I-10 Twin Peaks Traffic Interchange, Environmental Assessment

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Alternatives October 2005 CHAPTER 3: ALTERNATIVES Chapter 3 describes the project alternatives, including the preferred alternative and the no build alternative. The analysis of alternatives was documented in the Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista Alternatives Selection Report, dated May 3, 2004. Although readers are referred to the Alternatives Selection Report for a more thorough discussion, the results of the analyses are summarized here. Alternative Corridor Evaluation A series of analyses were applied to improvement alternatives considered for this project. These analyses began with very simple qualitative criteria and progressed through a series of successively more detailed analyses as the alternatives were refined. The first analysis was to determine if a proposed alternative met the project purpose and need as defined in the previous chapter. If the alternative met the purpose and need, it was advanced for additional study, if it did not, it was eliminated from additional consideration. Both the 1989 Marana Master Transportation Plan and ADOT’s 1990 I-10 Design Concept Study, Ruthrauff Road to the Pima/Pinal County Line recommended a new I-10 TI between the I-10 interchanges at Cortaro and Avra Valley Roads. This location served the Town of Marana’s objective to serve as part of a planned parkway system and achieved ADOT’s goals for desirable interchange spacing and for traffic congestion relief at the Cortaro Road TI. During the Public Open House of October 20, 2003, several individuals suggested that the new interchange with I-10 be constructed north of Twin Peaks Road; therefore, this alternative corridor was considered early in the analysis of alternatives. It was found that the construction of a new TI north of Twin Peaks Road would have two major disadvantages: 1) it would not provide proper interchange spacing along I-10; and, 2) it would not adequately relieve traffic on Cortaro Road and Silverbell Road. According to AASHTO’s A Policy of Geometric Design of Highways and Streets (2001) (AASHTO Green Book), interchange spacing has a pronounced effect on freeway operations. A general rule of thumb for minimum interchange spacing is 1 mile from onramp to off-ramp in urban areas. Placement of an interchange north of Twin Peaks Road would result in less than recommended spacing between the ramps of the new interchange and the Avra Valley Road TI ramps. A grade-separated railroad crossing at the Twin Peaks Road TI location would produce desirable spacing of approximately five miles between grade separated railroad crossings at the Prince Road/La Cholla Road, Orange Grove Road, and Twin Peaks Road TIs. A TI has been recommended at the approximate location of Twin Peaks Road for many years because of the desirable design characteristics discussed above and because many trips originate and end in Continental Ranch and the surrounding area. If the TI were constructed north of Twin Peaks Road, Continental Ranch residents would be required to drive farther to access the new TI. As a result, the TI would be used less, and congestion Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista 3-1 Project No.: NH-010-D (AIW) TRACS No.: 10 PM 236 H5838 01D
Alternatives October 2005 and safety on Cortaro Road and Silverbell Road would not be improved sufficiently. For these reasons, an alternative corridor north of Twin Peaks Road would not meet the project purpose and need and was eliminated from further consideration. TI Alternatives After meeting the purpose and need analysis, alternatives were subjected to a more detailed level of analysis. Several different alternatives were selected for additional analysis. These included alternative TI configurations, alternative TI design options, and alternative TI alignments. Each of these are discussed below. Alternative TI Configurations Several alternative TI configuration options were considered during the study, including a roundabout traffic interchange, a tight diamond interchange, and a Single Point Urban Interchange (SPUI). Each of these configuration options is discussed below. Alternative TI Configurations – Roundabout Traffic Interchange A roundabout traffic interchange consists of a circular intersection in which entering traffic yields to circulating traffic. The intersection is not signalized. Entering traffic aims at the central island and is deflected slowly around it. These characteristics may increase the capacity of the intersection, reduce maintenance costs, slow traffic speeds, and may reduce the frequency and severity of crashes. A typical roundabout interchange is presented in Figure 3-1. Recently ADOT has considered roundabout intersections as possible alternatives for intersections of crossroads and frontage roads. Roundabouts have been constructed on freeway corridors in the Phoenix metropolitan area and were considered at the request of the Technical Advisory Committee for the Twin Peaks Road TI. Because the intersections of the crossroad and frontage roads at the proposed interchange would be elevated on fill, the construction of roundabouts on one or both sides of I-10 would present substantial design and construction challenges and would require additional rightof-way acquisition from property owners (including the railroad) in excess of that required by a tight-diamond interchange. As a result, the roundabout was eliminated from additional consideration. Alternative TI Configurations – Tight Diamond Interchange The diamond interchange is one of the most prevalent TI designs and is commonly used throughout Arizona. A full diamond is formed when a one-way diagonal ramp (either an on-ramp or off-ramp) is provided in each quadrant of the interchange. The ramps terminate at the crossroad and at-grade left turns are confined to the crossroad. The diamond interchange has several advantages: traffic can enter and leave the major road at relatively high speeds; left-turning maneuvers entail little extra travel; and, a relatively narrow right-of-way is needed. A typical diamond interchange is presented in Figure 3-2. Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista 3-2 Project No.: NH-010-D (AIW) TRACS No.: 10 PM 236 H5838 01D
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Bibliography October 2005 CHAPTER 8
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Bibliography October 2005 Tetra Tec
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I-10 Twin Peaks Traffic Interchange, Environmental Assessment

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