Mary Barra: GM's next CEO?

FORTUNE -- On Friday afternoons, a group of top GM (GM) executives and engineers leave headquarters in downtown Detroit and heads for GM's proving grounds in Milford, Mich. for "knothole drives." Originated by Mark Reuss, head of GM's North American operations, the drives allow the executives to test new vehicles -- along with their competitors -- in successive stages of development and offer an assessment. The drives act like gates. MORE