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CDL Study Guides · 2026
General Knowledge is the one section every CDL applicant must pass, no matter which class or endorsement you're after. Get this right and most of the rest of the test gets easier.
Last reviewed 2026-06-20 · Based on FMCSA standards (see sources)
Safe driving in a commercial vehicle starts with understanding how different a large truck or bus handles compared to a passenger car. Commercial vehicles are heavier, longer, and taller. They take longer to stop, need more room to turn, and have larger blind spots. As a CDL driver, you are held to a higher standard because the consequences of a crash are far more severe.
Following distance is one of the most critical safety habits. The general rule is at least one second of following distance for every ten feet of vehicle length at speeds below 40 mph, plus one extra second. Above 40 mph, add an additional second. For a 60-foot truck at 55 mph, that means at least 7 seconds of following distance. In poor conditions such as rain, snow, or fog, double the distance.
Speed management saves lives. The number one cause of truck rollovers is excessive speed in curves and on ramps. When approaching a curve, slow down before you enter it, not while you are in it. Posted speed limits are designed for passenger vehicles — your loaded truck needs a lower speed. On downgrades, use the snub-braking technique: apply brakes firmly when your speed is 5 mph above your safe speed, then release until speed builds again. This prevents brake fade, which is the loss of braking power from overheated brakes.
Blind spots, also called No-Zones, surround every commercial vehicle. The right-side blind spot extends roughly two full lanes and is the largest and most dangerous. The left-side blind spot is smaller but still significant. The front blind spot can extend 20 feet in front of the cab, and the rear blind spot means you cannot see anything directly behind the trailer. Adjust your mirrors before every trip, and check them every 5 to 8 seconds while driving.
When making right turns, remember that rear wheels follow a shorter path than front wheels. This is called off-tracking. Turn wide as you complete the turn, but do not swing left before turning right — that creates a dangerous squeeze point for other vehicles. Left turns in an intersection should be completed past the center of the intersection before turning.
Key points to remember
A thorough pre-trip inspection is required by federal law before every trip. This inspection protects you, other drivers, and your cargo. It is also a significant portion of the CDL skills test.
Start your inspection by approaching the vehicle and looking for overall condition: leaks under the vehicle, vehicle tilt (which may indicate suspension or load problems), and damage to the body. Check tire condition on every axle. Steer tires must have a minimum tread depth of 4/32 inch — all other tires require at least 2/32 inch. Look for cuts, bulges, and foreign objects in the tread or sidewalls.
Inspect the wheels and lug nuts carefully. Rust streaks radiating outward from lug nuts are a telltale sign that the nuts are loose. Missing or broken lug nuts mean the vehicle should not be driven. Wheel seals should not be leaking oil, and hub oil levels should be adequate.
The brake system inspection is critical. Check brake drums and linings through the inspection ports. Brake linings must be at least 1/4 inch thick. Push rods should not move more than 1 inch on disc brakes or more than the maximum allowed stroke for the chamber size. Look for cracked or worn brake hoses and tubing, and check for air leaks by listening for escaping air.
Under the hood, check all fluid levels: engine oil, coolant, power steering fluid, and windshield washer fluid. Inspect belts and hoses for cracks, fraying, or leaks. The steering system should have no more than about 2 inches (10 degrees) of free play in a 20-inch steering wheel with power steering. Check that the exhaust system is secure, not leaking, and not rubbing against fuel lines or tires — a leaking exhaust can let carbon monoxide into the cab.
Windshield and window condition matters for safety and compliance. A cracked windshield is a violation if the crack is in the area cleaned by the wipers on the driver's side, or if there are intersecting cracks. Clean all glass, mirrors, and lights before departure.
Key points to remember
Proper cargo securement is essential to safe commercial driving. Cargo that shifts or falls can cause you to lose control of the vehicle, damage property, or injure other motorists. Federal regulations require that all cargo be immobilized or secured so it cannot shift, leak, spill, blow off, or fall from the vehicle.
