It’s a sliding scale of urgency. With the lightbar on but sirens off, an emergency vehicle can still be visible and slow traffic around them, but without the extra emphasis of, and attention attracted by, the sirens. I’d imagine an officer might do this when in a quiet residential area or when responding to a minor incident (e.g. fender bender, no injuries), or when they want to respond rapidly to criminal activity without tipping off the perps that they’re nearby.
Obligatory disclaimer: I’m not a police officer. I *am* an EMT and drive an ambulance; while I can’t claim to speak for officers and therefore I can’t claim my answer is accurate, I can tell you there’s a few situations in which *I’d* run lights-only, and conceptually my answer is based on that.
Lots of reasons… Mostly because it’s unnecessary and disruptive in some cases. If traffic is already moving favorably, the siren can cause more issues than it solves. You learn how drivers react to different things and adjust accordingly. On the fire engines we have another option too with the growler that you can basically set at a certain level and let it spin which can be helpful for alerting pedestrians or cyclists or other cases where you don’t want the full siren. It’s a big part driver’s preference too… I’m personally on the less siren is better side; some guys leave it on the entire time. For me if I need you to move or I’m concerned about alerting at an intersection or blind turn, it’s on. Otherwise, and almost always at night, it’s off.
Source: drive fire engines.
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