Tuesday, March 18, 2014

A first look at the "37 billion mile" data challenge, with some maps

MAPC has put together a data set from MassDOT that consists of the entire Commonwealth's vehicle registration data (suitably anonymized), including odometer readings, and then summed it up in quarter-kilometer square blocks. I grabbed a copy of the data and plotted a few simple Google Fusion maps to check it out:

Household Vehicle Miles per Day

You can see that the data is presented in blocks that are 250 meters on a side. MAPC prepared an estimate of how many miles per day each household travels in one of their cars. That can range from zero, for households with no cars, all the way up to this one crazy person who apparently drives 646 miles per day. I have chosen a color scheme such that the miles driven is divided up into quintiles based on city of Boston map squares. A fifth of the Boston map squares fall into the category of "0 to 11.98 mi/day", the next fifth are "11.98 to 17.65", etc. Although the map shows regional data (and state-wide data is available in the complete set), I decided to focus the color scheme on Boston because I am most interested in the car-free and car-light households.

Vehicles per Household

The same largely goes for Vehicles per Household, which is obtained by taking the number of households in each map square and dividing it from the number of vehicles registered to an address in each map square. Note that "households" is defined as the number of households counted by Census 2010, and registered vehicles were geocoded into the various map squares but that process was not always successful. There have been some slight adjustments to the numbers, according to MAPC documentation, to account for the vehicles that could not be accurately pin-pointed. I will trust their models for now.

Car-free Household Percentage

The last one just highlights where the households with the least number of cars are found on the map. I don't think there are too many surprises here, especially for people familiar with my map based on Census/ACS data. The effect of the Green Line is pretty pronounced, it's easy to see the trace of Commonwealth Ave, Beacon Street, and Huntington/S. Huntington Aves. In fact, the abrupt end to blue-colored blocks just past Heath Street seem to indicate that the loss of the Arborway trolley has really taken a toll in terms of increased car ownership around Centre Street. Or maybe it was increased car ownership that led to the cutback. Another interesting pattern is around the new Fairmount/Indigo Line stations. Most of them seem to be near pockets of car-light or car-free households, even though the stations are relatively new. No doubt, that was a key motivation in the planning of the line.

Quick note on outliers: sometimes you may spot a square that is radically different from its surrounding squares. It could easily be a matter of small sample size, or just a local facility that is skewing the data. For example, a nursing home. Or the Four Seasons hotel, which seems to have about 170 vehicles registered between it and the Boston Public Garden. Sanity checks are always a good idea.

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