Audio problem: When Jack tells the bad guy he wants to get off the bus, he says "it's not against the rules." The bad guy then says "All right, but I want you back real fast." Look at his mouth while he says the line. You can see that the talking isn't similar to the moving mouth. This is because Dennis Hopper originally said "in ten minutes" instead of "real fast." Since Jack was gone longer than 10 minutes, the line was re-dubbed to something less specific. (01:10:37)
Audio problem: When Jack and Harry are running down the stairs to get to the elevator Harry radios his lieutenant and says "Mac we need more help up here now", but his mouth does not appear to be saying these words when it is visible. (00:14:40)
Audio problem: As Payne cries "My Money!" when shooting at Jack, who is on top of the train, his mouth doesn't match what he is saying. (01:38:50)
Audio problem: When Annie asks Jack what the plan is, he says something like, "Stay above 50," and we hear Annie reply, "Nice plan," but her lips don't move.
Audio problem: When Sam is being loaded onto the trailer, Jack's mouth is visibly closed when we hear him say, "You got him? You got him?" (00:58:30)
Audio problem: When Jack is under the bus trying to diffuse the bomb and the piece of rubber hits him he says "F*ck Me" but his lips don't move. (01:14:30)
Audio problem: Right before they get off on the first exit to city streets, there's a shot of one of the passengers (Helen) who makes a distressed moaning noise. Her moan has an odd noticeable echo that is very out of place.
Audio problem: Annie is driving the wrong way down the street hitting cars. She starts mouthing "oh shit" but the audio suggests otherwise. She says "oh jeez."
Audio problem: After Sam gets shot several passengers are trying to help. One of them talks about how he's bleeding. Her mouth isn't moving.
Audio problem: When Keanu is pulling onto the freeway in his jeep, he starts honking his horn. The actor isn't pressing the horn in the wide shot. When we see Keanu from the front he is.
Audio problem: When Ortiz tackles a criminal in the bus aisle, a gun fires and injures Sam, but the blast can be heard a split second after the trigger is pulled. (00:41:45)
Audio problem: When the first bus blows up (the one that Bob was driving) you can hear the explosion before the bus actually blows up.
Answer: If they had, there might have been the possibility that to get to these routes they would have to go down a different road and depending on which route they take they might have to slow down to make the turn which would likely cause the bus to drop below 50 mph. Better to just keep going straight and keep the bus going. There's also the possibility that had they taken a different route, they would have to deal with a lot of traffic especially if there was traffic stopped at a red light. By using the highway, there'd be no traffic lights and less traffic to deal with.