Parking requirements are written into the Chicago zoning ordinance, but most large towers and even many medium-sized projects are done as Planned Developments, where pretty much every requirement is open for negotiation.
For as-of-right projects that don't go through PD, a 1:1 parking ratio is required, except in the densest zoning districts, where every unit after the first 100 can be parked at 0.6 spaces per unit.
The city recently passed a TOD ordinance to encourage developments with reduced or no parking near rail stations, but the applicable radius around each station is pretty small, leaving many obvious sites out of consideration for the parking reductions. Still, this has the potential to radically transform the city by making midrises financially feasible in outlying neighborhoods. The
first of these projects is now under construction.