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Dear Tom and Ray:

Lately, several carmakers, including BMW and Lexus, have been advertising their vehicles with "traction control." There have been ads showing a BMW plowing through the snow, a Lexus accelerating on an ice-covered pond with skaters in the background and an Acura Legend in a North Pole-like environment, all with traction control. Are these traction-control devices as good as the manufacturers claim, or are they just advertising glitz? - AnthonyTOM: Like most things, Anthony, some are good, and some are like my brother. That is, they work, but they don't work too well.

RAY: We've never driven a BMW or Acura Legend with traction control, so we don't know how well those work. But we have driven a Lexus LS400, an Infiniti Q45 and a Cadillac Concours. And we found the traction-control systems in those cars to be superb.

TOM: What traction control does is keep the drive wheels from spinning. And that's especially important in a rear-wheel-drive car, because rear-wheel-drive cars have always had more trouble on slippery roads than front-wheel-drive cars. When the rear wheels start to spin, traction in the back of the car is lost, and the rear end starts to "fish-tail." This can lead to a spin-out or loss of control.

RAY: Traction control continuously compares the wheel speeds. And when it senses that one wheel is moving a lot faster than the others, it concludes that the wheel has lost traction, and it slows that wheel down until it regains its grip.

TOM: How does it do that? Well, that's where the traction-control systems differ. The cheaper systems use only the anti-lock brake system (ABS) to grab the wheel and stop it. And while this might be good enough for cars with decent traction to begin with, it's only partially effective on cars that need the most help.

RAY: The better systems, like on the Lexus, Infiniti and Cadillac, use the ABS but also use the engine-control system to cut the power to the wheels when they're starting to spin. Most use the computer to shut off an increasing number of cylinders until the wheels are turning slowly enough to regain traction.

TOM: I should mention that this all happens in fractions of a second, so as the driver, all you know is that you stepped on the gas and the car went forward in a nice, straight line. You may feel the power being held back, and you may see a little indicator light on the dashboard, but other than that, you'd never know it was there.

TOM: And with a system like this, it is possible to stomp on the gas pedal on an ice-and-snow-covered pond and have the car start smoothly in a perfectly straight line. Of course, you wouldn't catch me driving on a pond if it were my Lexus.