Yes, STOPLIGHT is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, O 1, P 3, L 1, I 1, G 2, H 4, T 1.

Is it a valid word?

STOPLIGHT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a red light on the rear of a motor vehicle that signals when the brakes are applied to slow or stop
a visual signal to control the flow of traffic at intersections

Also: brake light, traffic light, traffic signal.