Like a lot of the national parks in the US, Zion is trying to reduce traffic and air polution by requiring some people to ride shuttles buses from P&R lots outside of the park. Here is the WWW site for this service.
The Stops
All of the pictures below are by G. Araki.
Visitors Center
You get a hint of what the park is like from the stop at the Visitors Center. Zion Canyon includes the headwaters of the Virgin River which feeds into Lake Mead.