If you are involved in an event (accident, crash, etc.) meeting certain criteria of the DOT agency, a post-accident drug test will be required. In most cases you will then have to take a drug and alcohol test if you are involved in a qualifying crash. Each DOT agency may have specific rules with regards to what is a post-accident test.
If you are involved in a qualifying accident, you are required to remain available for this testing and are not permitted to refuse testing.
Remember: Safety-sensitive employees are obligated by law to submit to and cooperate in drug & alcohol testing mandated by DOT regulations.