Pay people to walk...
Pay people or provide incentives to those who like to walk more which would create a domino effect of more people walking.
Realistically only a third (if that) of the population need to use vehicles because of handicaps or long distance travelling, so I would suggest setting up a system that would get people to walk more. It would help both traffic and would make for good daily exercise which would release more endorphins and lead to more innovations within the city. It's the simple things in life.
Other than that shuttle/bus services and carpooling seems like the best reliable option if implemented correctly.