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Combination vehicles are usually heavier, longer, and require more driving skill than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles. In this section, we talk about some important safety factors that apply specifically to combination vehicles.
More than half of truck driver deaths in crashes are the result of truck rollovers. When more cargo is piled up in a truck, the “center of gravity” moves higher up from the road. The truck becomes easier to turn over. Fully loaded rigs are ten times more likely to roll over in a crash than empty vehicles.
The following two things will help you prevent rollover, 1) keep the cargo as close to the ground as possible, and 2) drive slowly around turns. Keeping cargo low is even more important in combination vehicles than in straight trucks. Also, keep the load centered on your vehicle. If the load is to one side so it makes a trailer lean, a rollover is more likely.
Make sure your cargo is centered and spread out as much as possible (Cargo distribution is covered in another lesson of this course). Rollovers happen when you turn too fast. Drive slowly around corners, on ramps, and off ramps. Avoid quick lane changes, especially when fully loaded.
Trucks with trailers have a dangerous “crack-the-whip” effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many accidents where only the trailer has overturned.
Steer gently and smoothly when you are pulling trailers. If you make a sudden movement with your steering wheel, your trailer could tip over. Follow far enough behind other vehicles (at least 1 second for each 10 feet of your vehicle length, plus another second if going over 40 mph). Look far enough down the road to avoid being surprised and having to make a sudden lane change. At night, drive slowly enough to see obstacles with your headlights before it is too late to change lanes or stop gently. Slow down to a safe speed before going into a turn.
Control your speed whether fully loaded or empty. Large combination vehicles take longer to stop when they are empty than when they are fully loaded. When lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels. Your trailer can swing out and strike other vehicles. Your tractor can jackknife very quickly. You also must be very careful about driving “bobtail” tractors (tractors without semitrailers). Tests have shown that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.
In any combination vehicle, allow lots of following distance and look far ahead, so you can brake early. Don't be caught by surprise and have to make a “panic” stop.
Railroad-highway crossings can also cause problems, particularly when pulling trailers with low underneath clearance. These trailers can get stuck on raised crossings:
If for any reason you get stuck on the tracks, get out of the vehicle and away from the tracks. Check signposts or signal housing at the crossing for emergency notification information. Call 911 or other emergency number and give the location of the crossing using all identifiable landmarks, especially the DOT number, if posted.
When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a “trailer jackknife”
The procedure for stopping a trailer skid is:
Recognize the Skid: The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors. Any time you apply the brakes hard, check the mirrors to make sure the trailer is staying where it should be. Once the trailer swings out of your lane, it’s very difficult to prevent a jackknife.
Stop Using the Brake: Release the brakes to get traction back. Do not use the trailer hand brake (if you have one) to “straighten out the rig.” This is the wrong thing to do since the brakes on the trailer wheels caused the skid in the first place. Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out.
When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is called off-tracking or "cheating." Longer vehicles will off-track more. The rear wheels of the powered unit (truck or tractor) will off-track some, and the rear wheels of the trailer will off-track even more. If there is more than one trailer, the rear wheels of the last trailer will off-track the most. Steer the front end wide enough around a corner so the rear end does not run over the curb, pedestrians, etc. However, keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right.
If you cannot complete your turn without entering another traffic lane, turn wide as you complete the turn. This is better than swinging wide to the left before starting the turn because it will keep other drivers from passing you on the right.
Sometimes the road is too narrow and the turn at the intersection is too tight to utilize the Button Hook Method. On these occasions you will be forced to use the Jug Handle Method to complete the turn successfully; however, these instances are rare. You must use the Button Hook Method if at all possible, because it is the safest and most practical way to complete a turn successfully.
Highway Restrictions: Knowing what off-tracking is, and how it affects your vehicle, will help you understand the reason behind highway restrictions. Transportation engineers take into consideration several factors when determining the type and size of vehicles a highway can safely support. Two (2) of the factors engineers consider when setting restrictions on a highway, is the off- tracking and swept path characteristics of the various truck and trailer combinations. Narrow, mountain roads with steep grades and sharp curves present unique challenges for both the driver and the engineer.
Vehicles in excess of the listed dimensions may operate on a highway or section of highway with an over-legal permit. The permit will contain additional operating requirements that must be met such as off-tracking and overall length restrictions. These additional requirements help to ensure the vehicle configuration remains in the lane of travel and not drift into oncoming traffic.
When backing a car, straight truck, or bus, you turn the top of the steering wheel in the direction you want to go. When backing a trailer, you turn the steering wheel in the opposite direction. Once the trailer starts to turn, you must turn the wheel the other way to follow the trailer.
Whenever you back up with a trailer, try to position your vehicle so you can back in a straight line. If you must back on a curved path, back to the driver’s side so you can see.
Look at Your Path: Look at your line of travel before you begin. Get out and walk around the vehicle. Check your clearance to the sides and overhead, in and near the path your vehicle.
Use Mirrors on Both Sides: Check the outside mirrors on both sides frequently. Get out of the vehicle and re-inspect your path if you are unsure.
Back Slowly: This will let you make corrections before you get too far off course.
Correct Drift Immediately: As soon as you see the trailer getting off the proper path, correct it by turning the top of the steering wheel in the direction of the drift.
Pull Forward: When backing a trailer, make pull-ups to reposition your vehicle as needed.