Do HOV to HOT conversions decrease carpooling?

By Robbie Webber
A study from Texas A&M asks whether carpooling decreases when lanes are converted from high-occupancy vehicle (HOV) lanes to lanes allowing solo drivers to pay a toll. Their conclusion: it depends. The researchers studied eight roadway segments in six states and found that carpooling often declined when lanes were converted from HOV-only to high-occupancy toll (HOT) lanes that charge solo drivers a fee to use the supposedly faster restricted lanes. However, the results were varied and the researchers themselves admit that more study is needed.
Mark Burris and his team from Texas A&M’s Transportation Institute looked at the conversion of lanes from HOV to HOT in the metro regions of San Diego, Los Angeles, Houston (2 roadways), Seattle, Minneapolis, Miami, and Atlanta. Only in the case of SR-91—running between Anaheim and the Orange/Riverside County line—did carpooling increase in the restricted lanes. In all others carpooling went down or remained the same.
On I-405 in Seattle—a road not studied by the Texas researchers—the change has been startling. The original HOV lane was so popular that an additional lane was added to keep carpoolers moving. However, the new lane allowed solo drivers to pay a variable toll that would be adjusted every five minutes to keep traffic speeds up. Though many thought the maximum toll of $10 would only be paid by the very rich, that turned out to not be the case. A columnist noted that the $10 toll on the new HOT lanes was too low to keep traffic moving and, in a subsequent column, asked, “What happened to the carpoolers?” He cites an informal and admittedly small survey that showed 97 percent of people using the HOT lane had no passenger.
On SR-167 near Seattle—a roadway included in the Texas study—HOT lanes have existed since May 2008, just as gas prices were spiking. Indeed, the researchers found that carpooling in the SR-167 lanes followed gas prices. As gas got cheaper, fewer carpools were recorded.
The circumstances of each roadway in the Texas study and timeframes of the available data were so dissimilar that strong conclusions are hard to come by. Carpooling has fallen as a commuting option overall, and the alternatives to carpooling continue to multiply. As Washington State’s Commute Trip Reduction Manager mentioned a year ago, communities continue to invest in transit, walking, and biking, giving commuters more choices.
The researchers also noted the stranger phenomenon of carpoolers simply moving to the general purpose lanes, foregoing the supposed advantage of a faster travel time in the restricted lanes. Another glitch in the Texas study was that some roadway conversions simply changed the definition of a carpool. Instead of needing only two people to use the lane for free, cars must have three people during peak hour on studied roads in Atlanta, Miami, and one of the Houston roads. Since the most common carpool consists of two family members traveling together, the requirement of finding a third might have pushed these users out of the lane.
Regardless of the usefulness of this particular study, the research does raise the question of the goals of HOT lanes. If they are meant to encourage carpooling and increase average vehicle occupancy, they may not be achieving that goal. However, they do provide those willing to pay for convenience or share the ride a quicker trip. But those willing to pay the toll may be pushing the carpoolers right out of the lane.
Robbie Webber is a Senior Associate at SSTI.