CDL General Knowledge
 419 feet

419 feet to stop.
250 feet of headlight.

This is the substance of the General Knowledge test: the three separate legal duties people collapse into "the pre-trip", where the 419 feet at 55 mph actually goes and which part of it obeys the square law, how much space to keep in six directions, what happens to traction in weather and how each kind of skid is recovered, what makes a hazard readable before it becomes an emergency, what the law requires you to carry and where the triangles go, and the two cargo securement rules that have to be satisfied independently.

Read this part
The numbers on this page come from the CDL manual and from 49 CFR. They are minimums and rules of thumb, not guarantees: stopping distances assume dry pavement, good brakes and a driver who is paying attention, and every one of them gets worse in the real world. Get your own state's current CDL manual and read Sections 2 and 3 in full. This page is not a substitute for it and not a substitute for training.
ON THIS PAGE
Inspection: why it is three separate legal duties Speed, stopping distance and the square law Space management, in every direction Seeing, being seen, and driving at night Weather, traction and skids Hazard perception and emergency manoeuvres Stopped on the road, and what has to be aboard Cargo: weight, balance and securement
49 CFR 392.7, 396.11, 396.13

Inspection: why it is three separate legal duties

People treat the pre-trip as one thing. The regulations make it three, at different times, with different paperwork, and the test asks about all three.

  1. 1
    Before driving — 49 CFR 392.7.
    No commercial vehicle shall be driven unless the driver is satisfied that the service brakes and trailer brake connections, the parking brake, the steering mechanism, the lighting devices and reflectors, the tires, the horn, the windshield wipers, the rear-vision mirrors, the coupling devices, the wheels and rims, and the emergency equipment are in good working order. That is a personal duty on the driver and it is not discharged by somebody else having looked.
  2. 2
    At the start of the trip — 49 CFR 396.13.
    The driver must be satisfied the vehicle is in safe operating condition, must review the last driver vehicle inspection report, and must sign it only if defects were reported and certified as corrected or as not needing correction. Signing a clean report is not required; signing one that lists uncorrected defects is how a driver takes ownership of somebody else's problem.
  3. 3
    At the end of the day — 49 CFR 396.11.
    A driver vehicle inspection report is prepared at the completion of each day's work on each vehicle operated, covering at least the service brakes, parking brake, steering, lighting and reflectors, tires, horn, wipers, mirrors, coupling devices, wheels and rims, and emergency equipment. It must identify the vehicle and list any defect that would affect safety or result in a breakdown — but note the 2014 change: "Drivers are not required to prepare a report if no defect or deficiency is discovered by or reported to the driver." The old mandatory no-defect report is gone, and a lot of study material has not noticed.
  4. 4
    En route — 49 CFR 392.9(b).
    This one is a regulation too, and it has hard intervals. Inspect the cargo and its securing devices within the first 50 miles after beginning a trip, and again whenever you change duty status, or after 3 hours, or after 150 miles, whichever comes first. Cargo shifts and securement loosens most in the first miles after loading. Check tires, brakes and lights at the same time.
Two tread depths, and the smaller one is not the front
Steering axle tires need at least 4/32 inch of tread in every major groove. Every other tire needs at least 2/32 inch. People reverse these because 4/32 sounds like the more generous allowance; it is the stricter one, and it applies to the two tires whose failure you cannot steer out of. On retreads, read the scope carefully: 49 CFR 393.75(d) bans regrooved, recapped and retreaded tires on the front wheels of a bus. For a truck or truck tractor only 393.75(e) applies, and it reaches only regrooved tires of 4,920 lb capacity or more.
SECTION 2

Speed, stopping distance and the square law

Total stopping distance has three parts and they do not behave the same way. Perception distance is how far you travel from the hazard appearing to your brain registering it — about 1.75 seconds for an alert driver. Reaction distance is from your brain deciding to your foot arriving on the pedal — three quarters of a second to a second, which the manual costs at 61 feet at 55 mph. Braking distance is from the brakes taking hold to the vehicle stopping.

0 200 400 600 800 0 10 20 30 40 50 60 70 SPEED, MPH FEET 55 mph: 419 ft the manual's stated figure a football field, 300 ft AT 55 MPH Perception 142 ft about 1.75 s to see it Reaction 61 ft about 0.75 s to react Braking 216 ft energy, not time Total 419 ft Only the braking band obeys the square law. From 30 to 60 mph the braking distance goes from 64 ft to 257 ft -- exactly four times, because braking dissipates kinetic energy and that goes as v-squared. The TOTAL goes from 175 ft to 479 ft, a factor of 2.7, because perception and reaction are fixed times and their distances only double. Both bands are drawn from the manual's own 55 mph split.
The manual's figure for 55 mph is 419 feet, made up of 142 feet of perception, 61 feet of reaction and 216 feet of braking. The curve here is computed from that split: the first two bands are fixed times so they grow with speed, and the braking band is energy so it grows with the square of speed.

