Like most others, traffic economists also treat a vertical line for the capacity of a road. When traffic flow approaches the vertical line, there will be congestion, while the line itself is a gridlock. This book re-runs the traffic mathematics thoroughly, to derive some surprisingly new understanding of traffic-flow and capacity, which then help economize road size, prevent congestion, move cars smoothly and swiftly, object toll and speed control, and redefine the purpose of traffic.