Transport for London (TfL) is looking into the idea of a green travel budget. People could gain points on their Oyster cards for walking, cycling or taking public transport instead of driving.
These could then be redeemed to get money off the charge. It is just one idea that TfL is looking at to make charging more acceptable.
TfL managing director of planning Michele Dix admitted that public opinion is a big driver in future changes to the scheme.
"While we can make provision for potential future charging, our biggest issue is convincing people of the need to do so," she explained. "Charging that is linked more closely with actual use would be more acceptable for the public."
TfL is also looking at ways to ensure that the congestion charge is compatible with possible future national road pricing measures. A system in which tags on vehicles respond to radio signals from roadside beacons could be in place by 2010, Dix added.
Meanwhile, the government has shortlisted eight companies for next year's piloting of the technology that will be required to implement a national road user charging scheme.
Transport minister Rosie Winterton noted: "We will be working with the bidders to assess what their capabilities are and how they can best demonstrate them."