Just a guess but presumably, MK and AK have the most drivers arriving. All parks are accessible by bus obviously. MK is the primary destination park for WDW with parking at the TTC. The only other access is monorail or boat from the deluxe resorts or Epcot. AK access is bus or car only.
Epcot and HS are accessible between each other in addition to Skyliner from Pop, Animation, CBR and Riveria. Boardwalk, Swolphin, Yacht/Beach have walking or boat access to both parks. Epcot, of course, has access from the TTC via monorail, so tram service at TTC is helpful there to get to Epcot.