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Phoenix Opens Elevated Light-Rail to Airport

Ground view of the PHX Sky Train overhead transit crossing
The $644-million PHX Sky Train automatic people mover at Phoenix Sky Harbor International Airport features what may be the first overhead transit crossing of an active taxiway anywhere in the world. The 100 ft high bridge is tall enough to allow a 747 to pass beneath. City of Phoenix Aviation Department

The first segment of the eagerly anticipated PHX Sky Train, which gives Phoenix the distinction of having a mass transit system that extends above an airport taxiway, has opened for service.

May 7, 2013—In a city known more for sprawl than for mass transit, Phoenix has gradually attempted to improve its transportation options. In late 2008 the city opened the first leg of its light-rail system, a 20 mi line connecting the city to the neighboring cities of Tempe and Mesa. Now the city has opened the first leg of an automated people mover (APM) it calls PHX Sky Train, which links the light-rail system to Phoenix Sky Harbor International Airport, one of the busiest hubs in the United States.

The first of three stages of the $1.58-billion PHX Sky Train began service last month. This stage is a $644-million, 1.7 mi line connecting the light-rail line to the airport area called East Economy Parking and to Terminal 4, the airport’s newest and busiest terminal. The second stage, dubbed 1A, is under construction and will connect Terminal 4 to Terminal 3, a distance of 0.7 mi. The final, 2.5 mi stage will bypass Terminal 2, which is a short walk from Terminal 3, and connect directly to the airport’s rental car center.

The PHX Sky Train, funded by airport revenues and passenger fees, travels from the light-rail line’s 44th Street station to East Economy Parking in just three minutes, and after two more minutes passengers reach Terminal 4. (The platforms for the 44th Street station and the PHX Sky Train are separated by a 500 ft air-conditioned pedestrian bridge.) The driverless trains, which run on rubber tires and are electrically powered by a center guide rail, will run as frequently as every three minutes at an average speed of 23 mph. 

The station at 44th street connects the PHX Sky Train with the city's light-rail

The station at 44th street connects the PHX Sky Train with the
city’s light-rail; the 1.7 mi line allows passengers to reach the
airport in five minutes. Bob Perzel

Terminal 4, which opened in 1990, was designed with the idea that a mass transit connection would eventually be built beneath it. Early in the last decade, the airport conducted a series of studies to gauge the viability of a tunnel system that would carry some sort of mass transit line. “As they did some preliminary designs on the tunnel alternative, it became kind of clear that tunneling was going to be fairly high risk, and it was going to be fairly expensive,” says Mark Pilwallis, P.E., the facilities design leader on the project for the global infrastructure firm Gannett Fleming, Inc., which is conducting the geotechnical and structural engineering of the PHX Sky Train, having designed the airport’s Terminal 4. “Some of the connections with the terminals were going to be difficult from a level-change perspective,” Pilwallis says.

So planners stepped back to see what other options made sense. Extending light-rail to the terminals, running dedicated buses, and constructing an APM were considered. The bus option was discarded because there simply wasn’t enough room to expand the roadways near the airport to accommodate them. And the life-cycle costs of buses, even buses running on a dedicated roadway, were too high compared with the APM option.

When compared with light-rail, an APM system was found to give faster passenger ingress and egress. Planners also realized that having light-rail trains make several stops at the airport would significantly undermine the efficiency of the system. “Commuters from Mesa don’t want to stop every day three times at the airport,” says Kyle Kotchou, a project manager with the city’s Aviation Department.

Rendering of the PHX Sky Train which connects to the city's light-rail system with the airport's East Economy Parking Garage

The PHX Sky Train connects the city’s light-rail system with the
airport’s East Economy Parking Garage en route to the airport’s
Terminal 4. The train will eventually connect to Terminal 3, and
then onto the airport’s rental car center. City of Phoenix Aviation

In terms of initial costs, long-term costs, and service levels, the APM was the best solution. The airport conducted charettes to determine the proper number of stations and the best route alignment, the goal being to find a cost-effective solution that would have a minimal effect on the airport.

Since planners wanted to make a good connection into Terminal 4, they opted for an elevated station on the south side of the terminal. However, that meant building a bridge over one of Sky Harbor’s taxiways—perhaps the first aboveground crossing of an active taxiway by a mass transit line anywhere in the world. Airport staff members and engineers held numerous meetings with the Federal Aviation Administration, and they had to submit several technical documents to show what they thought the appropriate clearance should be. They also consulted air traffic controllers to ensure that there were no line-of-sight issues.

The taxiway was shut down for six months during the construction of the bridge. The cast-in-place concrete main span of the bridge runs 350 ft from center bearing to center bearing, and the clearance between the piers is 340 ft. The columns supporting the guideway are set 125 ft into the ground, the depth nearly equal to the columns’ aboveground height. The center of the bridge rises 100 ft above the taxiway, high enough to allow a 747 to pass beneath. (See “On Solid Footing,” Civil Engineering, May 2011.)

Another ground view of the PHX Sky Train overhead transit crossing

The driverless trains run on an electrically powered center-rail
guideway. The project is certified at the gold level in the LEED
program. Bob Perzel

The Federal Aviation Administration, Pilwallis says, was reasonable during the whole process, despite the novelty of the idea. “We provided all the right clearance envelopes,” he says, “and we didn’t impact any of the protected airspace.”

As Kotchou describes it, two main projects were proceeding simultaneously: the fixed-facility side of the project, which involved the infrastructure, and the train system contract, which was awarded to Montreal-based Bombardier and dealt with the assembly of the cars and the construction of the tracks. “The biggest challenge in that [was] just the coordination between the two,” he explains. “The tolerances for these train systems, because of the ride quality we require, are very tight. When you’re building massive concrete structures for the distance that we’re doing, to get those tolerances is very difficult—not only in design but also in construction.”

The superstructure was a flat deck when it was turned over to Bombardier, which in turn poured the running surfaces for the rubber-tired cars and built the steel guide rail. “You have a lot of change in temperature over the day here,” Kotchou says. “Those big structures are moving at those [expansion] joints. It’s critical we control the movement, significantly, especially when you have a steel guide rail running over that.”  

The first phase of the system met the requirements for gold certification in the U.S Green Building Council’s Leadership in Energy and Environmental Design (LEED) program; it is expected to reduce greenhouse gas emissions by 5,913 tons a year. The amount of water needed for the stations’ landscaping will be half of what is normally required thanks to a drop irrigation system outfitted with “smart” controllers, and the power used by the stations will be 30 percent below an established baseline. Furthermore, more than 50 percent of the construction demolition waste will be recycled or salvaged, and 10 percent of the building materials are recycled.

When the second stage of the PHX Sky Train is complete, the airport anticipates 40,000 riders a day. According to Julie Rodriguez, an airport spokesperson, “We had originally predicted seven thousand people a day. It’s currently higher than that. We know people are coming here to ride it because they just want to ride it. They’re not traveling, but it’s new and it’s fun. But we don’t know if that will settle down to seven thousand or if it will actually be higher than we projected.”



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