High Occupancy Vehicle/Toll Lanes: How Do They Operate and Where Do They Make Sense?

Motivated by the need for better utilization of existing high-occupancy vehicle (HOV) lanes and the potential of high-occupancy/toll (HOT) lanes to increase utilization, this paper examines the circumstances in which HOT lanes could provide a more desirable alternative to HOV lanes. First, it compares the HOT and HOV lane concepts, showing how they are similar and how they differ. Both involve finding some type of equilibrium in which no additional people are motivated to use the lane. However, in the case of the HOT lane, the agency operating the facility can affect the equilibrium point by varying the toll, thus having the potential to increase lane utilization and reduce overall delay. HOT lanes also offer travelers the additional choice of a delay-free trip if they choose to pay a toll. The question of how sensitive users are to the toll level was investigated using data from SR 91, in Orange County. During the first two years there was no toll for HOVs, so the toll lanes operated as true HOT lanes, even though they were officially named Variable Toll Express Lanes. Eastbound toll lane traffic showed a very low elasticity with respect to tolls-between -.02 and -.16. This means that if tolls are reduced by 50%, toll lane volumes will increase by between 1% to 8%. The westbound analysis found a positive relationship between tolls and toll lane traffic. This unreasonable result appears to be the result of a positive correlation between tolls freeway traffic volume that makes it impossible to separate the effects of the two variables. But this result also suggests that in the westbound direction toll lane volumes are not very sensitive to tolls. The final section of the paper uses a model to compare the performance of an HOV, HOT, and mixed flow lane in various circumstances. In all cases modeled, a HOT lane results in substantially less delay than an HOV lane. It also performs better or almost as well as a mixed flow lane. HOV lanes perform better than mixed flow lanes under extreme congestion and when there is a fairly high proportion of HOVs before the HOV lane is established. These conditions result in good utilization of the lane and provide the motivation for travelers to shift to HOVs. Under less extreme congestion, and when the initial proportion of HOVs is low, mixed flow lanes perform better than HOV lanes. These findings suggest that in locations where tolling would be feasible and where HOV lane utilization is low, HOTs might be a preferable alternative to the HOV lane. However, before conversion of a particular lane there should be more detailed modeling to confirm that an HOT lane would perform as expected and to determine the effects of the conversion on the network in which the HOV lane is located.