I think that the biggest catalysts for change in Dearborn Park and the area just south (including the Roosevelt Collection) will not be the usage of all northern lots. The lots to the north of Dearborn Park will take much longer than 25 years to be used up. Most of those lots are more optimally designed for 19th century buildings than 21st century buildings. I'm not saying it won't happen, and I'd love to see some of the clever techniques architects will use in these spaces, but it will take time. The biggest catalysts for change will be Riverline/Southbank (whatever they are calling it now) and the 78. Once these projects are completed and/or near completion there will be huge momentum in the South Loop. Dearborn Park will be steamrolled into submission.
