Myth vs. Fact: Where’s the Data? 1

Myth-understanding: OP is using the wrong vehicle data to justify its parking recommendations.

Fact: OP’s data about vehicle usage is derived from the U.S. Census Bureau, and we’re pretty confident in it. We’re labeling this one a “myth-understanding” because there appears to be some confusion around this issue, and reasonable people might have interpreted information differently. Here are the facts: in October 2012, a citizen asked OP some questions about vehicle data presented in a DDOT presentation. The slide, derived from Federal Highway Administration data, listed a figure of 150,000 vehicles in the District. OP uses Census data to estimate vehicle availability. And, different agencies using different sources of data seems to have caused some confusion. We want to clear up this myth-understanding, since we’ve heard some claims that OP has used “skewed facts and figures” to make “fallacious” claims that there “has been a significant reduction in registered passenger vehicles.”

Here are the numbers of vehicles available to District households, from 2005 to 2011 (all numbers are from the American Community Survey’s one-year data):

Year     Aggregate Number of Vehicles Available
2005                              218,961
2006                              227,209
2007                              229,069
2008                              225,490
2009                              223,642
2010                              226,783
2011                               224,073

Based on this data, it appears that the total number of vehicles (this includes SUVs, pick-up trucks, and motorcycles) available to District households peaked in or around 2007, and has been mostly decreasing since then. Over the same period, we’ve added more than 50,000 residents and more than 20,000 households. As a result, the number of vehicles per household has dropped slowly over several years – it was about 0.91 vehicles per household in 2010 and about 0.83 in 2011. In short, we’re adding far more new households than cars.

The total number of vehicles available to District households peaked in or around 2007

The total number of vehicles available to District households peaked in or around 2007

We also have large numbers of people who live in DC without cars. The percentage of households in the District without access to a private vehicle has remained relatively constant over the past several years: it was 36% in 2006 and 35% in 2010. According to the most recent data, it went back up to nearly 39% in 2011. Remember, those are citywide averages. In some Census block groups, that figure is under 20%. In others, it’s more than 80%. Not surprisingly, block groups with lower ownership rates tend to cluster around areas of high transit access (of course, income is also a major explanatory variable in car ownership).

So, when we say that the data supports the notion that reduced (or no) minimum parking requirements are justifiable in areas where transit, walking, and cycling are widely available and vehicle ownership rates are low, we’re on pretty solid ground. Regardless of whether the vehicle count is 150,000 vs. 231,000, the total number really isn’t what we’re basing our recommendations on. We have never said “DC only has 150,000 cars, and therefore we don’t need more parking.” What we’ve said is, “the number of cars per household is dropping, we have more families choosing to live car-free, and therefore each additional increment of development does not need to provide as much parking per dwelling unit/customer/employee as it did over the past 50 years.”

Of course, as we have pointed out repeatedly, the absence of a minimum parking requirement is not the same thing as the absence of parking. Some percentage of residents will continue to use private vehicles and will demand places to park them. Some will not. Both types of households will continue to find places that suit their needs. OP wants to ensure that DC residents can choose the living and parking situation that works best for them.

One comment

  1. The important thing to know about American Community Survey (ACS) 1 year data is that they involve extrapolations based very small samples of the relevant population and, as a result, they have a very high margin of error. In this particular series, in all but one instance (2005-06), the margin of error is much larger than the year-to-year variations.

    When the Census Bureau itself presents ACS data it always comes with a qualifier. Here that qualifier basically says “give or take 6,000 — 7,000
    cars. Oh and there’s a 10 percent chance the actual number falls outside even that range.”*

    The only credible claim that can be made from data this sketchy is that car ownership in DC has been relatively stable from 2007-2011.
    And the more trustworthy decennial census data supports that claim. In both 2000 and 2010, DC residents averaged .9 cars per household.

    As an aside, I’d be really hesitant about forecasting a long-term trend re
    car-ownership from post-2007 data. The domestic market for new cars tanked in 2008 (recession) and continued to decline in 2009. The market’s rebounding and this year sales are starting to approach 2007 levels. Next year’s sales are anticipated to be even higher.

    Sue Hemberger

    *I’m simplifying this for readability’s sake. Each estimate comes with a single margin-of-error stat — for this series, those numbers range from +/- 5886 to +/-7244. I’d also add that the 90% confidence interval used for ACS statistics is lower than usual. Media polls, for example, typically use a 95% interval (which means half as much chance — 5% — that they’re off by more than the margin of error.) Choosing a 95% confidence interval for this data would have raised the margin of
    error to as high as +/-8620 cars.

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