One complaint about congestion pricing is that it would push people out of the market. But in theory this is not necessarily so. The common sense story is that the price of driving rises, so the quantity of driving demanded decreases, and thus less people drive. But it depends how people shift their behavior in response to higher prices. There are many ways to respond to higher driving costs that don’t mean less total people travelling by car.
For instance, more individuals could begin carpooling. A new study points to an increasing trend in carpooling, and finds that gas prices are indeed inversely related to carpooling. The charts below show that the trend over the past few years has been of decreasing single-drivers, and that carpooling has been the predominant alternative to single-drivers, with the percent carpooling to work doubling from 4% to 8% (and shame on the author for not using 0% as the Y-axis intercept!):
Another way people could respond to higher prices is to drive during times when prices are lower, that is if prices are set in relation to congestion. Neither this nor carpooling represents any decrease in quantity, if quantity is taken to mean the number of people using non-public transportation automobiles for travel.
In addition, there are currently people who would prefer to drive places, but due to the time-cost of money decide not to drive there because they don’t want to wait in traffic. Driving could increase among these people.
In the end the number of people traveling by car in response to congestion prices is an empirical question, and there very well may be conclusive evidence that, contrary to the arguments I’ve presented here, the relationship is negative. I would be interested to see any such studies.