A New York Times article today highlights the perennial misunderstanding embedded in how transit agencies typically measure on-time performance.
By official accounts, 2009 was a banner year for the commuter railroads that serve New York City. Of all the trains that ran last year, the railroads said, nearly 96 percent were on time — one of the best performances since they began keeping records.
But the reality, as nearly any rider would tell you, can be considerably different, and vastly more frustrating.On weekday mornings, 1 in 10 trains entering Pennsylvania Station arrived late, two-thirds by 10 minutes or more. At the peak of the rush, from 8:30 to 9:30 a.m., about 25 percent of New Jersey Transit trains entering Manhattan arrived late; about 2 in 5 of the late trains were tardy by at least 15 minutes.
The explanation, of course, is that peak hour commuter trains are most likely to be late, because they run when the system has the least margin for error. And since most riders of commuter rail are on the peak, most riders experience the system at its least reliable.
In short, 96% on-time performance doesn't mean that 96% of customers get where they're going on time -- only that 96% of trains did, counting nearly empty trains at 10:00 PM when there's plenty of spare capacity on the rails, and a lot fewer causes of delay.
It would not be hard, of course, to calcuate and publicize the actual on-time performance for customers. Just multiply each train's performance by its ridership. The result, of course, would be a much lower number, but one that would give you a better indication of the odds that you'll be delayed when you use a service.