I was on a three-day first-aid course earlier this year and was given a list by a paramedic of four reasons why they might use “blues” but not “twos”
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for the benefit of patient on board whose condition means the sirens would cause “distress or discomfort”.
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to protect vulnerable road users ( cyclists, children, horses and riders etc) on public roads.
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When responding to an incident alongside police who have requested a silent approach
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When road conditions (eg double solid white lines) means it’s not legally possible to pass a vehicle and they actively want the vehicle to keep going until it’s safe for them to pass - they don’t want people to automatically stop without giving it proper thought, which too many people do apparently.