The Law Blog had an interesting entry on January 26th about this green arrow.
This was taken over from the German Democratic Republic. It permits a driver to turn right if the coast is clear (no pedestrians crossing; give way to those ahead turning left). But it does not oblige you to turn right.
bq. A GREEN ARROW may be provided in addition to the full green signal if movement in a certain direction is allowed before or after the full green phase. If the way is clear you may go but only in the direction shown by the arrow. You may do this whatever other lights may be showing. White light signals may be provided for trams.
No picture there, but I found one here:
It’s called a green filter. Did that come from the GDR? Apparently, it’s especially intended for cyclists.