One of the steps that the City of New York took in response to Wednesday’s rain-related subway trouble was to authorize livery cabs, which are usually permitted to respond only to telephone or radio dispatches, to pick up passengers who hailed the cars on the streets. It was a fine idea, but why only in the rain? If a free market in taxicabs is a good idea when the subway isn’t working well, it’s a good idea the rest of the time, too. It would help ease shortages those times in the city when it is hard to get a taxicab, whether it is New Year’s Eve or a snowstorm or when a big convention is in town or during the afternoon shift change. The city government doesn’t set limits on the number of restaurants or newspapers or clothing stores in the city, and as a result we have a healthy supply of all those. But the limit on the number of taxi medallions — and thus the number of vehicles authorized to accept street hails — assures a shortage of supply. The best solution would be to allow livery cars to pick up street hails even on a sunny day.