One of the Crain's Detroit bloggers, Daniel Duggan, wrote about Dan Gilbert's power play for parking in a downtown structure. He calculated that if Gilbert buys Chase tower and fills it maximum occupancy then the workforce would require roughly 2,000 parking spaces.

Buildings, obviously, need tons of parking in Detroit.

Following the rule of thumb on office occupancy, you need 250 square feet per person. That means that Chase Tower's 500,000 square feet of space needs 2,000 spaces at full occupancy — these are very rough numbers though.

http://www.crainsdetroit.com/article...OG08/110339974
Assuming that the calculation is about right, if Chase Tower needs 2,000 spaces then that means the building needs:
2,000 parking spaces x 160 square feet = 320,000 square feet for parking.

Chase Tower itself is 500,000 square feet, so that means the tower would need to have another structure that is nearly the size of Chase Tower just to accommodate parking. Extrapolate that to every building downtown and for each building you would have to build a dedicated parking structure of nearly the same size.

That then brings up productivity issues. How much of the potential productivity downtown is degraded by having to devote so much space to parking? By productivity I mean the amount of space that is generating revenue versus the space that just sits there. So for each square foot of space downtown, how much revenue could be generated if it was all devoted to workers? Now divide that in half because in Detroit you have to devote as much space to parking as you do for workers. The amount of space that needs to be dedicated to parking is degrading the potential productivity rates of downtown Detroit by almost half.

This is why downtown Detroit does not work. The lack of a transit system is why so many buildings downtown -- that would be highly sought real estate in other cities -- are effectively worthless.