Cargo securement uses three main tools: tie-downs (chains, straps, cables), blocking and bracing (materials that prevent movement within the trailer), and friction (the natural grip between the cargo and the trailer floor). For flatbed loads, the general rule is one tie-down for articles 5 feet or less in length, plus additional tie-downs for each 10 feet of length beyond the first 5 feet. The aggregate working load limit of all tie-downs must be at least half the weight of the cargo being secured.
Weight distribution is critical. Overloading any axle can cause mechanical failure, tire blowouts, and handling problems. Place the heaviest items on the bottom and distribute weight evenly side to side. Too much weight on the steer axle makes steering difficult and accelerates tire wear. Too much weight on the drive axles can cause the front to be too light, reducing steering control.
You must inspect your cargo within the first 50 miles of a trip, then every 150 miles or every 3 hours after that, whichever comes first. Sealed or containerized loads that you cannot inspect must have documentation confirming proper securement. For loads that touch the roof of the trailer, check the roof clearance at every overhead structure — bridges, wires, and overpasses can destroy improperly measured loads.
Hazardous conditions for cargo include sudden stops, sharp turns, and rough roads. Always be aware of your cargo when driving. A sloshing liquid tanker, a shifting flatbed load, or an unbalanced container all change your vehicle's handling characteristics and increase rollover risk.
Key points to remember
Federal Motor Carrier Safety Regulations (FMCSRs) govern all commercial vehicle operations in the United States. Understanding these regulations is essential for your CDL exam and your career as a professional driver.
Hours of Service (HOS) rules limit how long you can drive and work. For property-carrying drivers, the key limits are: 11 hours of driving after 10 consecutive hours off duty, a 14-hour window from the start of your workday (you cannot drive after the 14th hour even if you have driving time left), a mandatory 30-minute break after 8 cumulative hours of driving, and a 60-hour/7-day or 70-hour/8-day limit on total on-duty time. The sleeper berth provision allows you to split your 10-hour off-duty period into two periods of at least 7 and 3 hours each.
Your CDL can be disqualified for serious traffic violations. A BAC (blood alcohol concentration) of 0.04% or higher while operating a commercial vehicle results in disqualification — this is half the limit for passenger vehicle drivers in most states. You are also required to submit to alcohol and drug testing: pre-employment, random, reasonable suspicion, post-accident, return-to-duty, and follow-up testing.
The entry-level driver training (ELDT) rule, effective February 2022, requires all new CDL applicants to complete a prescribed training program from a registered training provider before taking the skills test. This includes both theory and behind-the-wheel instruction.
Roadside inspections can happen at any time. Officers may inspect your vehicle, your cargo, your paperwork, and your condition. Violations are recorded in the Compliance, Safety, Accountability (CSA) system and can result in out-of-service orders. An OOS order means you cannot drive until the violation is corrected.
Always carry your CDL, medical examiner's certificate, vehicle registration, proof of insurance, and any required permits. ELD (Electronic Logging Device) records must be available for inspection for the current day and the previous 7 days.
Key points to remember
Commercial drivers must be prepared to handle hazardous road and weather conditions safely. The key principle is always to reduce speed and increase following distance when conditions deteriorate.
Wet roads are the most common hazardous condition. Rain reduces tire traction and visibility. Hydroplaning — where your tires ride on a film of water rather than the road surface — can begin at speeds as low as 30 mph and is more likely with worn tires. When roads are wet, reduce speed by about one-third. If you start to hydroplane, release the accelerator and push in the clutch; do not brake.
Snow and ice require extreme caution. Reduce speed significantly and increase following distance to allow for longer stopping distances. Black ice, which looks like wet pavement, forms on bridges, overpasses, and shaded areas first. If you must drive in snow, use your low beams (high beams reflect off snowflakes and reduce visibility), and keep your windshield and mirrors clear.
