I would think >90% of driving, and pollution, is from daily commuting, where 5+ seater car/SUV/truck is carrying one person at a jerky 5mph. On a Pareto chart, this is the one that sticks out and needs to be addressed first. RDCanecutter is right about zoo trains. For minimal investment, these changes would make a big difference:
1. neighborhood zoo trains (with inflated tires, sorry RD): There seems to be no lack of Uber drivers. They can drive the zoo train for equivalent pay. These trains pass by within 2-3 minute walk of any home and circumnavigate an area of couple of square miles, connecting to local schools, shopping, and importantly, a terminus.
2. city-wide fast busses: These busses connect few termini from one area to another area. Each main street (equivalent of Lamar or Congress) will have dedicated central lanes for these busses and their signaling will be same as if they were on track a la BART. For highways, make the carpool lane for 3+ people and these busses.
3. zero pollution bikes/scooters/skateboards: All streets that are not main streets will have half the space (at least one full lane) allotted to these vehicles. With dozens of electric bicycle/scooters coming up, the last mile (or full commute, if you wish) should be easily and safely doable for all except infirm and disabled.
This should drastically reduce car traffic by at least 50%, and there is no reason to not do this in southern states.