When should carpool lanes be introduced in a multi‐lane highway?

High‐occupancy‐vehicle (HOV) lanes are often suggested as a cost‐effective alternative to address growing traffic congestion problems by providing priority treatment for buses and carpools. As a consequence of introduction of HOV lanes, some auto drivers would switch to buses and carpools, thereby reducing total vehicle demand, whereas there would be fewer lanes available for the remaining vehicles, thus possibly increasing the time cost of solo drivers. It is therefore an important issue to evaluate the efficiency of HOV lanes based on some system‐wide cost‐effectiveness measures. This short note presents a simple demand‐supply equilibrium model to evaluate the benefit resulting from HOV lanes, and therefore determine whether and how many HOV lanes should be introduced in a multilane highway.