Recently, while reading the final environmental impact statement for the Purple Line, I found table 3-1, “Total Daily Regional Transit Trips, 2030/2040.” The table projected that, in 2030, the Purple Line would carry 46,692 trips per day.

I was astonished by this number because almost no light-rail line in the country carries that many daily trips. The average is less than half that number, and the few that carry more than 46,000 daily trips all serve employment centers with well over 100,000 jobs. No employment center on the Purple Line has anywhere near that many jobs.

But now I’ve been taken to task for misreading the environmental impact statement. Apparently, table 3-3, which is titled “Year 2030/2040 Daily Purple Line Boardings," says that the Purple Line will carry, not 46,692 trips per day, but 64,538 trips per day in 2030. So a number that was already too high is supposed to be more believable if it is 38 percent greater.

Purple Line supporters embarrass themselves when they claim that 64,538 daily trips is a credible ridership prediction. In order to sell their projects, light-rail proponents routinely overestimate ridership while they underestimate costs. The Norfolk light-rail line was supposed to carry 10,400 people per weekday in its first year. It actually carried less than 4,400. Denver’s West light-rail line was supposed to carry 19,500 people per weekday in its first year. It actually carried 13,800. Improvements to Baltimore’s light-rail line in 2006 were supposed to generate 44,000 trips per day; they actually carried only 28,500. (Note that none of these come close to 64,000 trips per day.)

Even if the Purple Line managed to defy the experience of all other light-rail systems in the country and attract 64,500 riders per day, the environmental impact statement admits that more than 44,000 of those riders would have been riding transit anyway without the Purple Line. Construction of the Purple Line would result in less than 10,000 “new” daily round trips, that is trips that otherwise would not be taking transit. Yet the line is so expensive that it would be cheaper to give every one of those new daily round-trip transit riders a brand new Toyota Prius every other year for the next 30 years than to build the rail line.

If your goal is to waste a lot of money carrying very few people, then the Purple Line is a great solution. But if your goal is to cost-effectively increase personal mobility while reducing the congestion, pollution, and other social costs of transportation, the Purple Line will actually do more harm than good.