Fog is one of the most dangerous conditions because it can reduce visibility to near zero. Use low-beam headlights, reduce speed dramatically, and increase following distance. If fog becomes too thick to drive safely, pull completely off the road and turn on your four-way flashers. Do not stop on the travel lanes.
High winds are especially dangerous for high-profile vehicles like empty box trailers, dry vans, and flatbeds with light loads. Strong gusts can push your vehicle out of its lane or even tip it over. Keep both hands on the wheel, reduce speed, and be prepared for sudden gusts when emerging from cuts, tunnels, or from behind buildings and large vehicles.
Mountain driving presents unique challenges. On upgrades, shift to a lower gear before the hill, not while climbing. On downgrades, use the proper gear and snub-braking technique. Never coast in neutral on a downgrade. If your brakes fail, look for a truck escape ramp — these are built specifically to stop runaway trucks using deep gravel or uphill grades.
Key points to remember
Emergencies can happen without warning. Knowing how to respond can save your life and the lives of others. The most important principle is to stay calm and think through your options before acting.
Tire blowouts are common in commercial vehicles. A front-tire blowout will cause the vehicle to pull sharply to the side of the blowout. A rear-tire blowout is less dramatic but still dangerous. In any blowout: hold the steering wheel firmly, stay off the brake, stay in your lane, let the vehicle slow gradually, then brake gently and pull off the road. Never swerve or brake hard — this can cause a rollover or jackknife.
Brake failure on a downgrade is a critical emergency. Your best option is to use a truck escape ramp if one is available. If no ramp exists, look for an uphill road, soft terrain, or anything that will help slow the vehicle. Downshift to the lowest possible gear. As a last resort, rub the tires against the curb or guardrail to create friction, but avoid head-on collisions at all costs.
Skids occur when tires lose traction. The most common causes are overbraking, oversteering, and overaccelerating. In a rear-wheel skid, take your foot off the brake (or accelerator) and steer in the direction you want the front of the vehicle to go. This is called counter-steering. Once the vehicle straightens, counter-steer again if needed. Never use the engine brake or retarder on slippery surfaces — this can cause the drive wheels to lock.
If you must leave the road to avoid a collision, try to keep one set of wheels on the pavement. If you go onto the shoulder, do not try to return to the road immediately. Slow down, and when it is safe, turn back sharply to get back on the pavement.
In the event of an accident, protect the scene first. Turn on four-way flashers, set out warning devices (reflective triangles) within 10 minutes: one at 10 feet behind the vehicle, one at 100 feet, and one at 200 feet. On a divided highway or one-way road, place all three behind the vehicle. On a two-lane road, place one 100 feet ahead and one 100 feet behind. Report the accident and provide first aid if you are trained to do so.
Key points to remember
Run real practice questions on this section until you're comfortably past 80%.
Practice General Knowledge questionsThe General Knowledge test has 50 multiple-choice questions, and you need 80% — at least 40 correct — to pass. It's required for every class of CDL, so everyone takes it.
Under the FMCSA property-carrying rules, you can drive a maximum of 11 hours after 10 consecutive hours off duty, you cannot drive beyond the 14th hour after coming on duty, you must take a 30-minute break before 8 hours of driving, and a 34-hour off-duty period resets your weekly 60/70-hour limit. These numbers show up directly on the General Knowledge test.
The rule of thumb is one second of following distance for every 10 feet of vehicle length at speeds below 40 mph, plus one extra second above 40 mph. For a 60-foot rig at highway speed that's about 7 seconds. Double it in rain, snow, or fog.
Driving too fast for a curve or an off-ramp is the number-one cause of truck rollovers. Slow down before you enter the curve — posted limits are set for cars, and a loaded truck needs a lower speed.
See also CDL costs by state · Your state's rules · Sources
Based on the FMCSA CDL Manual, a U.S. federal government publication in the public domain. Independent study aid — not affiliated with or endorsed by FMCSA, AAMVA, or any state DMV. Always confirm current rules with your state DMV.