If you are operating an ambulance during an emergency, you could use the red flashing overhead lights and not the siren when:

When operating an ambulance during an emergency, the requirement to use both red flashing overhead lights and a siren is established to ensure the safety of both the emergency responders and other road users. The red lights serve to alert other motorists to the presence of the emergency vehicle, signaling that it may need to navigate through traffic or oncoming vehicles quickly.

The usage of the siren, along with the lights, enhances this alert by providing an audible warning, which is particularly important in environments where visual cues may be obstructed or not noticed by drivers and pedestrians. This combination is crucial in prompting other road users to yield the right of way, thereby allowing the ambulance to reach its destination as swiftly and safely as possible, especially in urgent situations.

While there are scenarios in which the siren might not be used due to particular conditions, such as in very light traffic or for operational reasons in certain environments, the law typically mandates the use of sirens in emergency responses to maintain consistency and ensure that all emergency vehicles are recognized as such. Therefore, it is imperative to adhere to the legal requirement of utilizing both the lights and the siren for effective operation during emergencies.