General Knowledge Study Guide
This study guide condenses the AAMVA CDL Manual chapter on General Knowledge into the concepts that show up most often on the knowledge exam. Read it through once, take the practice test, then come back to anything you missed.
Driver qualifications
You must be at least 18 to drive a commercial vehicle within your state and 21 to cross state lines or to haul hazardous materials. You must hold a valid non-commercial driver's license, pass a DOT medical exam (Medical Examiner's Certificate), and self-certify your driving category (most are interstate non-excepted). The federal blood-alcohol limit for CDL holders operating a commercial vehicle is .04 — half the .08 limit for non-commercial drivers in most states.
Pre-trip inspection
A complete pre-trip inspection takes about 30 minutes the first few times you do it; experienced drivers move through it in 15 to 20. Always do it in the same order so you do not miss anything. The seven-step method is: (1) approach and overview, (2) check the engine compartment, (3) start the engine and check inside, (4) turn off engine, check lights, (5) walk-around inspection, (6) check signal lights, (7) start engine and check brakes. The skills test will require a verbal pre-trip inspection in this order.
Stopping distance
Total stopping distance equals perception distance + reaction distance + braking distance. At 55 mph on dry pavement, perception distance is about 60 ft, reaction distance is about 60 ft, and braking distance for a fully loaded tractor-trailer is about 170 ft — for a total of roughly 300 ft, or the length of a football field. Doubling speed roughly quadruples braking distance.
Space management
Maintain at least one second of following distance per 10 feet of vehicle length below 40 mph, and add an extra second above 40 mph. So a 60-foot tractor-trailer needs 7 seconds at highway speed. Bad weather, heavy load, or poor visibility — add more.
Skid control
A skid happens when the tires lose grip on the road. Causes include over-braking, over-acceleration, over-steering, and driving too fast. To recover from a drive-wheel skid: stop accelerating, push in the clutch, counter-steer in the direction you want the front of the vehicle to go.
Railroad crossings
Buses, school buses, and vehicles carrying hazardous materials must stop at every railroad crossing whether or not a train is visible. All commercial drivers should never shift gears while crossing tracks. Never start across a crossing unless you have room to clear it without stopping on the rails.
How to use this guide before your exam
Read each section carefully and try to put the rule in your own words before moving on. The CDL knowledge exam tests recognition more than recall — you'll see the right answer in front of you and have to pick it from distractors that all sound plausible. The way to defeat distractors is to know the underlying rule cold.
Once you can read this guide and answer "what's the rule?" without checking, return to the full practice test. If you score 85% or higher across two consecutive runs, you are ready to schedule the official knowledge test at your DMV.