More people driving more cars equals more pollution, right? Not necessarily. It seems paradoxical to many people, but we can grow, and grow cleaner. Case in point from the EPA:
The same pattern holds for safety. Figure 12 from this rather large pdf depicts something very interesting. From 1967 to 2000 vehicle miles traveled shot up dramatically (from about 964 billion to 2.9 trillion), yet during the same period highway fatality rates shot down dramatically (from 5.4 to 1.5). For more on this, see the Peter Gordon paper I referenced earlier.