A sustainable society would not involve single use combustion engine motor cars.
You could look at some of the new more environmentally friendly car technologies such as electric cars, hydrogen powered cars or driverless cars. The entertaining vlog Fully Charged hosted by the wonderful Robert Llewellyn (Kryton from Red Dwarf!) will keep you up to date with latest developments.
Or you could look at different ways of using cars, for example there are many car-sharing sites springing up. For example, Zipcar is the UK’s biggest car-sharing company and provides a more sustainable and usually cheaper option to owning a car.
You could look at behaviours, for example in Cuba everyone is expected to give hitchers a lift and it is rare to see a car that isn’t full, people just stand by the road and get picked up almost straightaway.
You could have a society where almost everyone uses their bike, or has electric bikes or walks or where there is excellent public transport so private motorised transport doesn’t exist anymore.
See the video below for some cool new bike designs
‘Gridlock’ by Ben Elton was one of the first novels to highlight the absurdity of private transport and this issue is the centre of the story, and you could build a story around a plot based on green transport. But it is fine to simply have more sustainable transport options in the background of the story.
Or maybe your characters are enjoying slow travel – a holiday by bike or on a tandem or a young person on their gap year exploring the world by train.