We all know from experience that the severity of a 'traffic jam' is not linearly proportional to the number of cars. Instead, a traffic jam almost seems to be a product of a threshold effect. A given highway can handle any number of cars smoothly up to a certain point. Once that point is reached, additional cars rapidly cause the buildup of a traffic jam. Obviously this does not apply to jams caused by accidents or breakdowns, but most traffic jams aren't caused by those things. Most jams are caused by too many vehicles on too small of an artery.
The implication is that if we were to institute congestion pricing and reduce the overall number of cars by 20%, we may not be reducing the severity of the traffic jam by only 20%. The extra cars that don't show up because of congestion pricing may have been what pushed the highway over the traffic threshold had they been present. By getting rid of that extra 20% of the cars, we could potentially get rid of the majority of the traffic slowdown. This is sound policy on so many levels. Liberals and conservatives should all agree on this, but we don't have this policy yet. Too bad.