How planners are chasing better traffic flow with design
Few things are more frustrating than being stuck in traffic. Yet Americans spent an average of 42 hours waiting out gridlock in 2016, costing commuters a total of $300 billion, or about $1,400 per driver, according to connected car services and analytics company Inrix.
Sitting in traffic can also take a psychological toll on commuters, say researchers Louis-Philippe Beland and Daniel Brent, both economics professors at Louisiana State University. The pair found that the bigger the traffic jam, the more stress and aggravation drivers tend to suffer. Reported incidents of domestic violence went up as well.
One solution is to add more lanes to congested roadways, an option limited by space. "You cannot expand forever," said Deogratias Eustace, director of the Transportation Engineering Laboratory at the University of Dayton, in Ohio.
The pressure to give up more real estate to roadways is felt acutely in high-growth areas, where population growth is triggering building booms and as a result has planners barely able to keep up with demand.
"It's difficult to stay ahead of the population game," said Kurt Knebel, executive vice president of McCarthy Construction’s Texas division and head of the company’s civil business unit in the region. "In Texas, we've done multiple projects on the same stretch of roadway," he said. "By the time you get it designed and constructed, pretty soon you need some more."
Federal agencies funding such projects require lengthy environmental reviews that include detailed projections about future development near the proposed roadway as well as expected traffic conditions decades into the future. However, it can be hard to anticipate when new businesses or residential developments will sweep in and change that carefully thought-out dynamic, Eustace said.
Using standby designs
State transportation departments and their design teams can reduce the level of congestion and better control traffic flow using one of a handful of interchange layouts. Interchanges use ramps and connectors to direct traffic from one freeway to another or to move traffic from the freeway onto local roads.
The cloverleaf, the diamond and the single-point urban or diamond interchange are elements of conventional interchange design, according to the Federal Highway Administration (FHWA).
The cloverleaf is a two-level interchange that allows left turns by directing traffic over the desired road and then looping it back around onto the road via a right-hand exit. The diamond interchange has four ramps that exit and enter the freeway at angles so as to meet local roads at right angles, allowing a right or left turn at a stop sign or traffic light. The single-point urban or diamond interchange is a variation of the diamond configuration. The single-point urban interchange's ramps meet at one point either under or above the freeway, necessitating only one set of traffic signals.
Growth of hybrid models
While the traffic needs of many larger cities require unique solutions that don't fall squarely into one of these categories — along the lines of the Spaghetti Bowl in Las Vegas, which is now undergoing a $1.5 billion renovation — they represent the basics from which hybrids have emerged. One of those is the diverging diamond interchange (DDI), which has been gaining popularity since its introduction in the US. in 2009. Florida alone has 38 DDIs under construction or in development, according to Florida Today, six of which are part of the Ultimate I-4 project near Orlando.
One of the DDI's most sought-after characteristics is that it does away with left turns across traffic, increasing the pace of traffic flow and decreasing the chance of accidents. The country's first-ever DDI is in Springfield, MO. In its first year of operation, total crashes were down 46% in that location and left turn–related accidents were down 72%, according to the Missouri Department of Transportation (MODOT). MODOT also surveyed motorists and 80% reported better traffic flow and fewer delays.
"In Texas, we've done multiple projects on the same stretch of roadway. By the time you get it designed and constructed, pretty soon you need some more."
Executive vice president, McCarthy Construction
Knebel said contractors aren't involved much in the planning for these roadways unless they are being carried out through a design–build arrangement. However, DDIs seem to be popular when DOTs want traffic to easily access frontage roads and interstate overpasses with minimum impact.
Another, more unconventional method of reducing traffic congestion on local arterials is roundabouts, which are more prevalent in Europe and other parts of the world than they are in the U.S. Roundabouts, also known as traffic circles and rotaries, move traffic in a counter-clockwise motion around an island. There are no stop signs, and drivers circle the island until they are able to exit at their desired street. According to the FHWA, roundabouts have increased traffic capacity and reduced accidents where adopted in the U.S.
Challenges to implementation
Could it simply be drivers' fear of being stuck in an endless roundabout loop that is preventing wider implementation? David Tullis, design-build manager for Skanska USA's civil business, said roundabouts are more prevalent in the Midwest but that their development is often constrained by space limitations. As a result, their use in the U.S. tends to be limited to intersections with low or medium traffic volume, he said.
Americans' lack of familiarity with the road feature may have stymied its ability to gain traction in much of the country. With a little education, Eustace said, U.S. drivers could benefit from them.
Until then, planners' priority is to carry out traffic improvement projects with as little disruption to traffic flow as possible. That is no small feat when considering work being done on today's massive bridges and highways, which see high traffic volumes daily. This is particularly true in urban areas or within the environment of an infrastructure asset like an airport. "The goal during construction would be to at least maintain the current conditions, [where the] end result is better than where you started," Tullis said.
Despite being such a regulated environment, airports, specifically, are fraught with traffic-related logistical concerns given the ebb and flow of for-hire vehicles and buses, as well as the rush around flight schedules and the holidays, he said.
The pressure to deliver transit projects as soon as possible persists. Even the most well-thought-out plans can develop cracks along the way, such as starting a project without the necessary land acquisitions finalized or hiring a construction team before the final pieces are in place. "They push so hard to get the projects going that sometimes they're really not ready for it," Knebel said.
Until the country is ready to fully embrace alternatives like mass transit, contractors and DOTs will keep trying to make traffic flow a little smoother through new roadway design to handle increased capacity. "It's a never-ending challenge," Knebel said.
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