If u plan on going more than once a year or visiting other 6flags parks it's better to purchase a membership. For 2 memberships I pay $60 a month that includes the flash pass. As long as I visit 2x a month it pays for itself. That includes the $25 for parking and unlimited drinks. Plus admission into Any 6flags in the US and water parks.
$69.99 US + tax and fee, if you buy on their site at least a day ahead. There may be other deals out there.
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