i`ve been an interstate truck driver for a long long time.
you posess a COMMERCIAL drivers license, it is against federal law to test positive for mmj because of the medical card you have to posess requires a drug test. the local laws in cali might be different, but i`m almost sure that even if you posess a mmj card and a paper from your doctor, your class b license would be invalid, not allowing you to legally drive a commercial vehicle.
i`m not 100% on this as i`ve lost driving jobs for testing positive for marijuana. check your state laws, because i know basically just federal laws about this.