Monday, January 26, 2015

Cities and the Environment--A first order effect?

I was reading a story about peak driving over the weekend.  In the course of reading the story, I discerned that we here in California drive far less than the average American.  In fact, California ranks 41st among the states in per capita driving:

Date are from the Insurance Institute for Highway Safety.

Given the stereotype about California (as a place where everyone drives, always), this was a surprise to me.  But then it dawned on me--when one excludes the District of Columbia (which is kind of like a state, just without representation), California is the most urbanized state in the country.  And so I drew a scatter plot of VMT per capita against urbanization by state:

The negative correlation is quite apparent. To anyone who might be interested, here is the bivariate regression:

       mpc |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
       var4 |  -81.73815    14.1223    -5.79   0.000     -110.118   -53.35832
       cons |   15994.33   1066.959    14.99   0.000      13850.2    18138.47

So a one percentage point increase in urbanization is associated with an 81 mile per year reduction in driving.  I think the direction of causality is not too big a problem here (it is hard to tell a story that more driving causes a reduction in urbanization).  So Matt Kahn, Ed Glaeser and Richard Florida are all right--cities are environmentally friendly!

[BTW, a little Googling led me to a paper that relates to all this].

1 comment:

bakho said...

Driving and suburb/exurb living are mutually reinforcing. Ability to drive leads some to seek homes outside of the city.

The key is that urbanization create transportation alternatives to driving that non-urban areas cannot match because driving alternatives (walking, mass transit) require a high population density.

jonny bakho