I remember when Ken Livingstone - for yes, it was he, the genuinely visionary, genuinely London London mayor, and not the faux-buffoon Boris 'Benito' Johnson - first introduced the congestion charge, it reduced traffic on London's roads by something like 20% overnight.
It has of course crept back up, as people have become inured, but there's absolutely nothing to stop another genuinely visionary mayor (such as Ken Livingstone, who also, let's also not forget, introduced the so-called Boris bikes) from, say, doubling the charge, which would provide a healthy new revenue stream, along with providing the 'something' that needs to give.
Or, with a bit more political cojones, it would be relatively easy to use London's Stasi-like network of cctv cameras to empower even quite a complex system of exemptions (or the opposite). Eg, zero charges for full occupancy/double charges for single occupancy; exemptions for registered disabled, key social workers, midwives, organ couriers; quadruple charges for Chelsea Tractors and 10x charges for any car with an engine of over 3 litres, and/or whose original purchase price exceeded £100,000...oh yes, a visionary mayor could do quite a lot of good with the CC in one hand and cctv in t'other.