From the Pennsylvania Manual on Uniform Traffic Control Devices

Wait For Green (R10-21)
Justification:  The Wait For Green sign shall be authorized for use at traffic-control signal installations where a tendency is noted on the part of the motorist to start his vehicle moving before his green indication is lighted.  This occurs where opposing traffic has a leading green, and thus is allowed to move before the motorist in question.
Placement:  The R10-21 sign should be placed on the signal post or pedestal supporting the signals to which it applies.  It may also be placed in an overhead position on a signal mast arm or span wire.

So that's the justification, but I have to agree with your comments.

Personally my favorite roas sign is the H-1 sign in Hawaii . The concept of having an interstate highway in Hawaii is interesting to say the least.

Aloha

I going trough Philadelphia International Airport and saw that sign. I had a good chuckle and I also had to go through the odd traffic situation on i-95 on my way home. I am new to the Philly metro area and have the same conclusion you do they are terrible drivers.