The diagram below illustrates a set of drop-down visors which could be deployed for non-functioning traffic lights. On the face of the visor could be an out-of-order message or a warning for drivers to drive carefully.
The visors could be remotely electronically deployed, allowing rapid response to traffic light failure. Ideally the visor mechanism would be on a separate circuit to avoid both the traffic lights and the visors being out of action simultaneously. Alternatively the visors could be mechanically deployed.