Cabbies on heroin? Holy shit you couldn't drive a cab on heroin. I've never had to concentrate so hard on any job. There are live people in the back who might rob or kill you, or you're fighting traffic, or fatigue from long hours.
I knew a lot of cabbies in Denver. One day we were all sitting around at the bus station on a very slow day, and the conversation fell silent. The senior driver (30+ years) blurted out "Shit! If I was a junkie I'd go get high!" and we all busted up. The last thing any of us would do is narcotics.