Continuity mistake: The 2 SWAT team members are not visible on the preceding truck between two shots. (01:12:55 - 01:13:32)
Visible crew/equipment: During the passenger transfers at LAX, a cameraman is visible through the bus doorway two shots before Annie glances over her right shoulder twice. (01:27:35)
Revealing mistake: When Harry and Jack arrive at the building at the start in the police car, you can see the tire skidmarks on the road from previous takes. (00:05:05)
Continuity mistake: After Jack shoots Harry in the leg, Denis Hopper runs out the door which is wide open. The explosion shot (seconds later) happens and we see the door completely shut. (00:22:05)
Continuity mistake: In the cafe, Jack shouts to Bob "Have a good one" and holds his arm out with his coffee in his hand. When the shot changes and Jack is turning back to the counter, his coffee has swapped hands. (00:26:20)
Continuity mistake: When the crane breaks free from its bolts and starts flying towards some ventilation windows, it obliterates the upper pair. However in the next view from within an air duct, both of these windows just swing open freely as the crane busts through, and yet they barely appear to have suffered a scratch or bend in the process. (00:15:00)
Continuity mistake: As McMahon watches Harry stumble out of the bar, he takes a puff of his cigar. Between frames, his right hand quickly moves from the cigar to the counter-top. (00:26:30)
Continuity mistake: When the crane is torn out of the floor and flies towards the ventilation window, from a shot behind, the holes on the base (where the screws used to be) are gone. (00:14:30)
Continuity mistake: Payne has just forced his way into the subway car with Annie as his hostage. Jack shoots the lock off an emergency door in pursuit. The bullet holes on the two sides of this door do not match. (01:40:05)
Continuity mistake: When the cops arrive at Payne's house, the front passenger door of the silver car closes. However, in the next shot it is open. (01:19:45)
Continuity mistake: In the cafe, when saying goodbye to Bob, Jack's coffee cup changes hands between shots. (00:27:20)
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.