You can't decouple urban density from traffic of any kind, be it pedestrians, bicycles, cars, buses or even flying cars, they go hand in hand. There is no magic

You can't decouple urban density from traffic of any kind, be it pedestrians, bicycles, cars, buses or even flying cars, they go hand in hand. There is no magic trick to solve it no matter how much money you throw at the problem. Show me the city in the world with the worst traffic and I'll show you the city with the highest buildings. Only accepting that fact we can start planning better cities. Also worth considering, giving telecommute a high priority, since most of the office work done in these big cities can be easily done from home. Suburban sprawl becomes a problem only when those millions of people have to drive downtown to work clogging the driveways, but traffic is not an issue in suburban areas per se, lower urban density wins again.