Comment by tetha

It's both of that. It has been determined that 2 seconds of siren should be enough for everyone on an in intersection to realize what's going on.

And that's why there is a rule for drivers of emergency vehicles to sound the siren for at least 2 to 3 seconds when approaching an intersection with an intent to disregard other peoples right of way. The rule for normal traffic partakers is very much "If it flashes blue and has music, get the hell out of the way as fast as you can without endangering yourself"

This rule in turn is in contrast with the overall suggestion to keep siren usage low, because it's loud, disturbing for bystanders and not great for the hearing of the people inside the car.