If you have ever wondered how the decision is made whether or not to install a left turn arrow at a traffic light…. your question will be answered. I will be discussing how the state of Pennsylvania determines whether a left turn arrow is necessary; and the math behind it. My talk is based on an article “Waiting to Turn Left?” by Maureen Carroll, Elyn Rykken, and Jody Sorenson. This article works with a continuous model to interpret data using calculus in order to determine if a left turn arrow is needed.