50. At places where signal-lights are installed, drivers of vehicles must(a) when faced by a green light, proceed;
(b) when faced by an amber light, stop before the intersection unless already engaged therein or so close to it that it would be impossible to do so without danger;
(c) when faced by a red light, stop at the intersection;
(d) when faced by a green light in the shape of an arrow, they may proceed in the direction indicated.