It is often argued in the US that HOV (high occupancy vehicle) lanes are wasteful and should be converted to HOT (high occupancy vehicles and toll lanes). In this paper, we construct a simple model of commuters using a highway with multiple lanes, in which commuters are heterogeneous in their carpool organization costs. We first look at the HOV lanes and investigate under what conditions introducing HOV lanes is socially beneficial. Then we examine whether converting HOV lanes to HOT lanes improves the efficiency of road use. It is shown that the result depends on functional form and parameter values. We also discuss the effect of alternative policies: simple congestion pricing without lane division; and congestion pricing with HOV lanes. The analysis using specific functional form is presented to explicitly obtain the conditions determining the rankings of HOV, HOT, and other policies based on aggregate social cost.