This can be avoided by making sure employees get their loads checked. Also, new systems exist, such as Global Positioning Systems, that track the drivers and see how long they stop at each location.
Another way employees could act as enablers to employees is if there are drugs readily available, such as at a hospital. Employers of hospital staff could make certain drugs like Valium, Marijuana, and others easily accessible. Actually, this is a common problem today, and somewhere from 10-15% of doctors are projected to be using perscriptions illeagally (Guadagnino).
Some actions can be taken to prevent employers from acting as enablers to employees using drugs. One of these actions is simply having drug tests. These tests measure the amount of illegal drugs in the person's system, and they are very effective. However, some people argue that it isn't ethical, and it would only be successful for employers that want to stop the drug use, not the very few that encouage it.
Currently my job doesn't encourage much drug use, I am a telephone surveyer, and my hours are spend sitting at a desk, being monitored in my calling. However, because it is such a simple job, one could use drugs before going, and it might not be as noticable as other jobs since not much is required to work.