Many people drive a car in order not to use public transportation... And people having a car don't really care about saving a couple of euros to travel within Paris/suburbs.
Here's the thing. Reducing fares for transit in general everywhere would encourage usage and point out the need for more buses. An anonymous tracking app or transponder can be designed so that people can each have one, thereby illuminating where they like to go - the transit system can optimize routes to make the experience better (minimizing the pain). Overall, with more people in the buses, the roads become less congested and work better for people who have suboptimal trips. Using transit a lot does save people money, so they can afford better cars, perhaps hybrids and electric. Taking all measures to maximize transit usage worldwide would make a big difference in pollution.
The big question is how to pay for it. It's a bit of a catch 22. If people want to use transit more tax revenue could drop as people spend less on cars and carry less stuff out of the stores (how much can you carry from the bus stop?). However, smart retailers might hire strong teenagers or perhaps people who need exercise to be mules.
Retailers and road planners have made it easy for many car users to get from A to B in a fraction of an hour. Almost paradoxically the problem is being exacerbated by many people choosing to live in suburbs because of the great convenience of transportation. What could entice and accommodate people to live closer to work? The suburbs offer larger and more affordable homes, for one thing. I suggest that family-friendly neighborhoods with large multistory residential be built near the places where vast numbers of people work because that is the source of traffic. Land costs an arm and a leg in these places so the buildings have to be tall - not like Paris, more like Dubai. However, there has to be enough supply to make the homes affordable - not like Dubai.
Another thing is that the industry and hence the populace has to be diverse and flexible. Numerous, multistory but affordable residential neighborhoods require big money to kickstart. Many large cities have little houses relatively close to the city centre full of people believing one day they can sell their little gold mines yet they are still living there because developers are not into losing money. There needs to be foresight to rebuild the residential and move people closer to work.
In my city I see this happening here and there because the economy was strong at the time of the rebuild. Indeed, many rebuilds across the world can kill a couple of birds with one rock, paper, or scissors. The economy would be stimulated while pollution will be reduced. More people even save time on their commutes. A no-outsourcing job creation idea, Mr. Trump or Mr. Obama. How nice is that?