Today's WSJ includes "American Idle: On the Road" which describes the latest Texas Transportation Institute report on urban traffic congestion. Same old story. If price is not allowed to ration, crowding will. And if policy makers insist on using highway money for expensive rail transit and high-occupancy lane projects, we get worse crowding. (I know; it's supposed to work the other way.)
Traffic conditions are not even worse than TTI reports because land markets do what they are supposed to (when allowed) and prompt origins and destinations to co-locate and disperse from crowded centers.
My friends and I just presented this paper at the Transportation Research Board meetings in Washington DC. We tested the effects of tolling Los Angeles' freeways in the peak hours (we tested 10 cents and 30 cents per mile). It's a simulation on a real network and many substitutions occur. As expected, peak-hour freeway speeds increase, some people switch to surface streets and that traffic slows, some switch to off-peak hours and some (very few) travel less. And politicians take in a lot of money! That's for the 10-cent toll. The 30-cent toll overloads the surface streets. Many other options can be tested, including only tolling some of the freeways.
Planners have voiced concern that tolling the freeways would overload surface streets. There is probably a "sweet spot" that can easily be found. We also plan to look for effects on freight travel as well as travel by income groups.