Long range transportation studies in the Phoenix metropolitan area have identified the need for non-freeway restricted access facilities able to offer significantly greater travel capacity than that provided by major urban arterials. This roadway facility alternative would provide additional travel capacity without employing full grade-separations at intersections with arterial cross-streets. It would provide the benefit of increases in intersection capacity while maintaining the potential for direct driveway access to each quadrant of the intersection. And, where this facility has been implemented in other states, it has demonstrated significant safety benefits over standard intersection designs.
It has been termed the “Arizona Parkway.”
This innovative design alternative generally focuses on the provision of simple two-phase traffic signal operations at the intersections by eliminating left-turn movements at the intersection and accommodating them elsewhere. This has been done throughout the state of Michigan and is commonly referred to as the Michigan left-turn (MLT), or indirect left-turn, intersection design. In an MLT intersection, the left-turn movement is accommodated through a strategically placed U-turn break in the median on the far side of the main roadway. These U-turn breaks can be signalized in particularly high-volume areas. Shown below is a generalized layout for a typical MLT intersection.
Key: Green arterial to parkway, Red parkway to arterial — Source: MichiganHighways.org
The links below lead to animations portraying exactly how such an intersection would function.
Key advantages of this type of roadway over a typical urban arterial include:
Following the preliminary recommendations of the Hassayampa Valley Framework Study, the Maricopa County Department of Transportation led several studies to identify the operation and construction of the Arizona Parkway cross-section. The first set of documents are related to the MCDOT Enhanced Parkway Studies that report on technical studies that tested the traffic operation limits of the Arizona Parkway design in a simulated fully congested scenario. The second set are the product of MCDOT's Design Guidelines studies where guidance is provided on the potential construction of these facilities. Both MCDOT led studies were conducted in conjunction with the Maricopa Association of Governments, the City of Surprise, Arizona, and the Town of Buckeye Arizona.
The Maricopa County Department of Transportation launched a ground-breaking study to review the proposed Arizona Parkway network in the Hassayampa Valley and Hidden Valley study areas and establish at-grade parkway-parkway intersection thresholds. From these thresholds, the study determined which locations would remain at-grade and which would need grade separation. The project's results were developed to provide a framework for categorizing future parkway-parkway intersection treatments for the purpose of protecting future right-of-way (ROW) needs. An outcome of the study was the innovation of the "Parkway Grade Separated Interchange" or PGSI as an effective treatment for accommodating parkway-parkway intersections with volumes reaching 260,000 entering vehicles daily.
As demonstrated above, the Arizona Parkway has its roots in a design that has been implemented in other parts of the United States and operational for the past 40-years. The following documents provide additional information about the design behind the Arizona Parkway from the Federal Highway Administration and the Michigan Department of Transportation.