I understand that you shouldn't use cruise control on winter roads because you need to stay aware in case you hit black ice. But I've also heard that you shouldn't be using cruise control on hilly or winding roads any time of the year. That's basically all the roads here. So, what's the danger there, exactly? And is this advice still true with modern cruise control? I've got a 2015 Ford Expedition. – Suresh, Kelowna, B.C.
The danger with relying on cruise control when going downhill or approaching curves is that you could lose control entirely, manufacturers say.
"Do not use cruise control in heavy traffic, on winding roads or when the road surface is slippery," states the 2015 Ford Expedition owner's manual. "This could result in loss of vehicle control, serious injury or death."
While Chrysler called its first version of cruise control auto-pilot, the technology, even now, does one thing – it keeps you at the speed you set, no matter what's happening on the road.
"It's mechanical – 'this the speed you wanted, this is the speed I'm going to try to maintain,'" said Steve Elder, automotive instructor with the British Columbia Institute of Technology (BCIT). "Cruise control doesn't know what the road surface is, so it's going to try to maintain that speed regardless."
So, cruise control can't see road conditions or upcoming turns – so it won't slow down if roads are slippery or if you're about to hit a sharp curve.
"The car companies are putting these recommendations there for safety," Elder said. "They want the driver to be completely in control."
And hills? The cruise control can't see them either. Ford's manual states: "When you are going downhill, your vehicle speed may increase above the set speed. The system will not apply the brakes. Change down a gear to assist the system in maintaining the set speed."
That hill warning is there because the car companies are "worried about runaway vehicles." Elder said.
"It's generally more for going downhill than going uphill," he said. "It's all about safety."
Set it and forget it?
Some cars do shift gears to slow you down when you're going downhill, said Stu Miller, senior manager of auto service centre operations with the British Columbia Automobile Association (BCAA).
"Some cars today have the ability to do transmission braking and engine braking, but not all cars do," Miller said. "My truck will use engine braking to slow it down when I'm going downhill – but my wife's 2014 [SUV] doesn't have that ability."
Of course, you can turn off the cruise, and slow down, by hitting the brakes if you're going too fast – but you can only do that if you're paying attention.
And it can be tempting to take your focus off the road when the cruise control is on.
"There used to be an old joke in the industry about a couple with a motor home who set the cruise control and got up to make a sandwich," Miller said. "Cruise control definitely has limits."
But what about adaptive cruise control (ACC), which uses sensors to detect the cars in front of you and keep you at a steady distance from them?
It usually comes with the same warnings in the manual – avoid on curves, hills and slippery roads.
And that's true even for cars that also have lane-keeping assist, which keeps your car in your lane.
That's because, even if it can keep you in your lane on gentle curves or brake when the guy in front of you suddenly stops, it still won't know to slow down on slippery roads or on sharp curves, Elder said.
"Generally speaking, it's still using the same basic idea – it's still maintaining the vehicle at a set speed," he said.
And the advice also stands with technology like electronic stability control (ESC), which has been required on all cars made since 2012.
If the ESC senses that the vehicle is losing control, for instance, it will turn off the cruise control.
But you still have to be paying attention so you can regain control, Miller said.
"That's why on rainy or snowy days, it's not advisable to use cruise control," he said.
Some owner's manuals also warn about using cruise when towing.
"All the dynamics of driving change dramatically when you're towing something, so they want the driver to be completely in control," Elder said.
And, if you're towing on steep hills, the cruise control could make the engine work harder uphill than it would if your foot was on the pedal.
"If you're towing uphill on something like the Coquihalla [highway in British Columbia], that's pretty steep, the transmission will keep downshifting so it can maintain that set speed," Elder said. "You can see it getting close to the red line."
Could that extra work eventually lead to mechanical problems? Maybe.
"Using cruise control while driving through hilly terrain could cause the system to force multiple transmission shifts which could lead to overheating of the transmission fluid and premature component wear," said Stephen Leroux, automotive professor at Centennial College in Toronto.
Given manufacturer's safety concerns with cruise control on hills and curves, is it ever safe to use on mountain roads like the Coquihalla?
"I use my cruise control on the Coquihalla and I turn it off when I have to make sharp turns or if I'm going downhill," BCAA's Miller said. "I just want to have more direct control. There's nothing worse than heading into a corner going 120 km/h where you're going too fast to make it."
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