Why do taxis in Washington, D.C. say "call 911" on their sides?

This is a silent panic alarm. If the driver is carjacked, he can press a button which flashes the sign above the cab. This tells pedestrians to alert the police and arrest the thief.

People get used to the 911 call because it is a well known number. I never been to Washington D.C.
I believe Taxi drivers there have silent alarms special built somewhere on the dashboard, floorboards were it is closely reachable for them to tap in an emergency.

I live here in DC and have never seen this on a cab. @Vortico ‘s explanation puzzles me. What signal would passers-by recognize to alert them to call 911? And why would it be on the outside of the cab instead of a button the driver would push?

@anartist It doesn’t make a whole lot of sense to me to not have a GPS tracker or automated 911 call, but it’s what I’ve heard. I’ve also never been to DC, so I’m not sure of the taxi quality there—they could just not be able to afford a better system, but it still leaves confusion.

The buses here have that, too. A couple times I’ve seen metro buses go past with a scrolling sign that says, “Emergency… call 911.” If they’re being hijacked, nobody in the vehicle can talk to the 911 operator. If a pedestrian calls 911, at least they can give a description of the vehicle and a license plate number. If the bus or cab is at a busy intersection, GPS probably can’t find the exact vehicle on its own.