Background:
Many cities have serious shortages of buildable space in the most desirable areas—most land that is available for development has already been taken.
Proposal: Allow building of a “second level” of city on the un-used space above the streets
Fortunately, there actually is a large amount of unused space in each city (generally, approximately 5–10% of the total area in question). And it’s not even privately owned!
Specifically, it is the area above the sidewalks and public streets. (See Figure 2 for an example of a four-way intersection.)
All we have to do is allow structures to be built on stilts on top of the roadway. This will shield pedestrians and cars from rain, snow, and hail, and will keep the roadway comfortable and cool even on the hottest days. Additionally, it will increase the number of structures built in the city, which will help fund public education via property taxes.
This idea has extensive historical support in dystopian and cyberpunk fiction, so presumably most of the details of it have already been worked out.
Fig 1: Building a second layer of city above the roadway would be a great way to increase the tax base and allow people to live closer to their place of work.
A) are supports for the second level, which can be placed on the sidewalk just like telephone poles / utility poles. B) is the second level of the city and C) are the houses that are built on top of this platform. D) is the original roadway. E) shows the minimum clearance for the second level (in this example, the dashed line shows 30 feet from the roadway, although the specific heights may be different, depending on local requirements).
Fig 2: In this overhead view of a four-way intersection, there are buildings (dark gray) on all four corners and a sidewalk (light gray). There is significant un-built (but highly desirable) space being occupied by the roadway and sidewalks.
Fig 3: In this terrible diagram, we see the elevated area (blue) above the street (white). Sidewalks are in green for some reason. Features of interest: A) Staircase leading from the upper level to the ground-level sidewalk. B and C) Elevators. D and E: a skybridge connecting the buildings at near points D and E. F: a skybridge connecting the upper-level buildings directly to the building at F (perhaps it is a shopping center). Who made this diagram anyway, it is awful. Oh well!
PROS: Makes the city more like a cyberpunk dystopia. Adds new buildings that will pay property tax, increasing city revenue. By increasing population density and decreasing commute distance, we create a more eco-friendly city.
CONS: Doesn’t work in areas with significant over-the-road infrastructure. Also, this idea never seems to work out in works of fiction.
WorstPlans: updated every Monday since July 2014! 
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