We can use the data here to normalize against the decrease in auto fatalities across the entire population: http://www.census.gov/compendia/statab/2012/tables/12s1103.p...

From 2000 to 2009, the # of traffic-related deaths per 100,000 resident population dropped by 26%. Assuming the decay is exponential, that's about 3% per year. In contrast, the # of teen deaths dropped by about 4.5% per year.

So we can reasonably conclude that teenage auto-related deaths are falling at a faster rate than overall auto-related deaths.