Free CDL Practice Tests (2026)

The rule: at least 1 second for every 10 feet of vehicle length at speeds below 40 mph, plus 1 second above 40 mph. A 40-foot vehicle needs 4 seconds; at 50 mph you add 1, for 5 seconds. Picking 4 means you forgot the over-40 add-on.

Brake until you're about 5 mph below your safe speed — here, 35 mph — then release. When you're back up to 40 mph, repeat. Each application should last about 3 seconds. Dragging the speed way down (to 20–30 mph) means longer, hotter brake applications and more risk of fade.

Loads settle and straps loosen early, so check within the first 50 miles, then after every 3 hours or 150 miles, whichever comes first, and after every break. Checking only at the start and end is how loads end up on the road.

More than about 1 inch of movement where the push rod attaches means the brake probably needs adjustment. Brakes that are out of adjustment cost you stopping power, and 1/4 or 1/2 inch of movement is not the problem point.

In dual air systems, pressure should build from 85 to 100 psi within 45 seconds (vehicles with larger-than-minimum tanks may take longer — check the manufacturer's specs). "3 minutes" comes from the older single-system guideline of 50 to 90 psi at idle, a different test.

Stop and park safely right away while there's still enough air to use the service brakes. If you wait for the spring brakes to kick in, a loaded truck may take a long distance to stop, and a light truck or slick road can put you into a skid. Pumping the pedal uses up air.

Locked trailer wheels caused the skid, so release the brakes to get traction back; once the tires grip, the trailer will follow the tractor and straighten out. Using the trailer hand brake is the classic wrong move — it's more braking on the wheels that are already sliding.

Loss of pressure in the emergency line makes the trailer emergency brakes come on and the tractor protection valve close (the air supply knob pops out), saving the tractor's air. The valve closes — it doesn't open — so the tractor keeps its brakes.

The jaws must close around the shank of the kingpin, not the head. Also confirm there's no space between the upper and lower fifth wheel and that the locking lever is in the lock position with its safety catch engaged. A gap usually means the kingpin is sitting on top of closed jaws.

Class 8 is a Table 2 material. In non-bulk packages, Table 2 materials need placards only when the total reaches 1,001 pounds or more. At 900 pounds, no placards (and no H endorsement) are required — though shipping papers, labels, and markings still apply. "Any amount" is the rule for Table 1 materials.

The distance is 300 feet. You also can never park these explosives within 5 feet of the traveled part of the road, and someone must always watch the vehicle.

Never touch leaking hazmat, and never try to identify it by smell — toxic gases can destroy your sense of smell and can injure or kill you even without an odor. Don't drive a leaking vehicle any farther than needed to get safely off the road.

In a partially filled tank, the liquid keeps moving after you stop. When the wave hits the end of the tank, it pushes the truck in the direction the wave is moving. On ice or other slick surfaces, that can shove a stopped truck into the intersection — so stop smoothly and allow extra room.

Baffles are bulkheads with holes that slow the liquid's forward-and-back movement. They don't stop side-to-side surge, which can still tip a tanker in a curve or quick lane change.

Never load a tank completely full. Liquids expand as they warm, and the empty space you leave for that expansion is the outage. Different liquids expand by different amounts, so outage requirements vary by product.

Buses designed for standing riders must have a standee line — a line on the floor or another way of showing riders they can't stand forward of the rear of the driver's seat. All standing riders stay behind it so they don't block your view or controls.

Stop 15 to 50 feet before the tracks, look and listen in both directions, and open the forward door if that helps you see or hear. After a train passes, make sure no other train is coming on another track, and don't shift gears while crossing.

The interlock applies the brakes and holds the throttle in idle when the rear door is open, and releases when the door closes. It's a safety backup, not a replacement for the parking brake.