I've thought a lot about how we can seperate bicyclists and pedestrians from vehicles on the road. I think it would be amazing to have all roads completely underground and replace the roads outside with parks, for example. I can just see kids running up and down the grass 'streets' playing soccer with skyscrapers all around.

With underground streets you could even collect the pollution the vehicles emit and store it instead of releasing it into the atomsphere.

But the huge downside which I have no idea how I could make this practical with is the construction costs. Maybe the first city to do this could get an increase in tourism thus offsetting the costs?