What is the legal blood alcohol concentration limit for operating a commercial vehicle under federal regulations?

Federal regulations set the blood alcohol concentration (BAC) limit for commercial drivers at 0.04 percent, half of the 0.08 percent limit that applies to non-commercial drivers in most states. A BAC of 0.04 or higher in a commercial vehicle is a per se violation, meaning the BAC alone proves impairment under the law. Below 0.04 but with any detectable alcohol, a driver must be removed from duty for 24 hours. Some states impose stricter limits, including zero tolerance for under-21 drivers. The lower commercial limit reflects the greater consequences of impaired commercial driving: heavier vehicles, longer stopping distances, and higher kinetic energy at any speed. Refusing a federally mandated alcohol test is treated the same as a positive test.