Actually not that many taxi drivers flunk the annual piss screen drug test which proves that there are no desperate out of control junkie cabbies in New York. Also not many or not any total morons like Garth Johnston and John del Signoer at the Gothamist news website.
Taxi drivers do the urine screen annually. Also not only is it not random, but the drivers themselves schedule the exam which happens the month before their service anniversary.This means the driver decides when he must stop smoking weed or snorting coke .
So that tiny fraction of a percent that do lose their right to drive a taxi are either haplessly out of hand possibly like Mr. del Signore or very low functioning which I suspect describes Mr. Johnston.