I'm not sure if this is a school question or a question that you asked because you want to know (Quora is better for this), but I would:
Look around and see what everyone else is doing. Are any of them on phones? How many? If a lot of them are, they're probably calling police and I'd wait for police to arrive. If there was a lot if distress I'd get out of my car and call in my loudest voice to wait for police to come regulate traffic.
If no one else is calling police, I'd call. Tell them to send someone over as fast as they can to regulate traffic, and tell them what I think the importance level is, based on how many cars are backed up.
If people are still trying to drive, I'd try to get everyone to stop and wait for police to come regulate traffic. Stand on my car, call out, use a sign, make a sign, whatever I could think of. Use a whistle if I had one.
When the police came, I'd do what they told me to.
If I was on my way to work, I'd call my boss and say, "you probably know by now, but the traffic lights aren't working. I don't think I can be there in time."
