Continuity mistake: Upon exiting the highway into downtown traffic and blasting through a red light, the bus has a couple of road signs lodged into its bumper which disappear between shots - right before it gets blindsided by a Ford Taurus. (00:47:15)
Continuity mistake: The paint that sprays on Payne's face changes splatter pattern throughout the rest of the scenes with Payne. (01:35:30)
Continuity mistake: When Payne shoots the train driver in the back, blood appears on his shirt. A few seconds later when he is falling down, his shirt is clean. (01:37:10)
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: 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: The bus hits a car and wrecks the driver's side indicator. Later on however, the indicator is fixed.
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: When Jack pulls the Jaguar alongside the bus and starts honking, Tuneman's right hand changes its position between shots. First he's got a grip on the vent window, but then all of a sudden his hand rests on the windshield's upper edge. (00:36:45)
Continuity mistake: Jack is lying on his horn trying to get onto the onramp. This is right before he drives past Tuneman. He is lying on the horn, and yet both of his hands are on the wheel.
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.