But you can't teleport out somewhere other than where you teleported it in. Your road rage fantasy wouldn't play out in your favor. You could fill the pocket dimension up overnight by teleporting in water from the bathtub faucet, and you could take a swim in it whenever you want to (and presumably teleport out completely dry since you and your clothes teleported in from different places). But when you teleport it out while on the highway, you'd just end up releasing 4,000 gallons of water into your bathroom at home where you teleported it in.
On a scary note, if you were to enter the pocket dimension while it was filled with water from another location, I would strongly recommend against drinking any of it, given the potentially fatal consequences of teleporting back out...