Except, If I see a red light, I expect it to turn green eventually. On the other hand, If I see a sign telling me I'm only allowed to turn right, I know exactly what's going on. Explicit is better than implicit.
Whether or not there is a sign, you still need the light.. A sign is not sufficient as it puts the decision of when to turn on the driver of the car, instead of being dictated by the light as is the desired behavior...