Commercial Driver’s License (CDL) · Knowledge Test Overview
Endorsement Tests
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In 30 seconds
Beyond the core tests, each endorsement has its own knowledge test: Hazardous Materials (H) with 30 questions and 24 to pass; Tank Vehicles (N), Passenger Transport (P), and School Bus (S) with 20 questions and 16 to pass; and Doubles/Triples (T), whose count and score vary. Every fixed-count test uses the same 80 percent bar. You take only the tests your job needs, and a few carry extra requirements beyond the written test.
Why this matters
Your CDL class says how big a vehicle you may drive, but an endorsement says what you may carry, and each one has its own knowledge test that must be passed before the letter is printed on your license.
The college version
In normal terms
- Each endorsement letter unlocks one special kind of driving and has its own test.
- H is the biggest at 30 questions and also needs a federal background check.
- N, P, and S are 20 questions each; T varies by state.
- X is not a separate test; you get it by passing both N and H.
Concepts in this outline
- Hazardous Materials (H): 30 questions; passing score: 24 (80%); required for: Hazmat endorsement — Longest endorsement test; hazard classes, placards, shipping papers, emergencies; also requires a TSA check and age 21.
- Tank Vehicles (N): 20 questions; passing score: 16 (80%); required for: Tanker endorsement — Surge, baffles, bulkheads, outage, rollover risk; N plus H yields the X endorsement.
- Passenger Transport (P): 20 questions; passing score: 16 (80%); required for: Passenger endorsement — For vehicles designed for 16 or more people including the driver; paired with a passenger-vehicle skills test.
- School Bus (S): 20 questions; passing score: 16 (80%); required for: School bus endorsement — Stacks on P; danger zones, student loading, railroad stops, emergency equipment, state background checks.
- Doubles/Triples (T): questions vary; passing score varies; required for: Doubles/triples endorsement — Each state sets its own count and score; covers converter dollies, multi-trailer inspection, and handling.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Picture a backpack covered in badges. A CDL endorsement is a badge like that: a single letter on your license. It permits driving the basic license does not cover. That means dangerous cargo, liquid cargo, people, children, or more than one trailer. Each letter is earned with its own knowledge test, and some need more.
The Hazardous Materials (H) badge has the biggest test: 30 questions, 24 correct, or 80 percent. It covers the nine hazard classes, the groups dangerous cargo is sorted into. It covers placards, the warning signs on the truck, plus shipping papers, loading rules, and emergencies. Federal law requires a TSA security threat assessment before a state may print the H. That means fingerprinting and a background check. It takes weeks, so start early. Hazmat also requires you to be 21.
The Tank Vehicles (N) test is 20 questions with 16 to pass. It covers liquid surge, which is liquid sloshing and shoving the truck. It covers baffled and unbaffled tanks, bulkheads, and outage. Baffles are walls with holes that slow sloshing. Bulkheads are solid walls. Outage is room left for liquid to expand. It covers the high center of gravity that tips a tanker easily. Pass both N and H and your license shows an X endorsement, the tank-plus-hazmat combination. There is no separate X test. It lets you haul hazardous liquids in a cargo tank.
The Passenger Transport (P) test is 20 questions with 16 to pass. You need it for any vehicle built to carry 16 or more people, counting the driver. It covers passenger inspection, loading, trips, and emergency exits. The School Bus (S) test is also 20 questions with 16 to pass. It stacks on top of P, because a school bus driver needs both letters. S covers the danger zone around the bus, loading and unloading students, railroad crossing stops, and emergency equipment. Both need a skills test in the matching vehicle. School bus drivers often face extra state background checks.
The Doubles/Triples (T) test is different. Its question count and passing score vary, because each state sets its own. Check your state's CDL manual. The content is steady. It covers converter dollies, the small axles that join trailers, and inspecting them. It covers the crack-the-whip effect, where the last trailer swings hardest. T is a Class A add-on, since you must already be able to pull one trailer.
Endorsement tests are usually on a computer, scored right away. Each is scored on its own, so failing N does not touch your P. Retake rules vary by state. Earn the badges you know you need with your core tests. Adding one later only costs a return trip.
Worked example
Tomas Beltran drives for a fuel distributor and needs to haul gasoline in a cargo tank, which means the X endorsement. Since X is not its own test, he schedules the 20-question Tank test and the 30-question Hazmat test for the same visit, and books his TSA fingerprint appointment three weeks earlier because the background check takes longer than the test.
On test day he passes N with 17 out of 20 and H with 26 out of 30, above the 16 and 24 he needed. The state holds the H until his TSA clearance arrives. When it does, his license prints with an X, and he is cleared to pull a placarded tanker.
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