This paper investigates how much spectrum should be available for Intelligent Transportation Systems (ITS), and whether part of that spectrum should be shared with unlicensed devices, as has been considered by the U.S. Federal Communications Commission (FCC), and if so, what sharing scheme should be adopted. We found that the ITS bandwidth that maximizes social welfare could be either much more or much less than what has already been allocated, because optimal bandwidth is sensitive to uncertain factors such as device penetration, future data rates, and spectrum opportunity cost. That uncertainty is offset if ITS spectrum is shared under a scheme of coexistence among equals. We also found that the bandwidth required to obtain given throughputs on shared spectrum can be considerably less than the bandwidth to obtain the same throughputs in separate bands. We conclude that the spectrum available for ITS should be maintained or increased, but much of ITS spectrum should be shared with non-ITS devices.