In the 1930s, New York City instituted a system of licensing for taxicabs: only taxis with a “medallion” were allowed to pick up passengers. Because this system was intended to assure quality, medallion owners were supposed to maintain certain standards, including safety and cleanliness. A total of 11,787 medallions were issued, with taxi owners paying $10 for each medallion.

In 1995, there were still only 11,787 licensed taxicabs in New York, even though the city had meanwhile become the financial capital of the world, a place where hundreds of thousands of people in a hurry tried to hail a cab every day. An additional 400 medallions were issued in 1995, and after several rounds of sales of additional medallions, today there are 15,000 medallions. The result of this restriction is that a New York city taxi medallion is a very valuable item: if you want to to operate a tax in the city, you must lease a medallion from someone else or buy one, with a current price today of over a million dollars.

It turns out that this story is not unique; other cities introduced similar medallion systems in the 1930s and, like New York, have issued few new medallions since. In San Francisco and Boston, as in New York, taxi medallions trade for seven-figure prices.

A taxi medallion system is a form of quantity control, or quota, by which the government regulates the quantity of a good that can be bought and sold rather than the price at which it is transacted. It is another way that government intervenes in markets along with price ceilings and price floors. The total amount of the good that can be transacted under the quantity control is called the quota limit. Typically, the government limits quantity in a market by issuing licenses; only people with a license can legally supply the good.

A taxi medallion is just such a license. The government of New York City limits the number of taxi rides that can be sold by limiting the number of taxis to only those who hold medallions. There are many other cases of quantity controls, ranging from limits on how much foreign currency (for instance, British pounds or Mexican pesos) people are allowed to buy to the quantity of clams New Jersey fishing boats are allowed to catch.

In the real world, quantity controls set an upper limit on the quantity of a good that can be transacted. Some attempts to control quantities are undertaken for good economic reasons, some for bad ones. In many cases, as we will see, quantity controls introduced to address a temporary problem become politically hard to remove later because the beneficiaries don’t want them abolished, even after the original reason for their existence is long gone. But whatever the reasons for such controls, they have certain predictable—and usually undesirable—economic consequences.