Misconceptions threaten some express toll lanes


More than 40 express toll lane projects are now in operation in large U.S. metro areas, bringing welcome relief from peak-period congestion. But a serious misconception about general-purpose (GP) lanes and express toll lanes (ETLs) threatens the relatively new ETLs on I-405 in the Seattle area and the nearly-finished ETLs on I-77 in the Charlotte area.

In Charlotte, opponents of the express lanes cite an “affidavit” submitted by a retired civil engineering professor which claims that opening the new ETLs on I-77 as GP lanes would permit 70 mph free-flowing traffic for the next 50 years, leading opponents to argue that the new lanes should be opened as GP lanes instead—or at least with one of the two lanes each way as GP instead of ETL.

In Seattle, opponents still argue that the new ETLs on I-405 have “failed” because there is so much demand to use them that they do not quite meet the federal standard of at least 45 mph average speed during peak periods. The only reason this occurs is a politically imposed cap on toll rates, which means more people crowd into the ETLs during the busiest peak than would do so at a higher price. An April report by Washington Policy Center claimed that converting one of the two ETLs each way to a GP lane would increase speeds in the GP lanes and hence reduce demand for the remaining ETL.

These assertions misunderstand the backward-bending speed/flow curve well-known to traffic engineers. As volume increases in a traffic lane beyond its maximum throughput capacity, traffic flow becomes unstable and drops into stop-and-go congestion. When that takes place, both speed and throughput (vehicles/lane/hour) decrease significantly. The only known way to prevent this in a traffic lane is to use variable pricing to limit the volume to an amount compatible with free-flow travel at some target speed (45 mph, 50 mph, or whatever the DOT selects).

Evidence of this phenomenon is available on hundreds of U.S. freeways that experience stop-and-go congestion during peak periods, while evidence that ETLs accomplish free-flow during peaks is ample (41 operational projects, only a handful of which have imposed price ceilings). But new evidence was posted online by Florida DOT in April. Its traffic modelers used data from the successful ETLs on I-95 in Miami to investigate what would happen if the two ETLs each way were converted to GP lanes. You can read the 18-page report yourself, but here is my summary of its most important findings:

AM travel speeds in the corridor would be up to 27% less for current GP lane users and up to 39% less for current ETL users. PM travel speeds would be up to 22% less for current GP lane users and up to 27% less for current ETL users. Overall AM traffic volume in the corridor would be up to 7% less than at present, while PM volume would be up to 8% less.

In other words, nearly all peak period users of I-95 in Miami would be made worse off by converting the express toll lanes to general-purpose lanes. The report is posted here.

Washington State DOT has also done computer modeling. I-405 project manager Kim Henry explains that they compared I-5 (four GP lanes and one HOV lane each way) with I-405 south of Bothell (three GP lanes and two ETLs each way). They found that I-405 in its new configuration significantly out-performs I-5. “Same number of total lanes, same daily traffic volume, and yet peak hours on 405 are performing at a much better level than peak hours on I-5, where we are seeing a much higher level of congestion,” Henry told reporters.

And in point of fact, both WSDOT and NCDOT are planning additional ETL capacity in those metro areas – because express toll lanes work.

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Robert Poole is director of transportation policy and Searle Freedom Trust Transportation Fellow at Reason Foundation. Poole, an MIT-trained engineer, has advised the Ronald Reagan, the George H.W. Bush, the Clinton, and the George W. Bush administrations.


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