It all starts with SH 161 in Irving, where the three lane road drops to two. That is the definition of a bottleneck, and a recipe for gridlock. TXDoT's solution: convert the shoulder into a third lane for use during peak hours. It cost $4.25 million. The congestion eased and The Dallas Morning News declared a success with a headline: "Once TxDOT opened up SH 161's Shoulders, Traffic Started Sailing."
Well, not so fast.
The initial data, collected in late 2015, indeed shows improvement. One PowerPoint, cited in local and national media, includes data from 2015 that shows the wider road has alleviated the congestion. Jason Crawford, region manager at the Texas A&M Transportation Institute, collected the data and put together the report. "We're seeing more volume at higher speeds," he tells the Dallas Observer.
The success of the change made engineers and transportation planners scratch their heads. "Numerous studies ... consistently show that adding capacity to roadways fails to alleviate congestion for long," writes Susan Handy, Department of Environmental Science and Policy University of California, Davis, summing up the conventional wisdom in a study.
"Some of the speed benefits we saw in late 2015 have degraded."
tweet this
The basic argument is that more people will use a road that is less prone to congestion, causing a return of slower traffic. The wonks call this phenomenon "induced travel” and compare it to lowering the price of product to increase its sales.
SH 161's third lane is no exception to the rule. Crawford says his followup study, due to be presented the first quarter of next year, will show some congestion returning. "Some of the speed benefits we saw in late 2015 have degraded as more drivers feed into the system," he says. "We're still seeing good movement of vehicles through the corridor, but the speeds have evened out."
So, the early verdict seems to be: Traffic is better on SH 161 because the bottleneck has been eliminated during peak hours, but this progress is limited by the influence of induced travel.
That brings us to the new plan for that stretch of SH 161. The proposed project would modify the state highway, from one mile south of SH 183 to one mile north of Beltline Road, to accommodate an eight-lane freeway. That's four freeway lanes in each direction. "The reconstructed portion of SH 161 would have 12-feet-wide lanes with a 10-foot-wide outside and inside shoulder," TXDoT says. "Eleven ramps would provide access between the frontage road lanes and the freeway lanes. ... No displacement or relocation of residential or commercial properties would be required."
The $50 million project is expected to begin in 2018 and finish in 2021.
So will this new project confound experts again? Likely, no. Fixing a bottleneck by opening a third lane is a specific and interim step, and even that suffered from induced travel. No one should expect for the state to double the size of the road and expect drivers not to take advantage. With population growing in North Texas and existing roads clogged with drivers, the new SH 161 could become as congested as the one it's replacing.
Every road, which transportation wonks call a facility, has a limit of how much use it can take. "Any facility can handle an increase in volume," Crawford says, "but only up to a breaking point."
