Where the GHG come from:
http://cait.wri.org/figures/US-FlowChart.pdf
Road traffic is the largest source, but not the only source. I suspect that the Director's comments were weighted towards WOW factor and that she may have laid the burden of the GHG reduction entirely on the automotive fleet. She could have said it a different way by saying the only way to hit those targets was to limit the US automotive fleet to roughly half what it is today. But then that wouldn't score any points with her boss now would it?