How ABS brake systems work and why San Diego cops don't like them

Dear Matthew Alice: How does the ABS system work on my ex-girlfriend's new Mercedes? And when she has to mash down on the brake pedal if, say, someone were to pull out in front of her as she dashes home from the pet grooming salon, how would the investigating officer determine the rate of speed if the anti-lock braking system were to leave no skid marks? — Perplexed, San Diego

Well, Perp, I’ll overlook the pungent odor of a plot simmering — perhaps something involving your ex, her Mercedes, a car/truck/“mysterious” pedestrian/overclipped poodle “just happening” to dart into her path, a subsequent accident/traffic citation/lawsuit? Sounds like she left you steamed when she zoomed out of your life, spewing gravel, 0 to 60 in 8.3 seconds. But if the authorities ask, I’ll tell them that for all I know, you are just a concerned consumer.

ABS systems, when they’re engaged, apply and release your brakes many times per second so your tires keep rolling and you can still steer your vehicle as you come to a stop. It’s the equivalent of the old practice of pumping your brakes to keep them from locking. ABS kicks in when special sensors detect a loss of traction in the wheels. They send the “mayday!” to the ABS computer system, which briefly releases braking pressure, reapplies it, and continues “pumping” until traction is restored or you take your foot off the brake pedal.

Despite the high-priced ride, if your girlfriend is just like the average driver, she just might take care of the accident scenario without your help. According to an SDPD traffic investigations officer and a multitude of studies completed since ABS has become popular, the system hasn’t made much of a difference in the accident rate. The system’s fine, they say. We just don’t know how to use it. We don’t need driver’s ed, we need braker’s ed, apparently. SDPD has even pulled the fuses on some of the ABS systems on their vehicles because they were more trouble than they were worth. Odds are, the first time your ex applies the brakes in a panic, that thump-thump feedback through the pedal will cause her to pull her foot off, thinking something’s wrong with them (though Mercedes, the first to offer passenger-car ABS, mid-’85, claims it has a much lower than average level of feedback). And it will probably take her slightly longer to come to a stop with ABS even though, theoretically, braking distance should be the same as without it. People don’t apply enough brake pressure once the ABS kicks in, the experts say, and don’t maintain the pressure long enough. But best of all (for you, anyway), the odds on your girlfriend steering her car off the road and into a ditch are pretty good. SDPD officers and the National Highway Traffic Safety Administration have both rioted that now that we are able to slam on the brakes and still steer the car, we’re oversteering our cars right off the road. We don’t hit the jerk in front of us (down nationally by 24 percent), we bash into a tree instead (up nationally by 28 percent).

Your ex’s Mercedes will leave tire marks on the road in a panic stop. They’ll just be very faint compared to the non-ABS marks, since the friction heat will be spread around the still-rotating tire, not concentrated in one place. In accident investigations nowadays, sez my SDPD source, one of the primary duties of the first officers on the scene is to mark the beginning and end of the skid marks so they’re not defaced before they can be measured. That’s the only way ABS has changed the classic American traffic accident.