We have always known intuitively that people who live downtown drive less, but this graph shows how dramatic the effect actually is: If you live in a single family detached house you are likely to drive 15,000 miles per year; if you live in townhouses a third less; if you live in highrises less than a third.
But the most interesting factor is that we don't all have to live in "Soviet-style concrete-block high-rises and be forced to take state-run streetcars to their little jobs at the mill", as one Bloomberg correspondent described it. Just moving to townhouses or midrises makes a huge difference.
But density isn't the only factor, or even the main one. You can't put a "high-rise in the middle of a cornfield." As Kaid Benfield writes in NRDC Switchboard:
Neighborhood density works best when combined with synergistic elements such as location, transit service, connected streets, and mixed land uses.
More at NRDC Switchboard