How many questions are on the District of Columbia CDL knowledge test?

The District of Columbia CDL general knowledge test has 50 multiple-choice questions, and you need 80% (40 correct) to pass. The Air Brakes test is shorter. Most states do not publish its question count, but the 80% pass mark is federal under 49 CFR 383.135 and applies to every CDL knowledge test.

What's the minimum age for a CDL in District of Columbia?

In District of Columbia, the minimum age for a CDL is 18 (intrastate); 21 (interstate). Federal law requires drivers to be 21+ to drive a CMV across state lines or to transport hazardous materials, regardless of state minimum age.

Is Air Brakes an endorsement?

No, and the difference matters. Air Brakes removes a RESTRICTION rather than adding an endorsement. Fail it or skip it and an air brake restriction is placed on the license, barring you from any vehicle with air brakes, which is most heavy trucks and buses. There is no letter code for it as there is for HazMat or Tanker.

Is air brakes a separate endorsement or a restriction removal?

Air brakes is technically a restriction removal rather than an endorsement. If you do not pass the air brakes test, your CDL will carry a restriction code (typically 'L' or 'Z') that limits you to vehicles without air brakes or with only an air-over-hydraulic system. Passing the test removes this restriction.

Do I need to take both a written and skills test for air brakes?

Yes. You must pass the air brakes knowledge test (written/computer) and demonstrate air brake proficiency during the CDL skills test. The skills test includes a pre-trip air brake inspection where you must identify components and perform pressure tests. If you take your road test in a vehicle with air brakes, both are covered.

What is brake lag and why is it important for the test?

Brake lag is the time delay between pressing the brake pedal and the brakes actually engaging. In air brake systems, this delay is about 0.4 seconds because air must travel through the lines to reach the brake chambers. This adds roughly 32 feet of travel distance at 55 mph compared to hydraulic brakes. The test frequently asks about this concept.

What PSI numbers do I need to memorize for the air brakes test?

Key numbers include: the governor cut-out pressure (around 125 PSI), cut-in pressure (around 100 PSI), low-pressure warning activation (60 PSI), emergency spring brake engagement (20-45 PSI), and the maximum air pressure loss rates during the static and applied leak tests (2 PSI per minute for single vehicles, 3 PSI for combinations).

Can I get a CDL without the air brakes qualification?

Yes, but your CDL will carry an air brake restriction, severely limiting the vehicles you can drive. Most tractor-trailers, buses, and large trucks use air brakes, so the restriction effectively excludes you from the majority of CDL jobs. Most CDL training programs include air brake training as standard.

District of Columbia CDL air brakes practice test

Updated for 2026 · 45 air brakes questions of 472 across the full DC CDL path · Aligned to the commercial driver manual · Part of the DC CDL path

Start with general knowledge first. We recommend completing it before drilling air brakes questions.

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Removes the air brake restriction, which most heavy trucks and buses need. Covers the system components, the inspection sequence and how air brakes lengthen your stopping distance.

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2 tests, each drawing from a different question pool. Take all 2 to cover the full 45 questions.

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20 random questions from the air brakes pool. Try it free.

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40 questions covering the full air brakes bank. Take when you're ready for everything.

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About the District of Columbia Air Brakes test

Passing removes the air brake restriction rather than adding an endorsement. The pass mark is the federal 80%, and most states do not publish a question count for it. Questions cover the components of an air brake system (compressor, governor, reservoirs, brake chambers, slack adjusters), how to inspect air brakes, the proper procedure for testing low-pressure warning devices, the brake lag distance unique to air brakes, and emergency and parking brake systems. You will also be tested on the air brake inspection portion of the CDL pre-trip test. This is a separate knowledge test from General Knowledge, required in every state under 49 CFR 383.71, and the DC DMV publishes the District of Columbia CDL manual as the study source for it.