This doesn't apply to vehicles with traction- or stability control

Vehicles equipped with these driver aids fortunately have much less to fear, as the stability- or traction control will automatically inhibit any wheelspinning antics, and in the event of a skid developing against the odds, it will try to rectify the situation. Either way, when the stability control notices the first sign of a loss of traction, it will instruct the cruise control to disengage, thus preventing further wheel spin.

That said, it's actually never a good idea to use cruise control in adverse driving conditions, simply because unfavourable conditions demand extra control and concentration from the driver. Rather leave the cruise control for sunny days and dry roads – it's never a good idea to relax too much behind the wheel, but when it's wet outside, you really need to pay even more attention to the proceedings.