The Census Bureau's recent report on the growth of bike commuting over the 2000-2010 period got me thinking about the criticism of the data, of which there are basically two (not counting sample size):
1. The ACS only counts the “primary” mode. It does not count commuters as bicyclists if they rode only part of the week.
2. The ACS does not count commuters as bicyclists if they rode their bicycle to/from transit and the transit portion was longer.
I don't see the first criticism as much of an issue. If there is reason to believe that there a signficantly more 1 and 2 day bike commuters than there are 3 and 4 day bike commuters (like me), I haven't heard it. In other words, it should all be worked out in the averaging and I don't believe it distorts the results.
But the 2nd criticism is pretty fair, and Brian McKenzie of the Census Bureau admits that during this recent interview, which I only partially listened to.
So is there a way to capture that? Not really, but one piece of data from the 2012 Metrorail Passenger Survey is that 1% of Metrorail users are arriving by bicycle might help us estimate it. aAcouple of caveats first, that data is not availble by jurisdiction the way it is for walking, but since it's unclear to me if it is underreporting or overreporting the number for DC. Another flaw is that it only studies how people get to Metro, not how they leave.
The number of daily DC metrorail users, according to the ACS, was 70,781. Of which 1% would be 708. Adding that to DC's current estimated number of bike commuters would bump DC up to 4.4% from 4.1%. That's a floor.
Of course, that doesn't count people who bike to Metrobus, Amtrak, MARC or VRE. [I'm going to assume that very few (~0) people use their bike in combintation with driving, carpooling, taxis, motorcycles or walking]. But if we assume that a similar 1% of all transit users access the system by bike, then the percentage of DC commuters who use a bike for all or part of their commute goes up to 4.5%. And, that's still undereporting, because it only counts people who have a bike-transit-anything commute, but not people who have an (anything but bike)-transit-bike commute.
I'll be the first to admit that this is not a particularly robust calculation and that much better data would be needed to really tease out the percentage. Furthermore, it does involve allowing for someone to be simultaneosly a bike commuter and a transit commuter, (when I used to do a combo commute, that's how I thought of myself) which means that the total of all reported groups will be more than 100%. but I think the idea that the Census Bureau's measurement is undercounting bike commuters, by ignoring combo-commuters, by 10% is not too far off.
BTW, using the same methodology walk commuters are being undercounted by much more as 32% of all Metrorail riders are arriving on foot (and again that is metro-wide. I'd expect that number to be much higher in the city where there are fewer parking garages). If we add them in, assuming the same breakdown across all transit users, the percentage of DC residents who walk part or all of the way to work goes up from 11.9% to 24.3%.
And it's only fair to mention that this means that car commuters are likely being undercounted too. I'm not sure how many DC residents drive to transit, but there must be some, especially when considering "drop offs".