A party where guests wear the the colors of the traffic signal to denote their relationship status: green means they're single, red means they're taken, and yellow means their relationship status is "complicated"

At a red light you put the car in park then everyone gets out and starts dancing like crazy to whatever music works happens to be on. People tend to dance ON the car.

A stoplight party is where you wear the colors of your relationship status. If you wear green it means you're single, red is taken, yellow means you're into someone at the party.