Basically, there is no free lunch, a traffic jam is a capacity problem where demand outstrips supply, so unless you assume autonomous vehicles (once collectively networked as part of a national system) are somehow going to be less efficient at using road capacity than the same vehicles being driven by independent humans who have no global information, they will be a better solution. The fact that autonomous vehicles will be able to utilize alternative routes off-highway pre-emptively before actual congestion occurs alone is huge. Nevermind the fact that a large reason traffic jams occur is actually not a capacity problem per se, but due to a propagation of small jams into larger ones, in both time and space, due to human beings controlling the vehicle and propagating traffic waves:
http://en.wikipedia.org/wiki/Traffic_wave
In other words, traffic jams caused by accidents, road blockages, merging, or any other non-capacity related issues will likely no longer occur if the vehicles are autonomously controlled.