This is an interesting idea. It would require a sea change in the city. You'd have to fight a lot of business owners. Most transit in Manhattan is by foot, and

This is an interesting idea. It would require a sea change in the city. You'd have to fight a lot of business owners. Most transit in Manhattan is by foot, and you will have separated pedestrians from commercial entryways. As it stands, a great deal of storefronts—even in the most commercial and highly trafficked parts of the city—are in mixed-use buildings, with apartments in the upper stories. Now most potential customers, instead of walking past shop after shop, are walking past residences. Signs may point out what lies below. Presumably this would cut into businesses which rely on customers popping in without planning to; if you weren't going to exit onto that block for something else anyway, why make the climb down to commercial level just for a little something that caught your eye? And how did it catch your eye? Stores could no longer entice with a storefront, by showing off their inventory and sales. Restaurants, bakeries, and coffee shops could no longer waft out onto potential guests. They'd be in virtual basements, and would be much less attractive for that reason alone. Manhattan has a history of severing its residents from one thing or another—waterfronts are the first to come to mind—but I would be surprised to see them do this. Of course, not everything has to remain as is and the city may evolve (as it has before), but this sort of plan would require much more work than building a tiered roadway. Maybe in the future, when 'brick and mortar' starts to fail even in densely populated areas, the problems described above won't be problems anymore. But then, neither would the traffic; and the tiered roadway would be redundant, and not worth the money to maintain (nor the daylight lost).