Can you hear me now? The use of lights and sirens is a topic that surfaces with regularity, but we seem to let it drop at usage… do we or don’t we. Monty asked a great question in another thread: Can we outrun our sirens? At first blush, it sounds unlikely. We don’t go faster than the speed of sound. On exploration, I found the answer is surprising. I followed up on his post, and searching ‘siren effective distance and emergency’ found the same source, “Lights and Siren: A review of emergency vehicle warning systems.” at www.naemd.org/articles/warningsystems1.htm that is worth reading. 10 to 15 mph is the recommended safe speed to enter an intersection. Another, under the title “The Alarming Sounds of Silence” is also a good read. It comes to the same conclusion. The information is old, but the physics hasn’t changed. Sound pressure drops off as a square of distance from its source [as does light]. It is diminished [attenuated] as it passes through objects. The sound threshold is the range where a sound is audible up to where it inflicts pain. For this reason sirens are usually restricted to 118 dB at 30 feet. [to protect the user]. To be detectable, sound needs to be 8 to 12 dB above ambient noise which in an otherwise quiet car [70 dB] with windows rolled up is about 78 dB. A screaming child is 90 dB [ I know, we all thought it was more than that.], so close to 100 dB is needed for detection of a siren. The math that falls out of this is that, best case, sirens will only be effective [inside a quiet, closed car] up to 30 to 50 meters [90 to 150 feet]. If it is a well insulated car or the kids in the back seat don’t like the movie, it will be less than that. Assume a driver in a closed car with no distractions hears a siren when it is 150 feet away. Perception/Reaction time to unexpected events is roughly 1.5 seconds. [ So if the vehicle with the siren is closing on a common point with another vehicle at a speed of 100 feet per second, then the vehicles collide at the moment the second driver reacts to the emergency. That is definitely outrunning the sirens.] If one assumes that a total of 5 seconds is necessary to perceive [1.5 secs] and to execute [3.5 secs] a ‘see and avoid’ maneuver, then the maximum closing speed over 150 feet, [if the collision is to be avoided] can only average 30 feet per second. That’s 20 mph. Other braking distance models put the initial speed closer to 30 mph. Wow. Anything faster than that as a differential speed between a closed car and a siren may technically be outrunning the ability of sirens to inform the general public of our approach in time for them to take meaningful evasive action. Assume any distractions, assume a little weather, assume there’s noise in the vehicle, assume an older driver, or one eating lunch, and all of a sudden a maximum entry speed into an intersection of 10 to 15 mph sounds reasonable, and a full stop sounds even better. Thanks, Monty, for bringing it up. Does anyone have any better numbers on the subject? .