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Public Works Financing

Time to Rethink Carpool Lanes

Census shows declining numbers

Robert Poole
August 21, 2002

The recently released journey-to-work figures from the 2000 census reveal what many of us have long suspected: carpooling is a flop. Despite the expenditure of billions of dollars adding carpool lanes to congested freeways, carpooling declined from 13.4% of work trips in 1990 to 11.2% in 2000. Carpooling's mode share declined in 36 of the largest 40 metro areas—including highly congested Los Angeles and San Francisco. So what do we do now?

Some transit advocates have resented the use of these lanes by carpools from the outset, reminding us that high-occupancy vehicle (HOV) lanes were originally conceived as busways and should be converted, accordingly. Some highway advocates argue vociferously for converting most carpool lanes to general purpose (GP) lanes. And the Federal Highway Administration's Value Pricing office tries valiantly to persuade metro areas to convert underperforming HOV lanes to HOT lanes.

What's been missing from this discussion has been serious quantitative analysis of the tradeoffs involved. That gap has recently been filled by a little-noticed UCLA Ph.D. dissertation by Eugene Kim, "HOT Lanes: A Comparative Evaluation of Costs, Benefits, and Performance." Kim used a logit travel-demand model to estimate the comparative travel times that would come about by converting an existing HOV lane on a congested freeway to either (a) a GP lane, (b) a HOT lane, or (c) a Toll lane. He also estimated long-term (20-year) costs and benefits of each alternative, as well as environmental impacts.

The results are striking. In almost all cases, HOT or Toll lanes provide a greater degree of fiscal, consumer welfare, and environmental benefits than any other expressway investments. Echoing previous research by UC Berkeley's Joy Dahlgren, Kim shows that there is a very limited set of conditions under which HOV lanes can be the best option. But in most cases, society would be better off if the lanes were converted. Converting to GP lanes is most defensible when HOV use is less than 7% of all corridor trips, and there are under 700 vehicles/hour in the HOV lane. But in almost all cases, converting to a Toll lane produces greater benefits, primarily because it can preserve free-flow conditions as traffic continues to grow and freeway congestion worsens. And because Toll lanes generate substantial revenues for the highway system.

Whether to convert only to HOT (carpools still go for free) or go all the way to Toll receives detailed attention in Kim's work. Intuitively, one might expect that conversion to Toll would produce less delay-reduction than conversion to HOT, because fewer people will continue to carpool if those vehicles have to pay. But the modeling shows that conversion to Toll produces large delay-reduction benefits "regardless of whether the conversion . . . results in a significant increase or decrease in the initial proportion of HOVs."

These results clearly support the idea that many of today's HOV lanes are candidates for conversion to Toll. As Kim points out, tolling indirectly preserves economic incentives to ride-share, by (1) spreading the toll over more than one person, and (2) by providing insurance against travel-time uncertainty in the event that a carpool participant unexpectedly cancels—an effect already observed on the I-15 HOT lanes. But there's a way of guaranteeing that these Toll lanes would continue to serve large numbers of people in high-occupancy vehicles: let express buses use the Toll lanes at no charge. One 60-paggenger express bus takes up no more room than three cars, but carries as many people as 20 three-person carpools.

What about the operating characteristics of Toll lanes? Both existing California HOT Lanes (91 Express and I-15) use variable pricing to maintain 65 mph speeds during peak hours. Critics have argued that this is excessive, because (1) maximum throughput is obtained in the 35-50 mph range and (2) because emissions are significantly worse at 65 mph than in the lower speed range. Recent research challenges both beliefs.

Pravin Varaiya at UC Berkeley analyzed speed-flow data on congested LA-area freeways, using the new Caltrans Performance Measurement System. Measuring actual speed and throughput during entire rush-hour cycles, Varaiya validated the traditional bullet-nose-shaped speed/flow curve—i.e., as volume increases, speed gradually decreases until reaching a point of instability at around 2100 vehicles/lane/hour, at which point both speed and flow decrease significantly (with speeds dropping to 15 mph and flows cut to as low as 1300/hour). He concludes that 60 mph is the most efficient rush-hour speed and that lower speeds such as 45 mph are not sustainable (because flow becomes unstable).

In his dissertation, Kim compared the environmental impacts of his four alternatives using the EMFAC 2000 model. He concludes that the HOV case "produces a greater output of ROG, NOx, and CO than converting to either GP or toll lanes." And "a Toll lane produces the largest emissions reductions because it eliminates some vehicle trips (like an HOV lane) while reducing congested conditions more effectively than a GP lane." This is especially the case when the analysis encompasses a 20-year period, as Kim's does.

Kim's results are reinforced by a recent study of Houston's now-rejected 55 mph freeway speed limit, imposed briefly as an air-quality remedy. A new study by Environ International Corp., using the EPA's recent MOBILE6 model, found no measurable effects of the 55 mph limit on ozone and only modest impact on NOx (with most of that impact coming from heavy vehicles, not cars).

Thus, a growing body of research supports the case for converting most of America's HOV lanes into value-priced Toll/Bus lanes, operating at a throughput-maximizing

60-65 mph design speed. How about it, FHWA and FTA?

Robert W. Poole Jr. is director of transportation studies and founder of the Reason Foundation.


Robert Poole is Searle Freedom Trust Transportation Fellow and Director of Transportation Policy


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