This would be a pennywhistle, stuck through the roof of the car, so the wind would catch it. If the siren fails for some reason, the driver could work the pennywhistle's slide to warn other drivers to move over.
The pennywhistle would close all the way when the slide is pushed all the way up, so
it wouldn't sound off all the time; only in emergency emergencies.-- CaptainClapper,
Oct 24 2007random,