Not exactly a steady decline, quote from the NPR article linked above:

In its own early estimates that were released late in 2015, NHTSA reported seeing a 3.5 percent rise in vehicle miles traveled in the U.S. But it also said the fatality rate — the number of deaths per 100 million vehicle miles traveled — was rising at a slightly higher pace of 4.4 percent.

Maybe the average number of people riding the same vehicle has increased?