When it comes to "green" personal (rather than public) transport solutions, the only real options seem to be bicycles and electric cars, but I think the continuum in between those deserves more attention.
Two things have prompted thinking about this: observing the scooter/motorbike traffic in Ho Chi Minh City (Saigon) when I visited in 2009 and visiting the "Electric Transport" shop just around the corner from us here in Oxford.
Having spent a year as a regular cyclist, I'm definitely sold on bicycles as a means of transport. But they have limitations when there are steep hills, for the elderly or infirm, in bad weather, or for moving goods around.
Electric cars may emit less carbon than conventional cars, but otherwise preserve car culture and infrastructure pretty much unchanged; they still involve intensive resource and energy use and multi-ton steel objects moving around at high speed, killing hundreds of thousands of people a year — or regularly spending hours in traffic jams.
The bicycles sold by the Electric Transport shop are not full-on electric bicycles, but power-assisted push-bikes. For legal reasons they are limited to 200 watts (in the UK and Australia, 250 in Europe), so they will help you get up a hill but won't manage that on their own.
The next step up would be the very low power scooters that fill the streets of Saigon (Ho Chi Minh City). Accidents are common here - I saw a few myself just in the couple of days I was there - but happen at such low speeds that people just pick themselves up and get back on their scooters.
In between this and a full-sized car (or even a "Smart car") is an empty niche, for what one might call a buggy or "light car". I'm thinking here of something small and light and cheap, not built to the same standards as an ordinary car but restricted to 40km/hr zoned streets, and maybe physically limited to 50km/hr or even less. These could be electric, and would be small enough to use different sized lanes and parking places to a full-sized car. They would also be designed with co-existence with bicycles and pedestrians in mind.
Making either of these options practical might involve new kinds of licensing specifically for low-powered bicycles/scooters or light cars. It would also require the restriction of suitable areas to speeds of 30km/hr (20mph). Obviously this is not a practical option for dispersed suburbs in Sydney, but it might be feasible for much of London.