As we saw in How would a gas tax reduce consumption? there are a number of ways that higher gas taxes reduce gasoline usage - it was not simply limited to people purchasing new cars.
Some of those ways, such as carpooling and taking less trips would, in fact, reduce the number of cars on the road and in so doing would reduce the number of cars in the road.
A higher cost of gasoline also raises the price of idling, so that should cause some to alter the time of their trip to non-peak hours.
If your main concern is road congestion, however, a raise in the gas tax is a fairly inefficient way of reaching that goal. A better method would be a "congestion tax" such as the one being used in London, England. The tax is not without it's difficulties, such as costs in monitoring, collecting fees, and issues revolving around privacy and civil liberties. But if congestion is your only concern, it is probably a better system.
Living in London, Ontario, reducing road congestion is not a primary (or even secondary) concern of mine - though if I lived in Boston like Dr. Mankiw, I suspect I would feel differently. My concerns lie more with air pollution and greenhouse gas emissions, but there are many reasons to support Pigovian taxes on gasoline.
Next: How would a gas tax reduce consumption by enough to make it worthwhile?
Previous: How would a gas tax reduce consumption?
Main: Gas Tax and Carbon Tax FAQ
