The Design Exception

Because of the limited cross-sectional width and the unacceptable impacts of widening to the outside, the Iowa DOT decided to pursue a design exception for either shoulder width alone or both lane and shoulder widths. Working cooperatively with the FHWA Division Office, the designers investigated various combinations of lane and shoulder widths for the available width that would optimize safety and operations (see the following section on Mitigation Measures).

The cross section that was eventually selected narrowed the two inside traveled lanes on both sides by 6 inches (FIGURE 76). This provided enough space to provide 8-foot inside shoulders. The resulting design represented a compromise in both the lane width and shoulder width values. The consensus was that although this design did not meet the adopted design criteria, it would function well operationally and would most effectively use the available cross-sectional width to optimize safety. A design exception was prepared for both the lane and shoulder widths.

FIGURE 76 Cross section within the area of restricted width. Note the 11.5-foot (3.5-m) inside lanes and 8-foot (2.4-m) inside shoulders.

Figure 76 is a drawing of the design cross section used through the design exception location, where the cross section was constrained.  The inside (median) shoulders are 8 feet (2.4 meters) wide.  The two inside lanes in each direction are 11.5 feet (3.45 meters) wide.
 
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