Date for Change in Signal Colors?

[nw-mailing-list at nwhs.org]

Has anyone figured out from the literature the date that the N&W  made the 
switch from
 
WHITE for "clear"
GREEN for "approach" or "permissive"
RED for "stop
 
to
 
GREEN for "clear"
YELLOW for "approach" or "permissive" 
RED for "stop" ?
 
The ICC investigation report covering the rear end collision at Belspring  on 
July 16, 1915, seems to indicate that the change had not yet been made, as  
the operator at Pepper wrote "Block Clear" on the Clearance Card when he should 
 have written "Block Green" (i.e. "permissive") to advance Ex 1425 West into 
an  occupied block.
 
I think I recall that it was only about 1912 that Corning Glass Co. figured  
out how to make yellow glass that was suitable for railroad signaling purposes 
 (i.e. chromatically distinct from any shade of red, and having adequate  
"transmissibility".)
 
The answer surely lies in the timetables at the Archives.  Git busy,  Harry 
B. and James B. !!!!
 
-- abram burnett,
         writing from the La-La  Land of senile old fuddyduddies
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