Cars aren't going anywhere. Achieveing a 20% reduction would be great, but people in developing countries are only now getting started with cars. The only choice: what kind of cars?
Public transport is good, the nearest city to where I live has free public transport (Tallinn, capital of Estonia) but people still use cars. Public transport cannot get everywhere.
I propose a few test cases: try transporting someone old and frail, or a sick child or pet. Go by public transport, walk 10 minutes to the stop, switch lines, wait, walk 5 minutes to the hospital / clinic. If the old person tires, you can't carry them. Now try the same route with 20 cm of snow on ground. Now try with ice on ground. Now try in a storm. If you have a car, you'll be starting it up (if you don't, you'll be asking a friend or hiring a cab).
The question will be "which type of car", "whose car" and "how often".
Also, there will always be people working in the other end of the city, or in the countryside (where cars are practically required since public transport may be miles and hours away). People often have to decide whether to move near their job or move near their relatives (moving is an big hassle, it is not always possible to sell / buy / rent when needed, property prices differ, moving into a rich neighbourhood may be unrealistic) or commute. Smarter planning may reduce the flow, but there will be a flow. And industries are hard to integrate into living districts.