Mass Balance on Interstate Traffic

Today, as I was driving home I got stuck in a traffic jam on the interstate due to a car wreck. While I was in standstill traffic for 20+ minutes, I realized that I could write a mass balance for the traffic flow on the interstate. The traffic jam was due to an accumulation of cars in the few miles of interstate that I defined as my control volume. There were hundreds of cars driving into the control volume and only a few cars driving out. Since mass is conserved there must be an accumulation of cars in the control volume (a.k.a the traffic jam).