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.
Continuity mistake: At the beginning of the movie we see Keanu Reeves taking Jeff Daniels "out of the equation" by shooting him in the leg. We realise it's a serious injury as he requires the use of a cane when receiving his medal. However later on in the movie when they move in on Dennis Hopper's home, which takes place the next day, Daniels is part of the assault team and has no cane, no limp and is obviously very mobile.
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.