Equinox overheating when sitting in traffic cools when driving.... problem?

I think the cooling fan is not working,

Theres a few possibilities here. Like the comment before the cooling fan is the likely culprit. Understanding the "why's" behind problems helps you to understand the many different systems in a vehicle. The "why" in this case is because of airflow. When you're sitting still that usually means your engine is eating the exhaust of the car in front of you and the fan isnt helping to eliminate this aspect. If you haven't remedied the problem yet, try leaving a full car length between your car and the next to allow air to cool the engine off until you get moving. When you dove it cools off because you're force- feeding it fresh air which takes the heat off the engine as long as you're still moving it will do this. But if you dont get the problem fixed it will end badly with a blown engine, blown head gasket, or a warped head. This is the difference in $100(give or take $50) in repairs now and up to 2 grand if it blows the motor, several hundred for a new head so best bet, get the fan working even if you've got to have someone wire the fan directly to the radio contacts of the fuse box so it runs constantly while the key is turned over. Or wire it straight to a toggle switch and tape it to the dash so you control when yo cool the engine down. Good luck!