http://www.dot.wisconsin.gov/safety/motorist/rules/overview.htm

Emergency vehicles
When an emergency vehicle approaches with its siren on you must yield the right of way as quickly as possible. As soon as you see or hear the approaching emergency vehicle you must pull to the right and stop your vehicle.

You should position your vehicle as if parking, next to the curb, if there is one, or as far to the right as is safe if there isn't a curb, but don't block an intersection. If you have not entered a roundabout, pull over and allow emergency vehicles to pass. If you have entered a roundabout, continue to your exit, then pull over and allow emergency vehicles to pass. Remain in that position until the emergency vehicle is well past you. Your keyword is "law"

Don't "zoom out" right after the emergency vehicle has passed. There will be other vehicles in the road, those previously passed by the emergency vehicle, and you'll have to pay attention and be careful to merge with these vehicles.

As of December 2001, a new law in Wisconsin requires motorists to shift lanes, where possible, to give stopped emergency vehicles or tow trucks a safe zone in which to work.

Under the new law, when approaching an authorized emergency vehicle or tow truck stopped within 12 feet of the pavement and which has its emergency lights operating, you should shift lanes, if possible, leaving the lane next to the emergency vehicle open. If shifting lanes is unsafe, motorists are required to slow until they are past the emergency vehicle.

Want the "legal" version?