Traffic Laws

Study questions with answers

11 sample Traffic Laws questions with the correct answer, a short explanation, and the official handbook reference. Read through them, then take the quiz above.

1. 1. How are your brake lights switched on when you apply the air brakes?

   Correct answer: By an electric switch operated by air pressure

   An air-pressure-operated electric switch turns on the brake lights so drivers behind know you are braking.

   Source: Massachusetts CDL Manual — Section 5.1.12: Stop Light Switch

2. 2. With the system charged, the engine off, and the foot brake fully applied, the pressure on a combination vehicle should not fall more than how much in one minute?

   Correct answer: 4 psi

   With the brakes applied, a combination vehicle should not lose more than 4 psi per minute (a single vehicle more than 3 psi).

   Source: Massachusetts CDL Manual — Section 5.3.3: Final Air Brake Check

3. 3. When checking manual slack adjusters on s-cam brakes, about how much movement where the push rod attaches suggests they need adjustment?

   Correct answer: More than about one inch

   If a slack adjuster moves more than roughly one inch where the push rod connects, the brakes probably need adjustment.

   Source: Massachusetts CDL Manual — Section 5.3.2: Walk-around Inspection

4. 4. On a dual air system running at operating rpm, roughly how long should it take pressure to climb from 85 to 100 psi?

   Correct answer: 45 seconds

   A properly working dual system builds from 85 to 100 psi in roughly 45 seconds at operating rpm.

   Source: Massachusetts CDL Manual — Section 5.3.3: Final Air Brake Check

5. 5. The low-air-pressure warning must come on before tank pressure drops below what level?

   Correct answer: 55 psi

   A visible low-air warning is required and must activate before pressure falls under 55 psi (or one-half the governor cut-out on some older vehicles).

   Source: Massachusetts CDL Manual — Section 5.1.11: Low Air Pressure Warning

6. 6. The safety relief valve sits in the first tank the compressor feeds. At about what pressure is it normally set to open?

   Correct answer: 150 psi

   The safety valve guards the system against over-pressure and is usually set to release at about 150 psi.

   Source: Massachusetts CDL Manual — Section 5.1.6: Safety Valve

7. 7. On a vehicle that has a dual air system, what pressure must the compressor reach in each system before you set out?

   Correct answer: 100 psi

   Allow the compressor to reach a minimum of 100 psi in both systems before you drive.

   Source: Massachusetts CDL Manual — Section 5.2: Dual Air Brake Systems

8. 8. On a trailer with ABS, where is the yellow malfunction lamp located?

   Correct answer: On the left side, at the front or rear corner

   Trailers carry the yellow ABS malfunction lamp on the left side, at either the front or rear corner.

   Source: Massachusetts CDL Manual — Section 5.1.16: Antilock Braking Systems

9. 9. Air-brake trucks, buses, trailers, and converter dollies (other than truck tractors) built on or after which date must have antilock brakes?

   Correct answer: March 1, 1998

   Truck tractors needed ABS from March 1, 1997; other air-brake vehicles such as trucks, buses, trailers, and dollies from March 1, 1998.

   Source: Massachusetts CDL Manual — Section 5.1.16: Antilock Braking Systems

10. 10. With the system fully charged, the engine off, and the parking brake released, the air loss on a single vehicle should stay under how much per minute?

    Correct answer: 2 psi

    In this static leak test, a single vehicle should lose less than 2 psi per minute (a combination vehicle less than 3 psi).

    Source: Massachusetts CDL Manual — Section 5.3.3: Final Air Brake Check

11. 11. During inspection, a brake drum or disc must not have a crack longer than what?

    Correct answer: Half the width of the friction surface

    Cracks in a drum or disc must not exceed one-half the width of the friction surface.

    Source: Massachusetts CDL Manual — Section 5.3.2: Walk-around Inspection