The rationale for converting HOV lanes to tolled express lanes is explained well in the environmental assessment for Texas DOT’s MoPac express lanes project in Austin:

“HOV lanes have often not maximized corridor capacity . . . . DOTs across the country have been unable to manage HOV lanes effectively so that reliability is maintained on the facility . . . . For example, HOV lanes set at two-person occupancy limit are often congested, while those with a three-person limit are often underutilized, causing resentment among travelers in congested GP lanes. Therefore, many DOTs have replaced HOV lanes with other options, such as express lanes or have added a tolling component to their HOV lanes (HOT lanes). . . . Without a dynamic pricing component that ensures a minimum level of service on the HOV, reliability of the facility cannot be assured. . . . In addition, occupancy enforcement can be a challenge. There is currently no reliable automated enforcement technology. . . . Furthermore, without a tolling component, the funds to implement this alternative currently are not available.” n