Final answer:
Maintain a distance of at least 500 feet or 5 to 6 car-lengths from an emergency vehicle, such as an ambulance, to ensure safety. The actual frequency of an ambulance's siren is observed when the vehicle is directly in front of you, due to the absence of relative motion.
Step-by-step explanation:
When considering safe distances from an emergency vehicle (often abbreviated as emergancy v.), there are a few things to keep in mind. Generally, when you hear the Doppler shift in an ambulance siren, indicating that it is approaching, it is a sign that you should prepare to yield and pull over.
The safe distance will depend on your speed, reaction time, and local traffic laws, but a good rule of thumb is to be at least 500 feet away from any emergency vehicle. That's about 5 to 6 car-lengths, depending on the size of the cars.
The GRASP CHECK question pertains to the observed frequency of a siren. The frequency matches the actual frequency of the siren when the ambulance is in front of you, as there is no relative motion between you and the source of the sound.