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U.S. federal safety regulators continue to tackle the sticky problem of autonomous vehicles, this time with a proposal that could signal to the public, quite literally, when a vehicle is operating autonomously.

The National Highway Traffic Safety Administration has proposed new lighting requirements for autonomous vehicles that include an option (dare I say a strong suggestion?) to use cyan-colored lights to signal when a vehicle is operating autonomously without a human occupant monitoring the road.

The agency is not proposing a mandate for blue-green lights.…