Rebar suggests that "more than 70% of most cities’ outdoor space is being dedicated to vehicles and only a fraction of that land is allocated to open space"
The 2006 City Centre Masterplan did an interesting audit of Brisbane's CBD spaces and the breakdown of land is quite fascinating. Depending on which suburb you audit the percentages would obviously change (eg the Valley would have hardly any green 'park').
Thanks to major CBD parklands like Roma St, South Bank and Botanic Gardens and if you count footpath space of streets in the public realm, it works out to be approximately 45-50% of the CBD outdoor space dedicated to vehicles. The CBD is a unique location, and I suspect that across the city it might likely average out to match what Rebar quote.
Public realm improvements and PARK(ing) is about supporting the better use of the yellow (street), increasing the green (park) and getting greater outcomes in the grey (development).