CDL Study Reference — Alabama

CDL Key Numbers to Know

Every critical number, limit, and threshold you need to memorize for the Alabama CDL knowledge exam — air brake pressures, weight limits, HOS rules, BAC limits, and more. Bookmark this page.

Test Yourself →
💡

How to use this reference

These are the numbers that appear most frequently on the Alabama CDL knowledge test. The exam tests specific values — not just concepts. Cover the right column and quiz yourself on each number until you can recall every one without looking. Then take the practice tests to confirm you understand the context.

📋 Alabama CDL Permit Test
50 Total questions on the knowledge exam
40 Correct answers needed to pass (80%)
$25 Knowledge test fee at ALEA
14 days Minimum time to hold CLP before skills test
No limit Time limit on the Alabama CDL knowledge test
3 attempts Maximum tries before waiting period may apply
🔧 Air Brake Pressure
125 psi Governor cut-out — compressor stops pumping
100 psi Governor cut-in — compressor resumes
60 psi Low air warning must activate BEFORE this point
20–45 psi Spring brakes automatically apply in this range
0.4 sec Air brake lag time — vehicle still moving during this delay
~32 ft Distance traveled during brake lag at 55 mph
💨 Air Brake Loss Rates
2 psi/min Max loss — single vehicle, brakes RELEASED
3 psi/min Max loss — combination vehicle, brakes RELEASED
3 psi/min Max loss — single vehicle, brakes APPLIED
4 psi/min Max loss — combination vehicle, brakes APPLIED
⚖️ Weight & Size Limits
80,000 lbs Maximum gross vehicle weight — Interstate highways
34,000 lbs Maximum tandem axle weight
20,000 lbs Maximum single axle weight
12,000 lbs Typical maximum front/steer axle weight
53 ft Maximum trailer length — most Interstate highways
8 ft 6 in Maximum vehicle width — federal standard
13 ft 6 in Standard maximum vehicle height
🔄 Tires & Steering
4/32 in Minimum tread depth — STEER axle tires
2/32 in Minimum tread depth — all other axle tires
2 inches Maximum steering wheel play — 20-inch wheel (10°)
1 inch Maximum slack adjuster movement under hand pressure
4.5 in Maximum steering wheel play — 22-inch wheel
🚛 Following Distance
1 sec / 10 ft Minimum following distance per 10 feet of vehicle length
7 seconds Minimum following distance for a 70-ft vehicle at 55 mph
4 seconds Add for speeds over 40 mph in adverse conditions
Double Multiply following distance in rain, fog, or snow
⚠️ Safety Distances
300 ft Minimum distance from explosives to bridges / tunnels / fires
15–50 ft Required stop distance from railroad tracks — buses & HazMat
5 ft Minimum distance from fire hydrant when parking
500 ft Minimum distance from other vehicles — explosive cargo in tunnels
🛢️ Tanker & Endorsements
1,000 gal Total tank capacity requiring Tanker (N) endorsement
119 gal Individual tank capacity threshold for N endorsement
16 passengers Threshold requiring Passenger (P) endorsement (incl. driver)
1,001 lbs Many HazMat categories require placard above this weight
26,001 lbs GVWR threshold — CDL required above this
10,000 lbs Trailer GVWR separating Class A from Class B
⏱️ Hours of Service
11 hours Maximum driving time after 10 consecutive hours off
14 hours Maximum on-duty window — cannot drive past this
10 hours Required consecutive off-duty time before driving
30 minutes Required break after 8 hours of driving
60 hours Maximum on-duty in any 7 consecutive days
70 hours Maximum on-duty in any 8 consecutive days
34 hours Restart — resets 60/70-hour clock after off-duty period
🚫 BAC & Legal Limits
0.04% CDL holder BAC limit while operating a CMV
0.08% Standard legal limit (CDL holders out of service at 0.04%)
24 hours Out-of-service if BAC is between 0.02–0.039%
0.00% Required BAC for school bus and HazMat operations (some states)
🔍 Pre-Trip Inspection
3 triangles Emergency reflective triangles required in cab
B:C rated Minimum fire extinguisher rating — HazMat vehicles
5 lbs Minimum fire extinguisher size for many commercial vehicles
~30–50 min Typical time for full CDL skills test pre-trip inspection
7 days How long the previous DVIR must be kept in the vehicle
📐 Vehicle Control Numbers
55 mph Reference speed used in most CDL distance calculations
20 mph Maximum speed to test hydraulic brake failure with hand valve
5 mph Maximum reverse speed during basic vehicle control skills test
2–3 sec Ideal braking application length on long downgrades
15 mph Approximate minimum speed for hydroplaning risk

Now Put the Numbers to the Test

Knowing the numbers is half the battle — understanding how to apply them in questions is the other half. Take the free practice tests to see both.

Take a Practice Test → Find a Training School