Blog Post: What the DOT says about Random Drug Testing of Drivers...

HOW DOES RANDOM ALCOHOL TESTING WORK?

Random alcohol testing must be conducted just before, during, or just after a driver's performance of safety-sensitive duties. The driver is randomly selected for testing from a "pool" of subject drivers. The testing dates and times are unannounced and are reasonably spread throughout the year. Each year, the number of random tests conducted by the employer must equal at least 10% of average number of driver positions subject to the regulations.

HOW DOES RANDOM DRUG TESTING WORK?

Employers are responsible for conducting random, unannounced drug tests. The total number conducted each year must equal at least 50% of the safety-sensitive drivers. Some drivers may be tested more than once each year; some may not be tested at all depending on the random selection. Random testing for drugs does not have to be conducted in immediate time proximity to performing safety-sensitive functions. Once notified of selection for testing, however, a driver must proceed immediately to a collection site to accomplish the urine specimen collection.

I got this information from my associate, Tony Holland at Expert Drug Testing.  Feel free to contact him if you need more info.