I-35 in Austin, Texas

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.

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.