Fig. 2From: Teen driver system modeling: a tool for policy analysisTeen Driver System Model. The single arrows (in blue) represent causal relationships. The rectangles (or boxes) represent stocks (which accumulate flows over time). The double arrows with a valve symbol represent flows. A double arrow going in a stock is called an inflow and a double arrow going out of a stock is called an outflow. Inflows increase the value of a stock whereas outflows decrease the value of a stockBack to article page