'Why urban freeway expansion is futile': A report that cites I-30 project | Arkansas Blog

Tuesday, August 15, 2017

'Why urban freeway expansion is futile': A report that cites I-30 project

Posted By on Tue, Aug 15, 2017 at 12:34 PM

click to enlarge A table from Smart Mobility
  • A table from Smart Mobility

Smart Mobility Inc. engineer Norm Marshall explains in this Congress for New Urbanism article why highway departments' arguments for highway widening fall short of reality (as the graphic above does as well): He says its because their modeling is out of whack, both in its estimates of the effect of expansion on travel times and the effectiveness of boulevards, and he uses the plan to widen Interstate 30 from six lanes of traffic to 12 or more as an example.

Despite what analysts like Marshall and others say about their modeling, the Arkansas Transportation Department (now ArDOT, thanks to legislation passed earlier this year that dumped the old Arkansas Highway and Transportation Department) will go ahead and spend more than $600 million to widen seven miles of I-30 so the traffic can jam up farther down the road. The road builders don't care much about the consequences to the atmosphere of encouraging transportation by car rather than less polluting means, either.

The crux of Marshall's argument against road-builder models is this:

Despite enormous advances in computer hardware and software, highway engineers are still relying on a 50-year-old algorithm called Static Traffic Assignment (STA). STA does not consider bottlenecks at all. Instead, it tries to estimate congestion delay by estimating delays for each road segment separately and adding them together.

click to enlarge I-35 in Austin, Texas
  • I-35 in Austin, Texas

STA does not work for congested freeways. For example, I-35 in central Austin, Texas, is one of the most congested in the US. For northbound traffic in the peak period on a weekday afternoon, STA estimates over twice the actual traffic throughput counted, and a speed of 39 mph—almost twice the 20 mph actual average speed. STA overestimates traffic throughput because it does not properly constrain traffic at bottlenecks. It underestimates delay because it does not account for traffic backup delays behind bottlenecks. These STA problems are present in every region in the US. 


'Why urban freeway expansion is futile': A report that cites I-30 projectThis is what Metroplan staff have been telling us for a couple of years now: Widening a section of the road only creates blockage on its arteries. That sounds like a description of a heart attack, something that can only be avoided by making the arteries wider: Hence Metroplan's estimate that the I-30 project would require another $4 billion to widen the highway network in Pulaski County to actually achieve its goal of reducing travel times. Marshall said road-builders should use a "more sophisticated" calculation, Dynamic Traffic Assignment (DTA), which includes bottlenecks and delays behind them in the models. "It does not allow traffic volumes to exceed capacity. DTA should replace STA in all freeway planning work," Marshall writes.

The I-30 example:

STA and DTA lead to completely different conclusions about freeway expansion. The Arkansas Highway and Transportation Department is proposing that the 6-lane I-30 bridge over the Arkansas River between Little Rock and North Little Rock be replaced with a 12-lane bridge. In addition to I-30, there also are 2 4-lane street bridges within a mile of I-30 and 2 6-lane beltway bridges in the region.

Peak period, peak direction I-30 traffic volume is already at capacity (southbound in the morning and northbound in the afternoon). Therefore, throughput cannot increase significantly unless capacity is increased. With DTA, total afternoon peak hour northbound bridge crossings increase by 14 percent in the 2040 No Build alternative relative to 2010, with 97 percent of the increase on the two beltway bridges. There is only a 1 percent increase in traffic on I-30. [Times' emphasis.] In the Build alternative, afternoon peak hour northbound bridge crossings increase by 20 percent relative to 2010, and 67 percent of the increased traffic volume is on I-30. With DTA, total regional travel time in 2040 is higher in the Build alternative than the No Build alternative because total travel distance is greater but travel speeds do not change. Therefore, the Build alternative fails to reduce regional congestion.

In contrast, STA shows significant traffic growth in peak direction, peak hour traffic on I-30 whether the bridge is expanded or not because it fails to constrain traffic volumes to capacity. This exaggerated traffic throughput in the 2040 No Build alternative translates into extreme congestion. This unrealistic congestion can then be mitigated in STA by widening I-30. STA alone produces false benefits, but STA + microsimulation is even worse. The model was fed traffic volumes 20 percent over capacity, and ridiculously long delays are calculated. 
ArDOT's sole mission is to build roads. You can't expect them to think "boulevard" when what they want to build is a swath of towering, blight-inducing concrete. You can't expect them to consider Marshall's analysis and see if it's right. Sadly, there's no one arguing with them, including the board of directors of Metroplan.

Tags: , ,

From the ArkTimes store

Favorite

Comments

Showing 1-1 of 1

Add a comment

 
Subscribe to this thread:
Showing 1-1 of 1

Add a comment

More by Leslie Newell Peacock

  • More land for East Capitol View, Rose Creek trail

    Jason Baxter's seeming pie-in-the-sky sort of plan to develop East Capitol View may come closer to earth when the City Board at its reconvened meeting Dec. 12 considers as resolution to accept the donation of properties by Leland B. and Janet Jones "for the extension of the Rose Creek Trail" and to designate the land for a public park.
    • Dec 9, 2017
  • The chamber has some words to put in your mouth to support 30 Crossing

    The Little Rock Regional Chamber of Commerce is helping its members gin up messages to Metroplan in support of the 1950s-style highway swath that the highwaymen want to build through the heart of Little Rock's downtown.
    • Dec 7, 2017
  • "Perfectly Purple Pie" takes the prize at Mosaic Templars

    Celebrity judges at the Mosaic Templars Cultural Center’s sixth annual “Say It Ain’t Say’s” sweet potato pie contest on Sunday awarded first place to Kelli Marks of Cathead’s Diner for her “Perfectly Purple Pie,” and second place to Anne Woods of Honey Pies for her “Bubba’s Sweet Potato Meringue Pie.”
    • Dec 6, 2017
  • More »

Readers also liked…

  • Saturday's open line

    Got any thoughts? Put them here.
    • May 21, 2016
  • Free Zinn book for Arkansas teachers

    Arkansas teachers! Get your free Howard Zinn book here! Whether Kim Hendren likes it or not.
    • Mar 3, 2017
  • Kenneth Starr: A comment from Betsey Wright

    Betsey Wright, former President Bill Clinton's chief of staff when he was Arkansas governor, responds bitterly to a New York Times article today quoting Whitewater Prosecutor Kenneth Starr's warm words about Clinton. She can't forget the lives Starr ruined in Arkansas.
    • May 24, 2016

Most Shared

  • Conflicts of interest in the legislatures

    The Center for Public Integrity and the Associated Press collaborated for a project aimed at highlighting state legislators whose lawmaking might be affected by private business interests.
  • Industrial hemp pilot program coming soon to Arkansas

    One of the booths at this week's Ark-La-Tex Medical Cannabis Expo was hosted by the Arkansas Hemp Association, a trade group founded to promote and expand non-intoxicating industrial hemp as an agricultural crop in the state. AHA Vice President Jeremy Fisher said the first licenses to grow experimental plots of hemp in the state should be issued by the Arkansas State Plant Board next spring.
  • Cats and dogs

    I've always been leery of people who dislike animals. To my wife and me, a house without dog hair in the corners and a cat perched on the windowsill is as barren as a highway rest stop. We're down to three dogs and two cats, the smallest menagerie we've had for years.

Most Recent Comments

Blogroll

 

© 2017 Arkansas Times | 201 East Markham, Suite 200, Little Rock, AR 72201
Powered by Foundation