The graph below shows the growth of High Occupancy/Toll lanes in the United States. From a slow start in the 1990s, growth has picked up in the past decade significantly, with a number of projects recently opened and many more underway. The graph shows projects that are open, under construction, or with contracts signed. The number of projects in planning stages are more still.

While still a small share of the total highway mileage in the United States (there are 4 million miles of streets and roads in the US, the National Highway System has 150,000 miles, the Interstates are 46,000 miles of that), it is growing rapidly, and should eventually absorb at a minimum the US HOV network, and perhaps eventually reach a parallel to the congested fraction of the Interstate Highway system.

We wrote in The Transportation Experience about HOT Lanes

In the late 1990s, High-Occupancy/Toll (HOT) lanes that would allow single occupant vehicles to use HOV lanes (after paying a toll which depends on the level of demand) began to be considered in many US metropolitan areas. HOT lanes were conceived of by Ward Elliott at Claremont McKenna College in the 1970s, and reinvented in the 1990s by Gordon Fielding and Dan Klein as part of a Reason Foundation study. HOT Lanes were introduced on I-15 in San Diego for instance, using the same facility that had been the testbed for the Automated Highway Systems experiment, and have generally been viewed as successful. The revenue generated after paying for operating the facility helps subsidize transit in the corridor.

Seeing the success of I-15, as well as a handful of other HOT lanes, Poole et al. (1999) calls for networks of HOT lanes (or HOT networks) in major US cities. They conclude the benefits of such a system (including congestion reduction for those not using the system) outweigh the costs.

While pricing every congested road using marginal cost pricing may increase system efficiency, providing differentiated services further enhances efficiency. We recognize that different types of freight have different priorities (overnight, two-day, and ground are choices for shipping); that same kind of differentiation applies to drivers. Different drivers have different values of time at different times of the day. Thus the ability to pay a premium and travel at a better level of service during peak times provides a service not currently available, a service enabled by bundling several ideas. The old idea is tolls themselves, whose original intent was simply to raise revenue. Electronic toll collection complements that; its original intent was simply to automate the collection of tolls at traditional tollbooths, reducing both traveler delay and agency operating costs. HOV lanes aimed at giving priority to vehicles carrying more passengers (vehicles which had a higher value of time, since two people are more than one).

The HOT networks also provide the ability to provide bus rapid transit (BRT) services in metropolitan areas, by providing the high speed limited access routes that give transit a travel time advantage over the automobiles not paying for the HOT lanes. In many situations, buses are more cost effective than fixed rail alternatives, but the lack of a fast right-of-way leads people to perceive rail as inherently faster than the bus. With BRT, that perception can be made to disappear. Several cities are now building-out HOT networks through a combination of new construction and conversion of HOV lanes.