Answer:
In many jurisdictions this is actually posted on the rear of emergency apparatus, but it changes from county-to-county and state-to-state depending on the laws that were passed by their legislature. 150 feet is usually a pretty standard distance.
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In the state of Massachusetts it is posted on the rear of all emergency vehicles. 300 feet is posted on all fire trucks, ambulances. It is interesting that the size of the type is so small that you cannot read it unless you are about 25 feet away. 300 feet is the length of a football field from goal post to goal post. I have never noticed this much open space behind an emergency vehicle even though it is easy to read at a close distance.
Police cars nearby are busy opening traffic lanes if it is congested. Enabling the emergency vehicles get to their destination is the most important thing a cop can do in this situation because the problem is in front of the moving emergency vehicle not the back.
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