March incentive spending has decreased 0.4 percent from last month and 1.3 percent from March 2014, according to TrueCar, signaling that most automakers aren't trying to buy additional share as the U.S. market levels off. Photo credit: JOHN WALSWORTH March could be just the sixth month since 2009 in which the U.S. auto industry posts a year-over-year sales decline, a sign the market is starting to flatten out after one of the longest stretches of growth ever.