Alcohol, Drugs, and Impaired Driving
Impaired-driving rules are strict, and the thresholds for commercial drivers are lower than for the general public.
The limits
49 CFR 392.5 bars alcohol within 4 hours of coming on duty and any use that leaves a 0.04 BAC or higher while on duty. A result of 0.02 or higher removes you from safety-sensitive duty for 24 hours. Refusing a test is treated as a positive. Under 392.4, no controlled substances, including marijuana even where state law permits it, and no prescription or over-the-counter drug that impairs driving.
Testing program
49 CFR Part 382 requires pre-employment, random (50 percent of drivers for drugs, 10 percent for alcohol), reasonable-suspicion, post-accident, return-to-duty, and follow-up testing. After a qualifying crash, alcohol testing must be attempted within 2 hours and completed within 8, and drug testing within 32 hours.
Consequences
A 0.04 BAC or a refusal disqualifies a CDL holder for one year, or three years if placarded hazardous materials were involved. A second offense is a lifetime disqualification.
Worked example
A driver has two beers at 10 p.m. and reports at 8 a.m. Elapsed time is not the test: the rule bars drinking within four hours of duty, and a 0.04 result is a violation whenever the drinking occurred. If a random test at 9 a.m. reads 0.03, he is removed from safety-sensitive duty for 24 hours and referred for evaluation even though he stayed under 0.04.
Traps
Believing coffee or a cold shower sobers you; relying on state marijuana law; refusing a test because you expect to pass; failing to disclose an impairing prescription.