The congestion charge has cut the number of cars travelling into central London by about 60,000 a day, according to a new study.
The survey revealed that 106,200 penalty notices are issued a month
But Transport for London (TfL) now accepts it will only raise about half of the £130m which was predicted for the scheme's first year.
In a round-up of the first six months of congestion charging, TfL found it had cut congestion in central London by 30%, making driving through the zone quicker.
More than half of the 60,000 missing drivers are thought to have switched to buses or the Tube, while the rest are either driving around the zone, have bought a motorbike or have started car shares.
The survey also rejected suggestions that falling trade within the zone was caused by the scheme, which charges drivers £5-a-day to travel into central London.
The report said the charge was only to blame for 7% of the drop-off in trade.
Other factors which it said were responsible included the three-month closure of the Central Line, fewer overseas tourists, the general "economic slowdown" and even a spell of hot weather.
Mayor of London Ken Livingstone, who staked his political career on the success of the charge, said those supporting the charge now outnumbered opponents two-to-one.
He said: "It has helped to get London moving again after years of choking traffic.
"London has become the first of the great world cities to set about substantially reducing congestion in the central area."
The survey also revealed that 106,200 penalty charge notices are being issued a month - about 2,000 of which are appealed.