Lack of more detailed data cripples the ability to discuss DC parking issues intelligently

In the thread of the GGW entry "Less parking needed for housing atop Metro, not more," which in turn inspired my entry "1953 brochure, Downtown Washington DC Parker's Guide," there is some cavalier use of DC car registration data by one of the commenters.  I use the word "cavalier," because I think there is an intent to use data not to illuminate the issue, but to obfuscate, because there is no attempt to provide context to the data.

According to one of the comments, this is DC car registration data for the past few years:

2009 268,974
2010 275,043
2011 279,787
2012 284,905

About 62% of DC households are multiunit.  There are 2.13 people/household on average.  It was reported that in 2012, DC added about 1,000 new residents each month,.  It's not likely that the average number of residents per new household is 2.13, so the following calculation is overstated, but the number of new households is 5,633 and so each new added household brought slightly less than one car per household, at the city's average.  Over time, new households in certain areas of the city are more likely to shed vehicles. 

The fact of the matter is that we don't have good data on what DC car registrations mean exactly.  Not to mention the point that I make frequently that lack of data on curb space inventory throughout the city also makes it close to impossible to make data-based recommendations.

Averages in the data cover up important specifics.  Car ownership demographics vary considerably, and factors influencing the number of cars per household include:

• type of dwelling: single family attached; single family detached; multiunit; the existence of off-street parking on site;

• neighborhood (rowhouse or apartments or single family, mixed);

• average household size, the number of working adults; age; and presence of young children;

• location, both in terms of "core" of the city vs. the more suburban "outer" part of the city (see "DC as a suburban agenda dominated city" and "Understanding why Upper Northwest DC residents don't buy into the sustainability mobility paradigm") as well as in more in-demand locations vs. lesser demand locations (e.g., Upper Northwest vs. Fort Lincoln);

• proximity to a heavy rail transit station.

In short, single family housing has greater car registration on average than mulitunit housing, older households may have fewer cars, there is more demand for parking in economically healthy neighborhoods; denser housing with more population has greater demand for parking than neighborhoods with fewer houses, etc.

Seattle has compiled data on the number of parking spaces per block and per neighborhood.  To the best of my knowledge, DC has not compiled or released comparable information.  Below: Parking survey data from the Seattle Transportation Strategic Plan


In the GGW thread, OtherMike writes:

Vehicle ownership in DC is increasing, and even if the average number of vehicles per household in DC is less than other cities, it is still much higher than the current minimum parking requirements.

Yes, that might be true, but I expect that the average household car ownership in DC is significantly higher in single family housing neighborhoods, and that shapes the city's average.  Basically, the data is almost meaningless without more specificity.  

Above: this household on my block has been parking this out-of-city registered vehicle on the back of their property for years.


We have to use 2011 data, because that Census data for the number of dwelling units is from that year.

2011 car registrations =   279,787
2011 households         =   298,902
                                    =   0.93 cars/dwelling unit.

(Note that some households might have cars not showing up in these statistics because they are registered in other states.  If residents hold a DC driver license, for the most part, this is illegal, at least in terms of registration and car insurance requirements.  On the other side of the block, which I didn't census because I don't know the residents there, two vehicles parked off-street on our shared alley don't have DC plates, one is a "take home" work vehicle.)

The national average of cars/household in 2008 before the start of the recession was 2.33. The national average is projected to be 2.07 cars/household through 2025.

So there is no question that DC has significantly fewer cars registered per household on a national basis.

Sunday morning and people going to church and doing other things reduces the number of cars normally parked during the evening and weekends on my block.

Recognizing that you can't definitively generalize from one block to the entire city, I did a census of car ownership on my face block.  There are 12 houses on each side of the block; the 4 sides of the block has 32 houses.  Corner houses may have off-street parking access (through a curb cut) even if they don't have alley access.  A couple houses don't have alley access and don't have curb cuts either.

The block is 0.8 miles from the Takoma Metro, is zoned R1--houses are detached single family dwellings with large lots, and is in Ward 4, which has numbers of car registrations than wards in the core of the city.

This car on my block hasn't been moved in many months.  There was a period when two or three of the four tires were flat.  The car registration is current.

There are 32 cars for 24 houses.  One vehicle doesn't work and is parked off-street (it probably doesn't have a current registration either).  3 houses don't have cars (ours, a vacant house, and a house owned by a guy in his 80s, although he has visitors and attendants who tend to have cars).  That's 1.5 cars per dwelling unit, which is 0.57 higher than the city average.  (The number was a little higher before when certain households stored cars on the street for other family members.)  The block doesn't have Residential Parking Permit restrictions, although overnight parkers are still subject to other city regulations about vehicles registered in the city.  A couple cars on the street haven't moved for months and months even though they are registered

This car is stored on the back part of this property and hasn't been in use for many years.  It overhangs slightly on public space (the alley), so the city could tow or cite it.

Likely this data is generalizable to similar blocks of R1 housing in the outer city.   Probably more blocks have more cars than ours, especially if the households are comprised of younger adults.  Our block has a big retiree population, and this impacts the number of cars per household.

Multiunit buildings have fewer car registrations per unit on average than single family housing.

That's why zoning requirements for parking minimums currently impose reduced requirements--it is not even one space per unit let along two spaces per unit.

Imposing higher requirements on multiunit buildings as argued by the most vociferous opponents of changes to DC's zoning requirements for parking, when multiunit housing generates far fewer car registrations per household than single family housing, makes no sense.