Saturday traffic is said to be one of the busiest overall (not just uni-directional commuting into or out of the city). For one, there's few weekend parking restrictions on the arterials so 3 lanes in each direction is often cut to an effective single lane due to parked cars and left turners. One N-S road (Main St.) is being rebuilt which may increase traffic on the other N-S arterials. It should not add more than 10-15 minutes but that could also be the margin of error or not being able to board.

The other consideration is that you have to line up to 1) check in, 2) clear U.S. immigration and customs, (dumping checked bags on the belt after this is pretty trivial by comparison) and then 3) clear security before you get to your gate.