Some Like It HOT: High-occupancy toll lanes work best on high-traffic roads. Without congestion, drivers have little incentive to pay the toll.

Anthony Downs
Anthony Downs Former Brookings Expert

May 1, 2002

Economists advocate using tolls to ration space on congested roads. Their theory goes like this: Since drivers during peak hours do not have to bear the costs of the delay their presence on highways imposes, many drivers enter roads during peak hours. If a toll were charged during congested periods—and if it were set high enough—the number of drivers entering the road could be reduced enough to maintain rapid traffic flow.

In practice, this idea would be unacceptable to most American drivers. They believe high tolls would force them off the best roads during the most convenient hours while wealthier commuters move rapidly on those roads—a situation that strikes most drivers as grossly unfair. As I see it, this sense of unfairness is now, and will always remain, an insurmountable obstacle to charging variable tolls on congested freeways.

High-occupancy vehicle lanes—highway lanes designated for use only by vehicles containing two, three or more occupants—are a widely used alternative. HOV restrictions can drastically reduce the number of vehicles using such lanes, permitting cars that qualify for the lanes to travel rapidly during peak hours. This should create an incentive for people to switch from driving alone to carpooling, thereby expanding road capacity.

However, experience shows that the number of vehicles using HOV lanes is usually well below the capacity of each lane. Thus, HOV lanes are often underutilized.

Another approach involves high-occupancy toll—HOT—lanes. These are lanes that can be used by both high-occupancy vehicles (either without charge or with a reduced toll) and single-occupancy vehicles (with a variable toll during peak hours). The toll is determined by hourly vehicle flows and is set high enough in peak hours to keep the number of users down and, consequently, speeds of vehicles on the road up.

HOT lanes, however, do not eliminate peak-hour congestion on a crowded expressway, since such lanes comprise only a limited part of the road’s total capacity. The normal lanes remain heavily congested during peak hours. But HOT lanes do provide all drivers with a choice of paying a toll and moving rapidly or using toll-free normal lanes and experiencing congestion. HOT lanes have been used successfully on State Route 91 in Southern California since 1995, where they have notably reduced commuting times on both the HOT and normal lanes.

HOT lanes work best on roads where there is heavy traffic and long delays during peak hours. Without such congestion, drivers would have little incentive to pay significant tolls.

Creation of both HOV and HOT lanes is much more acceptable if it is done by adding capacity to an existing road. Conversion of existing lanes reduces the overall capacity of the road, thereby increasing congestion on the remaining normal lanes. Yet the new HOV or HOT lanes are clearly less congested than the remaining normal lanes. So the peak-hour drivers still on normal lanes realize they have been penalized with greater delays to benefit people using the HOV or HOT lanes. This will enrage many drivers, who will vehemently protest to public officials, often causing speedy reversal of such conversions.

Converting existing HOV lanes to HOT lanes should be considered only if the HOV lanes are significantly underutilized to begin with. If an existing HOV lane is so heavily used by HOVs during peak hours that it is near the capacity it can handle while maintaining high speeds, converting it to a HOT lane may reduce the road?s efficiency. Such conversion would allow some SOVs onto that lane during peak hours, and tolls might drive some HOVs away. That would reduce the total number of persons using these lanes in peak hours.

Surveys of HOT lane users show that people with relatively high incomes are more likely to use them regularly than people with relatively low incomes. On the other hand, anyone can use a HOT lane for high-speed movement on a particular day when such movement is important to him or her—which would be impossible if the road was toll-free.

Since the per lane per hour capacity of HOT lanes is limited by the need to maintain high speed, there may be long lines of drivers waiting to get onto those lanes during peak hours. This would offset some of the benefits of the HOT lanes.

A few other pointers on HOT lanes: They should be created only as part of the entire highway network in which they will be located, with full recognition of how those lanes will affect the whole network. Also, HOT lanes will be more politically acceptable if the money collected from the tolls is clearly going to be spent improving the capacity of that road or others in the same basic network.

In the right circumstances, HOT lanes can be a means of at least offering an option during peak hours to drivers willing to pay for a fast-moving advantage. Such a choice would not exist without the HOT lanes.