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When calculating per capita deaths by vehicles or by drunk drivers, calculate it by per capita drivers, or registered cars; not population. This keeps cities with disproportionally high numbers of people with no access to cars from appearing to have fewer deaths by drunk drivers than they really do; e.g. New York City.
John Nelson analyzed car accidents and deaths by drunk drivers in big U.S. cities by per capita population. His analysis revealed that New York City had a huge population but a very low proportion of vehicle deaths and vehicle deaths by drunk drivers. See his blog at: http://uxblog.idvsolutions.com/2013/01/drunk-driving-in-american-cities.html
He suggested that perhaps NYC has such a low per capita vehicle death rate compared to other cities because it has fewer drivers than would be expected for a city of its size. In fact, about 5x more of metroNYC does not have access to a vehicle compared to other cities (22% vs 4%). This means using deaths per capita drivers is a measure that will more accurately indicate how metroNYC compares to other cities for deaths by drunk drivers.
Data Source: http://factfinder2.census.gov/faces/nav/jsf/pages/index.xhtml
Table: Means of Transportation to Work by Vehicles Available (Universe: Workers 16 years and over in households.) 1. American Community Survey 1-Year Estimates. I used this data as a proxy for drivers or car registrations by MSA. I aggregated it into 2 dimensions, people with access to a vehicle, and those without access. The data set also divides the population by people who use cars to get to work or not.
Prepared using Tableau