Downtown traffic can be tricky depending on the date/time of the event, but once you arrive at the Horton Plaza garage it shouldn't take long to find a parking spot. Be sure and validate your ticket at Balboa Theatre, especially during Comic Con. More info: https://sandiegotheatres.org/directions-parking/balboa-theatre-parking-and-directions/balboa-parking/
Do they require a clear plastic bag to bring wallets, cellphones and keys?
It should not take long to find a parking spot at Horton Plaza. You should be fine but do not leave items out in the open for safety sake.
Depending on what time you seek to arrive, keep in mind that you can park almost anywhere in San Diego and get on the trolley. Balboa Theatre is in the Gaslamp district along the green line. Once you exit the trolley, you even have the option of scooters if you don't want to walk.
I wouldn't recommend parking at Horton plaza. The building doesn't house as many stores anymore, so there's a lot of transient activity and unfortunately crime in the parking garage now. The security isn't what it use to be
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