Rollover! It's one of the most serious types of accidents, because drivers and other truck occupants are usually tossed around and badly injured or killed. And of course the truck is often totaled and its contents spilled, mangled and lost.

Most often a rollover occurs when the driver enters a tight-radius curve too fast and can't react quickly enough to stop it from happening. It also happens when he suddenly tries to avoid an object or person in the roadway. Hitting the brakes can sometimes slow the truck enough to keep it from going over, but the driver often doesn't realize what he's getting into until it's too late.

With a tractor-trailer, there's little seat-of-the-pants warning as the rig plows into a turn or goes through a sudden maneuver. That's because the trailer begins tipping first, at its rear. Little of its motion is transmitted forward through the fifth wheel for the driver to feel. Testing shows that the trailer rolls beyond where it could be stopped before the driver knows it. As it goes, the trailer pulls over the tractor until they're both on their sides or backs. It's messier with double- or triple-trailer combinations, which flop around semi-independently before going over.

Brake manufacturers have devised products that can sense that such an event is occurring and, in a fraction of a second, take steps to prevent it. Generically called stability control, the products use wheel-motion sensors and electronic controls already in anti-lock braking systems, and in some cases add accelerometers elsewhere on the chassis to give the controls a better idea of what's happening. They apply brakes and cut engine power to slow the vehicle, and in many cases prevent the rollover. They're available with many of today's hydraulic and air-braked vehicles. In previous issues we reported on Bendix's anti-rollover systems for tractor-trailers and straight trucks. Last month, we drove competitive systems offered by Meritor Wabco.

The venue was a ride-and-drive event in Washington, D.C., sponsored by the Motor and Equipment Manufacturers Association. Member companies displayed vehicle safety devices that are available to reduce accidents. The audience included legislators and their staffs, government agency staffers, and reporters. About two dozen manufacturers had exhibits set up around one of the massive parking lots at RFK Stadium on the city's east side.

Among the companies showing off stability control products were Bendix Spicer (whose devices I had experienced and have written about previously), Bosch and Continental. The latter two make stability control products for light trucks and SUVs, vehicles that have gained notoriety for their propensity to roll over and maim or kill their occupants. I sampled their systems during this MEMA event and can testify that they make top-heavy SUVs easier to control in violent maneuvers, so much so that I'd want them in my personal vehicles. Of course, the ideal situation is to stay alert and avoid getting into sticky situations in the first place, but the real world is far from ideal.

Meritor Wabco first demonstrated its anti-rollover products to the press almost three years ago, but didn't let us drive trucks with the systems. This time they did. The company has two products for trucks and tractors: Roll Stability Control (RSC), which concentrates on center of gravity and lateral and braking forces, and Electronic Stability Control (ESC), which has additional sensors to measure steering input and yaw, or rotation, motion.

There's also Roll Stability Support (RSS) for trailers. This keys on lateral movements and forces in the braking system and air-bag suspension. RSS acts alone to keep a trailer from doing its roll-and-pull act in dangerous maneuvers by applying its brakes, and is the only such system now available for trailers in North America, Meritor Wabco says.

RSC and ESC work on the power unit's brakes and also command its engine to reduce power to further cut speed. These things occur no matter what the driver does. He doesn't have to touch the brake pedal. In fact, he can be mashing the accelerator and it'll still operate and try to save him and his rig. This was demonstrated in two vehicles by Meritor Wabco test drivers, who took us through a course marked by orange cones. They showed how unprotected vehicles roll when they get into trouble, tipping sideways until wheeled outriggers arrested the movement. Company executives and those waiting for their turns in the trucks stood near the maneuver point to watch.

First I went through the course in a Stewart & Stevenson military cargo truck and observed how it began rolling while entering a tight left turn; its right-side outrigger stopped the roll. With the system switched on, the truck maintained its composure even if there was some tire howl. A GI at the wheel would've written it off as a no-sweat affair, except for what was being generated by the heat and humidity, which were both well into the 90s. The no-frills truck had no air conditioning, so I retreated from its cab without asking to drive it.

Instead I climbed into a Freightliner Century Class pulling a weighted flatbed trailer. Tim DiFalco, a company test technician, drove us through the course with the stability control systems off, and we watched the mirrors to see what the trailer did. We could see its rolling motions even if we couldn't feel them as the tractor plowed through the prescribed left turn at about 35 mph. He switched the system on for the next trip through the course and, as expected, the systems slowed us down and kept the rig under control.

Then I got behind the wheel. On my first trip with the system off, I balked at throwing the truck into the turn. I touched the brakes as I approached the turn point and then went into it, but the trailer barely lifted its inside wheels and there was not a lot of tire noise.

"Not much movement," DiFalco commented.

"I lost my nerve," I said a little sheepishly. "Next time..."

Coming out of my wimpy maneuver at about 20 mph, I downshifted the tractor's 10-speed transmission to 6th, accelerated, went to 7th and quickly returned to the turn-around area and did an easy 180. I downshifted again, got on the accelerator and worked back to 7th, then held us steady at about 38 mph. DiFalco noted that we were a little too far to the left, so I turned to the right – and headed momentarily toward the folks on the ground. They all instinctively stepped back, then relaxed as I cut left again.

Then I cranked the wheel to the left and the tractor tore into the turn, tires moaning. The trailer followed and began tipping to the right. I glanced in the left mirror and saw the bed heeled up; with the rig now bent, I couldn't see the right-side outrigger's tire touch the pavement, but we felt the bump and our tip-over stopped. The trailer settled back down as I eased us out of the turn.

"There ya go!" I said with a bit of satisfaction.

"A good one," he agreed. "Now let's turn on the system" He flipped the toggle on the dashboard and the indicator glowed "on."

I returned to the other end of the course, looped around, then accelerated again toward the test turn and went into it briskly. This time I kept my foot on the accelerator as I spun the wheel, but the system sensed the lateral force, knew what was happening and cut power. At about the same instant the brakes began applying. By about midway through the turn, the rig had slowed by maybe 12 to 15 mph – I wasn't watching the speedometer too closely this time – and whatever tipping had begun in the trailer had been arrested. I completed the turn at a safe speed and, while I had expected the system to work just as it had done, I couldn't help but be impressed.

A consistently conscientious driver might never experience the actions of any stability control system. But it could save a tired driver who exits a freeway and suddenly finds himself on a sharply curving ramp and has no time to react. The driver will be amazed. The rig's owner will be pleased, if he ever hears about the incident, because his investment of $2,000 to $3,000 – the amount varies with the systems and among vehicle builders – has been paid back many fold.