Final answer:
When hearing an emergency vehicle's siren or seeing its red light, one must pull over and stop, allowing the vehicle to pass. This is mandated by law to ensure that first responders can reach their destination promptly.
Step-by-step explanation:
When you hear the siren or see the red emergency light of a closely approaching ambulance, police vehicle, or firetruck, you must pull over to the side of the road and stop, making way for the vehicle to pass. This is not only a legal requirement but also a critical action that can help first responders get to their destination more quickly, potentially saving lives. The Doppler effect can be observed here, where the sound of the siren changes pitch as the vehicle approaches and passes by due to the changes in the frequency of sound waves reaching the listener.