Continuity mistake: When Jack is under the bus there is a shot from the driver's side that shows the vehicle pacing the bus. The two people that were standing on that vehicle are gone. They are back the next time we see it (when the bolt snaps).
Continuity mistake: When Sam gets shot, Annie takes the wheel back. We see the bus swerving toward the wall as Annie is grabbing the wheel. In the next shot inside the bus, she repeats the action.
Continuity mistake: 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.
Continuity mistake: When Helen tries to get off the bus, Payne blows up the door. The door is open. However, in the next shot, when looking over Jack's shoulder, the door is closed.
Continuity mistake: As Jack's initial pursuit of the bus begins, you can see although both vehicles are supposed to be traveling north on the 110 freeway, the sun changes position during filming several times. Shining on all sides for several minutes. This lasts up until the moment that Jack boards the bus.
Continuity mistake: In the shots before the bus blows up the Pacific Courier Freight plane, you can see that the vehicle towing it and the plane itself are no longer moving, as opposed to a couple of shots before this where the tow vehicle and plane are moving at a good pace. You also can no longer see the man who is supposed to be driving the tow vehicle (who might have run for it, but there's minimal time).
Continuity mistake: In a camera shot from behind the bus, it shows Traven moving the Jaguar in front of the bus. In the next camera shot from the front, it shows Traven moving the Jaguar again in front of the bus. He'd already completed the move in the previous shot.
Continuity mistake: The bus hits a car and wrecks the driver's side indicator. Later on however, the indicator is fixed.
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.