Every winter it happens - SUV drivers careening across medians, sliding into ditches, slamming into stopped cars, you name it. And why? After endless commercials showing SUV's blasting down snow covered mountain roads, these poor souls actually believe their SUV's have "more traction" than other vehicles.

Even here in Minnesota, where you'd think people would have figured it out by now, my cop friends marvel at the number of reckless SUV drivers they encounter in their winter patrols - as one of them recently quipped, "Geez, how many stupid people ARE there, anyway?"

But don't SUV's and other "all-wheel drive" vehicles have more traction? The short answer is NO. The problem comes from a lack of understanding about what "traction" actually is. Traction is simply the TOTAL coefficient of friction between the tire and the road, and is the result of two major factors.

The first is pressure - the amount of WEIGHT each tire supports. This is the reason front drive vehicles can "get going" on a slippery road - there is nothing magical about front wheel drive, it just puts a bigger percentage of the car's total weight (primarily the engine) over the drive wheels, thus exerting more pressure on those tires. This is the same reason that a rear drive car like a Porsche 911, which has a REAR mounted engine, has a remarkable ability to accelerate on a slippery road surface.

Second, the friction between tires and the road. Friction is affected by the size, tread pattern, and tread compound of the tire itself, AND of course, the CONDITION of the ROAD SURFACE.

Now, you shouldn't have to be a rocket scientist to understand that a wet road will be more "slippery" than a dry road, and that a snowy, slushy, or worse, icy road, will be even worse. At least one would think so. So, why then, do these folks continue to barrel along at 50 or 60 miles per hour on roads that are the equivalent of skating rinks?

Because owners of front wheel drive and 4 wheel drive vehicles (incorrectly) attribute their ability to ACCELERATE more quickly on a slippery surface with having more "traction." But as stated, ANY vehicle with a certain set of tires and a certain weight will have the same actual TRACTION per tire, REGARDLESS of whether it is front drive, rear drive or all wheel drive.

What gives these vehicles the ability to "get going" is that they have the ability to distribute POWER more effectively, and thus USE the available traction of more tires more effectively. In the case of front drive, power is going to the tires with more traction (the fronts) and in an all-wheel drive vehicle, power can be distributed to all four tires.

This gets more complicated when you start introducing things like limited-slip differentials, which allows even a rear drive vehicle to double the number of tires available to accelerate - instead of having one wheel spinning, power can be divided between the two drive wheels. Drag racers learned this a long time ago.

But here's the problem: No matter what kind of drive-train your vehicle has, ONCE YOU ARE MOVING, ALL BETS ARE OFF. Because, as pointed out, the only thing keeping your car on the road is the friction between your TIRES and the road.

Car and Driver magazine did a wonderfully informative demonstration of this years ago - they compared 2 pairs of identical vehicles, an Audi (one with front wheel drive and one with 4-wheel drive) and a Mercedes (one with REAR drive and one with all wheel drive). They compared acceleration, cornering and braking on a snow and ice covered test track. Oh, and they compared the vehicles first with standard, all-season radial tires, and then they they ran the same tests with winter-only snow and ice tires.

Not surprisingly, the front drive and 4-wheel drive cars out-accelerated the rear drive car, but when it came to CORNERING or STOPPING, the front drive and the 4-wheel drive cars had NO advantage at all. As a matter of fact, the REAR DRIVE car with snow and ice tires out-cornered and out-stopped BOTH 4-wheel drive vehicles when they were running on the all-season radials.

So, at the end of the day, the only real way to increase your car's TRACTION, is to install the best snow and ice tires available for your vehicle. However, while I recommend winter tires for anyone who lives in a place like Minnesota (even on your SUV) they will only give you a slight increase in traction, and even that improvement can be wiped out (pun intended) if you simply use them as an excuse to drive faster than you should.

The bottom line? Once your vehicle is moving, your front drive or 4-wheel drive BUYS YOU NOTHING.  So, if you let the improved ability of your SUV to pull away from a stoplight deceive you into thinking you can blast along without a care in the world, be forewarned - you may very well find yourself in that snowy ditch, dazed and confused (if you're lucky, that is).