definition, derivation  of  flag fall as in  taxi  flag fall

I haven't been in a real cab in a long time (in DC there are no
meters!) but don't they still drop the flag? Or is that another victim
of the digital age?

Nope, they still drop the flag!

meownow1, the "flag fall" is the initial $$ amount you see when the
driver drops the flag. This varies from city to city, country to
country, and presumably, planet to planet...

Meownow,
The expression "flag fall" dates back to the old taximeters that were
box with a lever that could be seen from outside the cab.  The lever
had a squarish "flag".  When it was up and visible, the cab was not
occupied.  When a passenger stepped in, the driver turned the lever
down  - the "flag fall" -  and the taximeter started counting  - with
the initial amount and then so much per tenth of a mile or waiting
time.  (This was back in the days when NYC cabs cost 20 cents for the
flag fall and 10 cents per 1/10 of a mile.   Cabby wasn't impressed
when I gave him a dollar for a 90 cent fare. :)  Cynthia probably
wasn't around back then.)

I believe that the flag fall also turned off the light in the "TAXI"
sign on the roof.

Does that help?  Myoarin

To the contrary myoarin, I have been in many more cabs that had the
old flags than recently with the electronic versions, but as usual,
you described it far better than I.  FYI, I'm 48 years of age.

Cynthia, I didn't ask, but should have known that all the personal
experience you bring to answers couldn't come from someone fitting the
image I project on all unseen females.  ;)  But maybe you do; I will
assume so.
Regards, Myoarin