When New York City first issued taxi medallions in 1937, they were just licenses, worth $150 in today's terms. In the years after, life was pretty good for cabbies, as it was for many low-skill employees in postwar America. Some drivers owned their cabs. The rest were unionized employees who worked on commission and received a full slate of employee benefits.
Crucially, the owners were in the taxi business and took on the risk that entailed. If gas prices went up, that came out of the owners' pockets. If drivers had a bad shift, the owners did too.
All that began to change in 1979. That year, New York's Taxi and Limousine Commission changed its rules to allow medallions to be leased out for 12-hour shifts, making cabdrivers "independent contractors" under federal labor laws. The move cost such drivers their benefits, but the real fallout was far more profound. Even for medallion owners who operated their own taxi fleets, the economic value of the right to pick up fares was now severed from the value of actually doing so.
It turns out that the former business is a hell of a lot better than the latter.
Under a medallion lease, the medallion owner has a constant stream of income. Drivers are the ones who suffer when gas prices rise or a cab gets stuck in traffic--they've still got to make their daily lease payments. More importantly, New York's tight limits on the number of medallions in circulation has suppressed the supply of cabs. There are 13,237 medallions now outstanding, a few hundred fewer than in 1937, but a huge supply of drivers competing to lease them.
It's amazing that NYC has the same number of medallions now as in 1937, but apparently SF has only 1,500! Which is more than I thought, I'd have guessed 60.