When encountering emergency vehicles parked on the shoulder of a highway with their lights flashing, it is the law that you must move over one lane.
If you are unable to do so, then you must slow down significantly and try to stay as far from the parked vehicle that your lane allows. See the following videos for more information:
Two Lane Highways – Video Describing What You Need to Do When Encountering Emergency Vehicles
Multi-Lane Highways – Video Showing The Correct Driving Proceedure When Encountering Emergency Vehicles