The problem with incentives is that it's hard to tell that someone who avoided the traffic jam would have (but for the incentive) driven during the traffic jam.

In other words, let's say someone doesn't do the traditional 9-5, but works nights, or from home, or doesn't work at all, or lives 3 states away, or whatever. Do they get the incentive? If you bring them into the pool, then all of a sudden the budget would need to be a lot bigger.

There are some roundabout ways to try to measure this, but it seems much easier to just charge people for driving at peak hours. But that creates other problems, like installing tracking devices in peoples' cars.