'Call 911' On top of taxi cabs

Factual Questions
1

Last time I was in D.C. I noticed this on top of a lot of cabs. It was on the little light on top of the car that usually is on if they are looking for a fare, or light up the “off duty.” When I was in NYC I didn’t see any cabs with a “call 911” light on the roof. I’m guessing that the “call 911” part of the sign is if the cabbie is being robbed/carjacked s/he can turn it on and alert someone for help.

Has this worked and why don’t other cities (at least not NYC) use this?

2

NYC has this too, but it’s a retrofit system that was begun about 10 years ago. Barnacled on the back (usually on the trunk lip) of most NYC cabs is a round, amber light fixture (about the size of a tuna can). If the driver turns it on, the police will stop the car.