That difference is the whole reason the rule of thumb has to be stated carefully. Doubling your speed quadruples the braking distance, and quadruples the force of a collision, because both are proportional to kinetic energy and kinetic energy goes as the square of speed. It does not quadruple the total stopping distance, because two of the three components are reaction times and those only double. At low speed most of your stopping distance is you; at high speed most of it is the brakes.

SECTION 2

Space management, in every direction

You cannot steer or brake out of a situation you have no room for. Of all the space around the vehicle, the manual is emphatic that the most important is ahead, because that is the space you are about to occupy.

Direction The rule Why
Ahead One second for every 10 feet of vehicle length below 40 mph. Above 40 mph, add one second. A 60-foot rig therefore needs 6 seconds under 40 and 7 seconds above it. Count the seconds between the vehicle ahead passing a fixed object and your front bumper reaching it.
Behind You cannot control it, so keep to the right and stay at a steady speed where you can. Being tailgated is dealt with by making it easy to pass you, not by speeding up.
To the sides Keep to the centre of your lane. Do not travel alongside another vehicle. Alongside, you have nowhere to go, and you may be in a blind spot.
Overhead Know your height. Empty is taller than loaded. A trailer that cleared a bridge loaded may not clear it empty, because the suspension rises. Repaved roads reduce clearance too, and posted clearances are not guaranteed.
Below Watch for soft ground, drainage grates and railroad crossings. Low clearance under the trailer is what hangs a vehicle up on a raised crossing.
For turns Turn wide as you complete the turn, never as you begin it. Beginning wide invites a car up your right side; you then turn across it. Keep the rear of your vehicle close to the kerb.
To cross or enter traffic Account for length and acceleration, not just the gap. A loaded rig needs far more of a gap than the same driver's car does, and it takes that gap for longer.
Off-tracking, and why the trailer does not follow the tractor
In a turn, the rear wheels of a long vehicle follow a tighter path than the front wheels — they cut the corner. The longer the vehicle, the more they cut. That is why the manual has you steer the tractor well past the point you would in a car before turning, and why the correct technique is to keep the rear of the vehicle close to the kerb and swing wide at the end of the turn rather than the start.
SECTION 2

Seeing, being seen, and driving at night

Do not out-drive your headlights
This is the one place on this page where two numbers from two different sections collide usefully. Low beams show you about 250 feet. Total stopping distance at 55 mph is 419 feet. If a hazard appears at the edge of your low beams at 55 mph you will hit it, and no amount of skill changes that — the arithmetic was decided before you saw anything. The fix is a lower speed, not sharper eyes.
SECTION 2

Weather, traction and skids

Condition What happens What to do
Wet road Stopping distance roughly doubles. Reduce speed by about a third — from 55 to about 35 mph.
Packed snow Traction falls much further. Reduce speed by about a half or more.
Ice Very little traction is available at all. Reduce speed to a crawl and stop driving as soon as you can do so safely.
Hydroplaning The tires ride on a film of water and lose contact with the road. It can happen at speeds as low as 30 mph if there is enough water, and low tire pressure or worn tread makes it worse. Do not brake and do not steer sharply. Release the accelerator and push in the clutch to let the wheels turn freely and regain contact.
Black ice A thin clear layer that makes the road look merely wet. Watch the mirror brackets, antennas and the front of the trailer for ice building up — these ice before the road does. So does the spray stopping when the road is still wet: that means the water is freezing.
Bridges They freeze first, because cold air passes above and below. Treat any bridge as more slippery than the road either side of it.
  1. 1
    Drive-wheel skid, the common one.
    Usually caused by over-braking or over-accelerating on a slippery surface. On a tractor-trailer, the trailer pushes the drive wheels sideways and the rig begins to jackknife. Stop braking. Release the brakes to let the wheels roll and regain traction, then turn quickly in the direction you want the vehicle to go, and be ready to counter-steer as it comes back — that recovery swing is what puts an unprepared driver into the other skid.
  2. 2
    Front-wheel skid.
    Almost always caused by driving too fast for the conditions, and sometimes by worn front tires or a load with too little weight on the front axle. The vehicle goes straight regardless of steering input. The manual is blunt about the remedy: "the only way to stop the skid is to let the vehicle slow down. Stop turning and/or braking so hard."
SECTION 2

Hazard perception and emergency manoeuvres

A hazard, in the manual's words, is any road condition or other road user — driver, cyclist, pedestrian — that is a possible danger. The point of the section is that seeing hazards early converts an emergency into an adjustment, and that most of them are readable in advance.

