If you are driving to San Jose for Worldcon, note that while all of the hotels have parking, none of it is free, and in some cases it is valet parking only and quite expensive, and in all cases, the hotel-provided parking is in parking garages, not lots, and not suitable for over-height vehicles. For a full rundown of vehicle parking available in downtown San Jose, see the ParkSJ web site.
The San Jose Convention Center parking garage is $20/day, no in-and-out privileges, but overnight parking is permitted. Clearance is 6'6" (approx. 2 m). The entrances to the parking garage are off Almaden Blvd (Hilton end) and S Market St (Marriott end).
There are several lots (open-to-the-sky lots, not garages) where over-height vehicles can park. The lot at Almaden Blvd/Woz Way is across the street from the Hilton end of the Convention Center and costs $7/day. It is also open 24 hours. If you do not need ready access to your vehicle, another option is to park at the Tamien light rail station (parking is free) or one of the other San Jose VTA Park & Ride Lots and ride light rail ($2.25 one-way) to Convention Center Station, located in front of the Convention Center. Some Park & Ride lots have a limited number of long-term parking spaces available.
Transit & Trains
If you are commuting to the convention, VTA light rail and buses serve the San Jose downtown area extensively. From San Francisco and the Peninsula, your best alternative is to take Caltrain to San Jose Tamien Station and either ride light rail to Convention Center or take the free (weekdays only) DASH shuttle bus from Diridon Station to downtown points.
From Sacramento and Oakland, the Amtrak Capitol Corridor trains run to San Jose Diridon Station (see above for connections to downtown from the station).
Bay Area Rapid Transit (BART) does not run to downtown San Jose. To use BART to get to Worldcon, ride to Fremont station and take VTA express bus route 181 to downtown San Jose.