They shut down some of the walkways in Disneyland for parades and the fireworks. When they do hordes of employees come out and redirect you, making sure no one goes under the ropes and takes the short cut. Their job, for however long it lasts (hours, sometimes) is to say... you can't get there from here, you need to go around.
The autopia is less crowded during pyro (fireworks). You can see the fireworks through the trees as you drive. It is a little distracting. Don't crash in to the car in front of you!