Final answer:
When an emergency vehicle approaches with its siren on, pull over to the curb and stop. The frequency of the siren sounds higher as it approaches due to the Doppler effect and matches the actual frequency when it is directly in front of you.
Step-by-step explanation:
When you hear the siren or see the flashing lights of an emergency vehicle, you should pull as close as possible to the curb or edge of the roadway and stop. Doing this allows the emergency vehicle to pass by quickly and safely. It is important to stay stationary until the emergency vehicle has completely passed. Regarding the Doppler shift, as an emergency vehicle with a siren approaches you, the frequency of the siren's sound is higher, and as it moves away, the frequency becomes lower. If you are standing still and an emergency vehicle drives by with its siren blaring, the frequency you observe will most closely match the actual frequency of the siren when it is directly in front of you because there is no relative motion between you and the siren at that point.
In a situation where you're driving and hear a Doppler shift in an ambulance siren, you can determine when it's getting closer or if it has passed by, helping you to decide when to pull over. If standing in the center of a circular road and an ambulance drives around with its siren blaring, the pitch of the siren will continuously change due to the Doppler effect, being higher as the ambulance approaches and lower as it moves away from you.