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The Hidden Benefits of HOV Lanes

A little smoothing would help here...

HOV, or “carpool” lanes, are, like everything, the subject of much controversy in the world of traffic. Do they make congestion worse for everyone while only aiding a few? Do they cause more crashes? Needless to say, I could have written a book only on the complexities of the Diamond Lanes (though I’m not sure who would want to read it).

A new paper, “The Smoothing Effect on Freeway Bottlenecks: Experimental Verification and Theoretical Implications,” from Michael J. Cassidy, Kitae Jung, and Carlos F. Daganzo at the University of California-Berkeley, presented at last week’s TRB (I’ll be data-mining the huge trove of research from that conference over the coming weeks), suggests that HOV lanes can provide overall benefits to highway traffic flow — even when the lane itself is underutilized. As with many things about traffic flow, this is beyond the grasp of the average driver, who may simply look over at the HOV lane, see that fewer cars are in it than his own (even if, of course, they’re carrying more people), and begin to grouse about misguided government policy.

The authors looked at a particular stretch of California freeway with a regularly occurring bottleneck, essentially a “merge bottleneck” resulting from increased volumes of entering traffic. A carpool lane becomes active during the morning and evening rushes; and one might be led to think the activation of the lane somehow causes the bottleneck. But the authors note, “previous analysis established that the carpool lane did not contribute to the bottleneck formation and capacity drop. Instead, and as is typical of merge bottlenecks without carpool lanes, the queue first formed in the shoulder lane and then spread to all lanes.” But even as the capacity of the carpool lane began to drop, the researchers observed an interesting pattern: The “discharge” of vehicles from the bottleneck in other lanes actually began to increase.

They looked at video footage, taken from a pedestrian overpass, to figure out what was going on. Interestingly, it was all about changing lanes. In lane 2, as pictured above (the one next to the HOV), drivers made fewer changes in and out of it when the carpool lane was activated (some of those drivers may have previously been jumping back and forth between lane 1, the ‘fast lane,’ and 2).

And here’s where it gets really interesting:

The same phenomenon was observed a few minutes earlier in lane 3, as drivers started
anticipating the impending carpool restriction. The video data reveal that: (i) drivers’ tendencies to avoid the median (carpool) lane as its activation time approached created crowded conditions in adjacent lane 2, starting at about 14:52 hrs; and (ii) although this crowding temporarily induced some drivers to migrate to lane 3, its more significant impact was to dampen the entries made into lane 2 from lane 3; (see Cassidy, et al, 2008). The net result: lane-changing maneuvers between lanes 2 and 3 diminished at 14:52 hrs, as revealed by the boldfaced oblique cumulative curve in Fig. 5. As in the case of lane 2, this reduction in lane changing was accompanied by a sudden and sustained increase in lane 3’s discharge flow.

The authors observed what they called a “smoothing effect.” Drivers were less tempted to change lanes, because there were fewer options available, and the “discharge flows” actually increased. When the carpool lane wasn’t activated, lanes saw anywhere from 9% to 13% worse performance in VPH (vehicles per hour). When considered in terms of “people hours traveled,” the activation of the carpool lane provided a benefit on the order of 30%. The authors note that if this “smoothing effect” is not observed and quantified, long highway delays might be incorrectly attributed to the carpool lane.

What’s interesting about this is that for many of those individual drivers, they were presumably changing lanes to try and improve their own position. But in doing so they were actually reducing the overall performance of the highway.

As the authors note, it’s worth investigating how signage and striping might reduce “disruptive” lane changing. “Disruptive lane changing,” they add, “might also be reduced in some cases by sorting drivers (and vehicle classes) across lanes according to their preferred travel speeds; or in other cases by inducing a more even distribution of flows across lanes.”) We already have seen “variable speed limits” to help smooth out flow in a linear sense; maybe someday we’ll have “variable lane assignment” to smooth it out across the highway.

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This entry was posted on Monday, January 19th, 2009 at 11:24 am and is filed under Cars, Congestion, Traffic Engineering, Traffic safety, Traffic Signs, Traffic Wonkery, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

One Response to “The Hidden Benefits of HOV Lanes”

  1. Robert E. Clucas Says:

    Dear Mr. Vanderbuilt,

    I happen to live in Hayward and I am very familiar with this
    particular bottleneck. Although I have not read the article by Cassidy
    et. al., I think it is worth pointing out that there are unique
    features that affect this particular stretch of interstate – to my
    mind, it is not a particularly representative sample. What makes this
    section of highway unique is that:

    1.) The shoulder lane north of Tennyson on I-880 is also a heavily
    used exit lane from I-880 for the off-ramp immediately north of the
    study location (the Highway 92/Jackson Street cloverleaf);

    2.) The distance from where the Tennyson traffic enters and Highway
    92/Jackson Street exists is very short (less than ¾ mile),
    complicating both entry to and exit from I-880;

    3.) Many cars entering from Tennyson immediately exit onto Highway
    92/Jackson Street and never merge into the through lanes on I-880;

    4.) There has been major reconstruction at the Highway 92/Jackson
    Street cloverleaf and lane alignment modifications in the study area
    in recent months, which continues to complicate navigating the study
    area (I have no idea when the study took place, so this may or may not
    be a factor); and, most importantly

    5.) The bottleneck on I-880 along this stretch of highway, and
    particularly during rush hours, is caused by a heavy stream of drivers
    attempting to exit at Highway 92/Jackson Street, which is a major
    shortcut across Hayward to I-580 (check it out on Google Maps/Earth).
    In other words, this backup at rush hour is caused by drivers on I-880
    exiting at the next off-ramp immediately north of the study area (at
    Highway 92/Jackson Street), rather than drivers merging onto I-880
    from Tennyson. The real reason drivers stay in their lanes during rush
    hour is because many of them do not want to lose position for their
    exit at the following Highway 92/Jackson Street cloverleaf, which can
    be tricky under any circumstances, but is particularly bad at rush
    hour when traffic restrictions in the diamond lanes are in effect
    (rush hour begins early in Hayward because of the widespread use of
    flex-time in Silicon Valley). How do I know this? Because I drive this
    section of I-880 on a regular basis and I am one of those frozen
    drivers who are just praying they make it into the shoulder lane and
    off the freeway at Jackson.

    I am a regular user of Diamond Lanes and I would like to believe that
    they help to reduce traffic. Because of the unique features of this
    particular stretch of I-880, I would be reluctant to draw such a
    conclusion from this study.

    Thank you for your informative and well written blog.

    Yours truly,
    Robert E. Clucas

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Traffic Tom Vanderbilt

How We Drive is the companion blog to Tom Vanderbilt’s New York Times bestselling book, Traffic: Why We Drive the Way We Do (and What It Says About Us), published by Alfred A. Knopf in the U.S. and Canada, Penguin in the U.K, and in languages other than English by a number of other fine publishers worldwide.

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