There is a discussion on the Carfree Network list about induced
demand, which is based on a theory that 'congestion is the only true
limiting factor in unregulated traffic'.

"Amsterdam does not have congestion charging, but it has successfully
reduced traffic to and in the inner city by implementing three
measures, preceded by a referendum early nineties. Introduction of
ever increasing parking rates, approaching € 5 per hour now; reducing
the number of parking places; introducing lots of one way streets. The
results are very good."

"On the other hand, where data for shrinkage is tough to come by,
evidence for the concept of induced capacity, whereby new roadway
space attracts more cars, is thoroughly described in journals and
traffic studies and is far more available."

"As long as people keep packing into the cities, you will have an ever
increasing problem. Traffic congestion is only one of them.

One thing some engineers here in the US have figured out is that there
is no possable solution. It boils down to a simple rule of human
behavior.If you give some one a space, they will fill it... ie. If you
add a road or tunnel to relieve congestion, more cars will show up. If
you cut back on the cars ( high parking fees etc.), then the space
will be filled with some thing else ( bikes, peddacabs, padestrians
etc.) If you want any chance of controling the problem, you have to
work with in this parrameter."