Like Ken says, and I think I mentioned at some point, not all ABS systems are created equally.
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Originally Posted by kevinp
Someone also said that ABS was so that you could turn while braking. WRONG. ABS is so that when Average Driver jams the brake pedal to the ground, the back tires won't lock and send the rear of the car to where the front is supposed to be.
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Kevin, I'm sure they mentioned the effects of going outside the traction threshold in that driving course you mentioned. I'm sure they also mentioned that while a tire is skidding it is incapable of providing any directional influence because there is not enough grip to influence vector. A good example of this would be what happens when you toss a small fuzzy animal across a freshly waxed floor
You watch them scramble to move their own direction but their momentum keeps them going in a straight line because they have no grip.
There are two purposes behind ABS. First is because a machine that can monitor whether a tire is turning or skidding is going to be able to ride the traction threshold much closer than a person can, there's just no two ways around that. All we have to work with is our butt dyno, ABS can actually monitor whether the tire is turning or not and adjust accordingly. Older ABS systems were not precise enough in this process to exceed the capability of a skilled driver but that hasn't been true for years now. The grip of a tire that exceeds the traction threshold gives you much less grip than a tire that is still inside that threshold.
The second reason for ABS is indeed to maintain the driver's ability to steer a car during a hard stop. If a tire is skidding it's like that fuzzy creature skimming across the waxed floor and it doesn't matter what you do with the steering wheel.
As is demonstrated in the preceeding graphic not only does braking require a measure of a tires finite available grip but so does turning. This means that if you want the tire to not only slow down the car but also provide direction you're going to only have a percentage of the total grip available to each task. Managing how much grip each task is consuming is very difficult for even good drivers; that is why good driving instructors will advise especially their newer students to avoid trail braking (braking while turning). The second benefit of ABS is that if a driver is required to manuever while braking there is very minimal chance of locking up the tires, skidding, and taking away both the ability to stop and to steer.