Can High-Occupancy/Toll Lanes Encourage Carpooling? Case Study of Carpooling Behavior on the 91 Express Lanes

A case study of carpooling behavior on the 91 express lanes is presented. The 91 express lanes are the nation’s first high-occupancy/toll (HOT) lanes on which carpools with three or more passengers could use the lanes for free (at the time the data for this study were collected) and others paid a toll that varied by time of day to use the premium express lane. One concern over such a policy is that people will not carpool if they can just pay for the travel time savings that they would normally obtain by carpooling and using a high-occupancy vehicle (HOV) lane. Survey data show that the rate of carpooling has not changed much since the opening of the express lanes, a lot of changing between modes occurs (increases and decreases in the number of passengers), a large number of people carpool a few times a week, and two-person HOVs use both the regular lanes and the express lanes. It was further investigated whether HOT lanes encourage carpooling by modeling carpool formation with discrete choice models. The results show that mode choice behavior in the corridor is similar to carpooling behavior in other locations, and carpooling in the corridor is not discouraged.