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I've just watched the scene a few times, and I think that the three parked cars (Jesse's pickup, Swamp Molly's pickup and the General) appear to be in the right places when the angle changes. There's room behind them for the truck to enter the water.
A couple of things I did notice:
1) Rosco's driving a '74 Plymouth Satellite when he gives Enos permission to make the arrest, but in subsequent shots he's driving a '75 Plymouth Fury.
2) From when Rosco speeds past Enos to when he hits the truck takes about 12 seconds. Even if he was only going at 30mph, he would still travel 176 yards in that time, yet when he gets out of his crashed car, Enos and the walkie talkie in the road are no more than 20 yards behind him.