Final answer:
Upon encountering an emergency vehicle with active sirens or lights on a highway, one should move over and stop to yield a clear path for the vehicle, as indicated by the Doppler shift in the siren's sound.
Step-by-step explanation:
When approaching an emergency vehicle with its lights and/or siren on a single-lane highway, you should safely slow down and move over to the side of the road, coming to a complete stop if necessary, to allow the emergency vehicle to pass. It is critical to remain stationary and avoid any sudden movements that could endanger yourself or obstruct the emergency responders. This action is in line with the signals provided by the Doppler shift in the sound of the siren. When the siren's pitch is increasing, the ambulance is approaching you; when the pitch is constant, the ambulance is passing by you; and when the pitch decreases, it has already passed.