However, a car sitting in traffic will produce much more pollution per mile than one travelling at 70mph on a clear road, so the thinking behind this may be flawed.

If the roads are free they fill up until gridlock makes them more painful than the alternatives. If you charge money then the expense becomes part of the cost/benefit calculation. It favors car pooling, rich people (who don't care about the price) and other people who urgently need to get somewhere during rush hour (who can pay the price that one time).

You can also just do car pool lanes, which has some similar benefits.

It's good in cases where you can eventually meet that demand. Say induced demand for broadband leading to more fiber optic cables being laid down.

However there isn't enough space in cities to ever meet demand. What needs to happen is a shift away from the single-car culture in America and towards mass transit.