Mayor Ken Livingstone's congestion charge has been so successful that Transport for London (TfL) now faces a £65m budget shortfall.
Traffic levels are down 40%
Cars have kept away from the congestion charge zone in central London since the road toll scheme was introduced on 17 February.
Traffic levels have fallen by around 16%, the mayor's office said.
But the lack of drivers paying £5 each day for entering the zone means TfL's budget for the scheme will be £65m short by the end of the financial year, according to the mayor's latest estimates.
During a meeting of the London Assembly's budget committee, Mr Livingstone and TfL bosses acknowledged that the net income generated by the charge would be just half the £130m originally planned.
Liberal Democrat Sally Hamwee, who chairs both the committee and the Assembly, said she was concerned that the mayor seemed to have no rescue plan.
As a direct consequence of the scheme's success, we are likely to be slightly below our original estimates of how much we might make from it
"This is a substantial shortfall and I am quite alarmed," she said.
"The things that get cut in these situations are things that are less high profile, like road safety, provision for pedestrians and cyclists."
There is already a forecast £563m "black hole" in TfL's finances for 2005-6, she said.
A spokesman for the mayor said this year's spending plans would not be affected by the predicted shortfall.
"It is still early days for the congestion charge but the scheme was always designed to reduce congestion and not raise revenue," he said.
"But as a direct consequence of the scheme's success, we are likely to be slightly below our original estimates of how much we might make from it."