Yes, that's right, a traffic light. It is set up to randomly turn green for Go or red for Stop and Inspect. I've never seen it turn yellow, but you may. If you do get a red light, they will kindly ask to see the contents of your bags. Driving If you are driving into Mexico, check with your auto insurance carrier.
Most companies exclude coverage in a foreign country.
You're not in Kansas. Contact your Mexican insurance company immediately. For detailed information please read Driving Into Mexico article. When to Go The travel industry designates several business seasons during the year, High, Swing or Low. If you are coming from the Northern Climes, you probably consider winter the only time to visit.