When I wrote up all of the Transit Master Plan’s High Capacity Transit corridors, many of which the Council has killed for now, I tried to make the point that there is no one super-metric that is arbiter of which corridor is the most important one. On the other hand, I spilled many words explaining one metric, ANC/NR, that may have given people the impression that it was the truly important one.
ANC/NR is basically a measure of how much you’re spending to get another person to ride transit on one of their trips. While that’s a really important consideration, a simple thought experiment illustrates its limitations.
Imagine there are two lines. Both have only two stops, at each endpoint. One is 1 mile long and will attract 10,000 riders. Another is 2 miles long and will attract 16,000 riders.* As they are technically almost identical, line 2 costs the same per mile, or twice as much overall.
More after the jump.
So which is the best to build? There’s obviously a lot of “it depends” since I haven’t specified the problem very well. A lot of people here would, holding all else equal, say “line 1″. And that’s a valid viewpoint; ANC/NR, for sure, would tell you that line 1 is 20% more cost-effective.
But would-be customers of line 2 are almost certainly creating more congestion and spewing out more greenhouse gases as they keep driving. So you could instead measure passenger miles. But at some point you’re inhibiting the growth of dense, non-auto-oriented neighborhoods and applying centrifugal force to the population. The really long trips are commuter trips, and before long you’re building a peak-commute oriented network.
And if you’re a line 1 partisan, we can tweak the numbers to your breaking point. What if line 2 has 19,999 riders? Line 1 still has a miniscule edge in our cost-effectiveness, but do you still think it’s better? If not, you’re conceding that there are other metrics that enter the calculus.
Conversely, someone who prefers line 2 has to address increased ridership on line 2. If there are 30,000 boardings on line 1, that’s still fewer passenger-miles than line 2, but is it really worse?
This is not intended to endorse any particular line. It’s a way to clarify your thinking about what’s important when choosing between various alternatives.
* If you think 1 mile is too ridiculously short for a transit line, feel free to double the numbers.
** Athough the 4th/5th connector has some similarities to line 1 with respect to the other HCT corridors