DO
  • Steer, do not brake, when there is no room to stop. A vehicle can almost always turn more sharply than it can stop.
  • Keep both hands firmly on the wheel and use the least amount of steering that clears the hazard.
  • Countersteer immediately: turn back the other way as soon as you are clear.
  • Go right if you can choose — the roadside is a better place to end up than oncoming traffic.
  • If you leave the pavement, hold the wheel tightly, stay off the brake until you have slowed, then turn sharply enough to get right back on and countersteer at once.
DO NOT
  • Brake hard while steering hard. Combining maximum braking with maximum steering asks for more traction than the tires have.
  • Lock the wheels. On a vehicle without anti-lock brakes, locked wheels cannot steer at all.
  • Assume anti-lock brakes shorten your stopping distance. The manual says they do not necessarily shorten it; what they do is keep the wheels from locking so you can steer while braking.
  • Try to edge gradually back onto the pavement after dropping a wheel off the edge. The manual is explicit: "Don't try to edge gradually back on the road" — the tires can grab unexpectedly and you lose control.
What anti-lock brakes are and are not
ABS keeps the wheels from locking under hard braking, which preserves your ability to steer. The manual's wording is worth copying exactly: it "does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking." It adds no traction, so it does nothing on a surface that has none; it does not let you follow more closely or drive faster. If only the tractor has ABS, or only the trailer, you still get the benefit on that vehicle and you should brake normally.
49 CFR 392.22 AND 393.95

Stopped on the road, and what has to be aboard

Both the equipment and the placement are federal, and both come up on the test. The time limit is 10 minutes from stopping, and you must first switch on the hazard warning flashers immediately.

Where you are stopped Where the three devices go
Two-lane road, or any undivided road One within 10 feet of the vehicle on the traffic side, one about 100 feet behind in the centre of the lane or shoulder, and one about 100 feet ahead, also in the centre of the lane or shoulder.
Divided or one-way highway One within 10 feet of the rear on the traffic side, one about 100 feet back, and one about 200 feet back — all in the direction of approaching traffic. Nothing goes forward.
Within 500 feet of a curve, the crest of a hill, or anything else blocking the view The rearward device goes further out: between 100 and 500 feet from the vehicle, in the direction of the obstruction, so approaching drivers get warning before they can see you.
Fighting a fire, and when not to
Pull well off the road, and do not pull into a service station. Notify emergency services. Keep the fire from spreading before you try to put it out. Use the right extinguisher: B:C works on electrical and burning liquids, which is what you are equipped for. Water on a gasoline fire spreads it; water on an electrical fire will conduct. Stay upwind, keep as far back as the extinguisher's range allows, and aim at the base of the flame, not the flame itself. If it is a cargo fire in a sealed van body, keep the doors shut — opening them feeds it air. And if you cannot control it, get away from the vehicle: the only unrecoverable mistake here is being close to a tire or a tank when it goes.
SECTION 3 AND 49 CFR 393

Cargo: weight, balance and securement

You are responsible for the load whether or not you loaded it: for inspecting it, for knowing it is secure and not overweight, for recognising overloads and poorly balanced weight, and for keeping it that way for the whole trip. The one exception is a sealed load you have not been allowed to inspect — and even then you are still responsible for not exceeding the weight limits.

0 1 2 3 4 5 6 0 5 10 15 20 25 30 35 40 ARTICLE LENGTH, FEET TIEDOWNS 1 tiedown only if also 1,100 lb or less AND, SEPARATELY Aggregate working load limit of the tiedowns must be at least HALF the weight of the cargo. 393.106. Two rules, and passing one does not pass the other. The staircase is 49 CFR 393.110: two tiedowns up to 10 ft, then one more for every 10 ft or FRACTION of 10 ft beyond the first ten -- so a 31 ft article needs five, not four. The single-tiedown case needs the article to be 5 ft or less AND 1,100 lb or less; either one alone is not enough. The working-load-limit rule runs alongside all of it.
The minimum tiedown count from 49 CFR 393.110, drawn as the step function it is. The separate working-load-limit rule in 393.106 is not shown on the staircase because it is not a function of length at all — it is a second, independent test that the same tiedowns have to pass.
DO
  • Inspect the load and its securement before you move, again within the first 50 miles, and at every stop.
  • Use a header board or bulkhead where the cargo could shift forward into the cab.
  • Cover a load that could blow off, and know that some states require covering regardless.
  • Watch specific commodities: dry bulk shifts and rides high; hanging meat is a very unstable high centre of gravity; livestock moves; oversize loads may need permits, escorts and route restrictions.
  • Remember that a partly filled tank surges, and that a smooth-bore tank surges hardest.
DO NOT
  • Assume a sealed trailer removes your weight-limit responsibility.
  • Rely on the count of tiedowns without checking their working load limits.
  • Load the heavy items on top because they are easier to reach.
  • Forget that the securement rules apply for the whole trip, not just at loading. The manual asks for a check within the first 50 miles precisely because that is when it comes loose.
  • Drive a vehicle with a load that blocks your view ahead or to the sides, or prevents you from reaching the emergency equipment.