If London is anything to go by, when congestion charging was introduced many years ago now, traffic went down quite dramatically initially but then slowly built up over the years. The positive thing is that it doesn't feel like it went back to the same level as pre-congestion charging.
But I have no data to support any of this, just my feeling based on what I see from my office in central London, two streets inside the congestion charging zone. I imagine data exist somewhere...
PS - I cycle to/from work and always have. :-)