Trucks are not large cars. They're a different species of highway creature. Whether they're accelerating, braking, climbing a hill, switching lanes or turning onto a side street, tractor-trailer trucks must perform certain maneuvers that drivers of automobiles, or "4-wheelers" as truckers call them, are generally not familiar with.

A typical tractor-trailer combination -- a power unit pulling a loaded semi-trailer hinged to its rear end -- may weigh up to 120,000 pounds. Depending on the trailer length -- 40, 45, 48, or 53 feet -- the total length of the combination may exceed 70 feet. On the busiest intercity routes a motorist will encounter double or even triple-trailer combinations sometimes exceeding 100 feet in length.

Any motorist who's been stuck behind one of these trucks at a traffic light knows that a semi-trailer combination accelerates much slower than the typical late-model automobile. The truck may have to go through 15 gears--a relatively lengthy process--to reach the speed limit. The truck may have two or three times more power under the hood than a car does; but with up to 110,000 pounds of trailer and cargo behind it, a truck engine must move 30 or 50 times more weight than does a car engine.

Segment Index
Braking Turning Characteristics	A Trucker's Blind Spots Maneuverability

Braking

Most motorists understand that a semi-trailer will take more time and distance to brake to a stop than a car traveling at the same speed. But how much more? In a series of tests conducted by the Insurance Institute for Highway Safety (IIHS), a passenger sedan traveling 55 mph came to a stop in 133 feet while a sleeper- cab tractor with a loaded trailer required 196 feet.

Surprisingly, IIHS researchers also learned that it takes almost as long to stop a "bobtail" -- a tractor running without its trailer -- as it does to stop a loaded tractor-trailer combination. Without the weight of a trailer on its rear wheels, the bobtail came to a stop only after traveling 183 feet.

When motorists fail to understand the physical forces that act on trucks, the resulting confusion can have grave consequences. Using the example of a bobtail's braking distance, our "instincts" tell us that a lightly loaded truck should have less momentum and a shorter stopping distance than one carrying a heavy load -- and that a bobtail should stop fastest of all. That may be why the IIHS study showed that the fatal-accident rate for bobtails was more than 13 times higher than that for tractors pulling trailers. The absence of trailer weight on the tractor's two rear-powered axles drastically reduces tire-to-pavement contact and interferes with the driver's control of the vehicle.

In its defensive driving course for the professional truck driver, the National Safety Council (NSC) notes that braking distances are increased by two additional factors most people are unaware of: reaction time and brake lag. Reaction time is the number of seconds it takes a driver to recognize a problem and take the appropriate action. The NSC says the average reaction time of most people is three-quarters of a second. At 55 mph, a truck will travel 60 feet before the driver applies the brakes.

Brake lag, the period of time between the application of the foot valve and the time the brakes react to maximum brake efficiency, is generally unique to trucks. The hydraulic brakes on automobiles and smaller trucks engage almost immediately. In tractor-trailers, however, the air-brake systems that transmit braking power from the tractor to the trailer are subject to a lag that can add many feet to the stopping distance.

In a separate study, the National Safety Council found that a combination truck traveling at 55 mph with a full load under ideal conditions will travel a total of 335 feet before coming to a complete stop. With hot brakes, the distance will be even longer. Whether one cites the IIHS study or the National Safety Council study on braking distances, the point is clear -- a tractor-trailer of any kind takes far longer to stop than does a passenger car and motorists must take this into account while driving.

One final note about brakes: Cars have self-adjusting brakes, but many tractor-trailers do not, so truckers must regularly check their brakes every few hundred miles.

Braking characteristics aren't the only way in which motorists may be confused by a truck's actions. Turning characteristics have the same effect. For example, many motorists following a semi-trailer assume that if the truck moves to the left it is preparing to make a left turn.

Turning CharacteristicsRETURN TO INDEX

In fact, tractor-trailer trucks often swing out to the left as the first step in making a right turn. Particularly when making a tight turn at an urban intersection or when pulling off the street into a driveway, the tractor must swing to the left first to prevent the trailer from riding up over the curb or striking vehicles in the parking lane. Unprepared motorists, however, sometimes misinterpret this preliminary swing to the left as the beginning of a left turn that will allow them to pass the truck on the right. When the truck proves to be turning right, the auto is trapped between the truck and the curb in the "right-turn squeeze."

"One of the most common insurance claims we see are from motorists who get caught in the old 'squeeze' play," says one John Deere Transportation Insurance agent. "We always tell them, when following a truck, observe its turn signals before trying to pass. If it seems to be starting a left turn, don't try to pass on the right before checking to see which way the driver is signalling he's going to turn. It's like that old saying -- 'Do as I say, not as I do.' You should act according to what the truck driver says he wants to do -- not what you think the truck is doing."

One of the most serious misjudgments motorists make about trucks concerns a trucker's field of vision. Many motorists believe that because a truck driver sits twice as high as the driver of an auto, he can see farther ahead and can react more promptly to events as they develop. True, the trucker has a better view over the top of any cars ahead of him. But truckers have serious blind spots motorists do not have -- immediately in front, on either side of the cab, and up to 200 feet in the rear.

A trucker cannot see over or through another tractor trailer in front of him any better than a motorist can, and may have a limited view of the road directly in front of his cab. If he's driving a tractor with a long hood, a trucker may not be able to see the first 20 feet of concrete in front of his bumper - plenty of room for a car to slip unnoticed into a position of danger. Even on some of the more streamlined tractors with sloped hoods or with no front hood (called cab-overs), there can be a blind spot of up to ten feet.

The blind spots on the sides and in the rear hamper a trucker's ability to take evasive action to avoid accidents, so motorists must be careful to ensure their vehicles will be seen. An often-cited rule of thumb for motorists overtaking a semi-trailer is, if you can't see the truck driver in his side mirror, he can't see you.

ManeuverabilityRETURN TO INDEX

Trucks can neither speed up, slow down nor maneuver as nimbly as cars, so truck drivers sometimes must engage in procedures that seem puzzling -- and often infuriating -- to uninformed motorists.

For example, on an expressway or freeway of six or more lanes, motorists sometimes see a semi-trailer truck apparently "hogging" the center lane, even though its comparatively low speeds would seem to warrant use of the right lane. Actually, the trucker is playing it safe. Using the center lane doubles his options if he has to switch lanes in order to avoid an accident. Cars can brake--even swerve-- with relative ease and safety, because braking is quick. Because trucks take longer to brake, a premature shift to an adjacent lane can set up a crack-the-whip effect in which the tractor and trailer can sway out of control, possibly causing a collision or overturning the trailer. Therefore, from his position in the center lane a trucker can better control his vehicle by keeping both adjacent lanes open as escape routes if trouble develops. By doubling his options in this way, the driver protects himself, his cargo and the vehicles around him. It should be noted, however, that many states require trucks to remain in the right lane except when passing.

The enormous size-weight difference between cars and trucks is the major reason why car-truck crashes are so dangerous. When a passenger auto and a large truck are involved in a fatal collision, it's the people in the car who are likely to die. The IIHS reported that 98 percent of the people killed in 2-vehicle crashes involving a passenger vehicle and a large truck in 1995 were occupants of the passenger vehicle.

What these figures mean is that regardless of "whose fault it was," any accident between a large truck and a car is literally "weighted" in favor of the truck. Regardless of how the driver of a truck is behaving, the vehicle itself simply must be respected. Yet most driver education programs fail to inform motorists about the dangers of the car/truck weight differential and provide no instruction on how to deal with it. Motorists go through their driving careers fearing trucks, but without the knowhow that would help them share the road with a greater measure of safety and peace of mind.