Vienna convention:
unece.org/trans/conventn/signalse.pdf
Page:17 Chapter III Article 23
- In a three-colour system, the red, amber and green lights may be
replaced by arrows of the same colour on a black background. When lighted up,
these arrows have the same significance as the lights, but the prohibition or
authorization is restricted to the direction or directions indicated by the
arrow or arrows. Arrows signifying that traffic may or may not proceed
straight ahead shall point upwards. Black arrows on a red, amber or green
background may be used. These arrows have the same significance as the
above-mentioned arrows. - Where a signal of a three-colour system includes one or more additional
green lights showing one or more arrows, the lighting of such additional arrow
or arrows shall, no matter what phase the three-colour system may be in at the
time, mean that traffic may proceed in the direction or directions indicated
by the arrow or arrows; it shall also mean that, when vehicles are in a lane
reserved for traffic in the direction indicated by the arrow or the direction
such traffic is required to take, their drivers must proceed in the direction
indicated if by stopping they would obstruct the movement of vehicles behind
them in the same lane, provided always that vehicles in the traffic stream
they are joining must be allowed to pass and that pedestrians must not be
endangered. These additional green lights should preferably be placed at the
same level as the normal green light.