This is a great idea and feels like a good addition for the next generation of theme park simulation games. It's a gateway to eventually adding hotels and full-on resorts.
To add to what others have suggested, if this were in a career mode or scenario, I envision something like this...
First scenario begins with a road that connects your parking area to the outside world. Your starting parking lot might begin with 200 spaces and guests walk to the entrance.
As your park grows, you'll want to expand your parking area to accomidate more guests so you pay to build another 200 spaces and now more guests arrive by car.
As time passes and space is limited, you can build multi-story parking garages, bus stations, rail stations etc and hire staff to drive those buses.
Even more elaborate parking areas would have stations for hop-on-hop-off trams or monorails to get peeps from the parking area to the entrance quicker. They had the tram transport ride in RCT3 but no parking lots to drive through.
All this would add to general park management. You pay cash upfront for the infrastructure and continue to pay expenses for staff (drivers), and vehicle/facility maintenance. You get revenue from bus/rail tickets and parking fees.
As for making space for this, I think it would probably share the same space as the amusement park. Im sure the developers have a maximum park size in ind. Players would have a zoning tool designate which part of the map is for the amusement park and which part is for transport and parking and the exterior fencing of the park will move accordingly.
This sounds awesome. It can also provide another challenge in terms of money.
Bus routes coming into parks will be pretty neat, and the bus stops can also be upgradable.
Trams/buses connecting car parks to multiple locations of the theme park might be a good way of diverting peep traffic.