Here’s how I know that at least 2.5 percent of the land area in Chicago is covered in parking lots and garages, as of February 5, 2017.
That’s a lot of polluted water runoff.
I grabbed the land area of 227.3 from the Wikipedia page.
I grabbed all the parking lots from OpenStreetMap via Metro Extracts, which is going to be the most complete map of parking lots and garages.
Volunteer mappers, including me, drew these by tracing satellite imagery.
With the parking lots data in GIS, I can count their area in square feet, which comes out to 160,075,942.42. Convert that to square miles and you get 5.74.
5.74/227.3*100 = 2.5 percent
The last snapshot of parking lot data I have is from February 2016, when only 3.39 square miles of parking lots have been drawn.
There are still many more parking lots to be drawn!