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Third Infantry Division Highway Corridor Study

4.0 Design Levels

Per the FHWA Task Order and input from the EWG, three roadway design levels were considered along the proposed corridors: Interstate, Arterial, and Super-2. At this conceptual level of detail, any design level could be applied to any corridor segment. Design levels are described below and were applied to corridors passing the fatal flaw screening.

Interstate Design Level. The first design level complies with the typical design standards for an Interstate route. The actual cross-section can vary depending on the width, design speed, type of median, and the terrain. Interstate System design standards require that an Interstate has no at-grade intersections. For this planning level of analysis, it was assumed that connections would be made through grade-separated interchanges. Smaller cross streets would either be terminated on either side of the proposed alignment or passed over/under the proposed facility.

A variation of this design level is a special Interstate section specifically identified for areas of rugged terrain or in environmentally sensitive areas. The special Interstate design level incorporates tunnels or elevated roadway sections on viaduct to minimize impacts. This cross-section would reduce the roadway footprint by reducing the need for cut/fill sections. It also helps to address concerns with rock slides, which are inherent due to the region's geology.

Examples of this special Interstate design level are found on Interstates and parkways throughout the country: the 4,600-foot Cumberland Gap tunnel on US 25E near the Kentucky, Tennessee, and Virginia border; tunnels through the Allegheny Mountains along the Pennsylvania turnpike, I-70, and I-76; and I-70 through Glenwood Canyon in Colorado. Special Interstate sections may help eliminate the need for seasonal road closures through the GRSM or address existing landslide issues along I-40 and similar roadways. Special Interstate sections should be considered during future project development activities if any corridors warrant additional consideration as an Interstate-level route.

Figure 6A shows an example cross-section for a four-lane Interstate facility; Figure 6B shows an example cross-section for the special Interstate design level.

Arterial Design Level. The second design level is for a four-lane, divided highway with at-grade intersections. Grade-separated interchanges may be included at major arterial junctions with other arterial routes. Four lane arterials typically have 60 to 70 mph design speeds with 12-foot lanes and full width paved shoulders. The width of the median can vary, as shown in Figure 7.

Super-2 Design Level. The third design level is for a three-lane highway with at-grade intersections. Super-2 highways start with a standard two-lane cross sections and, as needed, a third lane is added for passing, truck climbing, turning, and other purposes. Research shows Super-2 highways are typically safer than two-lane highways and can be constructed at lower costs than traditional four-lane highways. Figure 8 illustrates an example cross-section for a Super-2 highway, showing sections of both a two-lane and three-lane segment.

As part of the cost estimating task, a lowest cost scenario was also applied to reuse existing routes as much as possible. The Minimal Build option is intended to present the minimum level of improvement necessary to implement a continuous two-lane 3rd Infantry Division Highway route within a given corridor.

Figure 6: Click for long description

The typical cross section for an arterial would carry two 12-foot travel lanes per direction with 2-foot wide paved inner shoulders and 6.5-foot wide paved outer shoulders.  Medians widths vary from 32 to 44 feet.

The typical cross section for a Super-2 highway varies between two and three travel lanes with 2-foot paved shoulders on both sides.  The third lane is added as needed for a passing lane or truck climbing lane, as appropriate.

Updated: 3/22/2013
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