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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies For Design Exceptions

Project Description and Context

Interstate 235 is an urban freeway approximately 14 miles (22 km) long that runs through the heart of Iowa’s capital city. The freeway serves downtown Des Moines, the state capital complex, Drake University, and other local destinations.

The original freeway, constructed in the 1960s, was reconstructed in the mid 2000s. The design speed selected for the reconstruction project was 60 mi/hr (100 km/h), with a posted speed limit of 55 mi/hr. The design year (2025) traffic volume was 151,000 vehicles per day. Because the project involved full reconstruction, design criteria from the current AASHTO guidance (A Policy on Design Standards, Interstate System) were used.

The reconstruction project focused on improving safety and operations through the corridor, as well as replacing aging infrastructure. The project elements included:

Figure 72. Interstate 235 before reconstruction.

FIGURE 72

Interstate 235 before reconstruction.

Figure 72 is a photo showing a view of the Interstate before construction.  There is a left-hand entrance, deteriorating pavement, and non-standard median guardrail mounted behind curb. Old, uninteresting bridges span the freeway.

Figure 73. Interstate 235 after reconstruction.

FIGURE 73

Interstate 235 after reconstruction.

Figure 73 is a photo showing a view of the Interstate after construction.  There are four lanes of traffic in each direction, shoulders wide enough to accommodate vehicles, and a concrete median barrier.  In the distance, there is an aesthetically pleasing pedestrian bridge with one span over the freeway and a blue "basket–handle" arch. The appearance, condition, and geometry of the freeway is much improved as compared to the "before" photo.

Site Constraints

The design cross section that was used through much of the freeway corridor is illustrated in Figures 74 and 75. Twelve-foot (3.6-m) lanes, 12-foot (3.6-m) outside shoulders, and 12-foot (3.6-m) inside shoulders were provided, consistent with FHWA’s adopted criteria from AASHTO’s A Policy on Design Standards, Interstate System.

Figure 74.  Cross section within the unconstrained locations. Note the 12-foot (3.6-m) outside shoulders, and 12-foot (3.6-m) inside shoulders.

FIGURE 74

Cross section within the unconstrained locations. Note the 12-foot (3.6-m) lanes, the 12-foot (3.6-m) outside shoulders, and 12-foot (3.6-m) inside shoulders.

Figure 74 is a drawing of the design cross section used throughout much of the I-235 corridor, where the cross section was not constrained.  All lanes and shoulders meet criteria and are 12 feet (3.6 meters) wide.

Through most of the corridor, the existing median width was 50 feet (15.2 m), which meant that the added lanes, full shoulders, ramp connections, and median barrier could be accommodated within the median. However, over a length of about 4.6 miles (7.4 km), the existing median was about 10 feet (3.0 m) narrower than the rest of the corridor, which was not enough space to meet full design criteria for all of the cross-sectional elements.

Within this constrained area, providing a cross section that fully met criteria would have significantly increased the costs and impacts of the project on adjacent land uses. In addition to the right-of-way impacts, nearly $28 million would have been incurred for construction costs to widen the freeway to the outside, including significant utility relocation.

Figure 75. The unconstrained cross section.

FIGURE 75

The unconstrained cross section.

Figure 75 is a photo showing the cross section used throughout much of the I-235 corridor, where the cross section was not constrained.  There are full-width lanes and shoulders and a 10-foot (3.0-meter) median with concrete barrier on each side and lighting down the middle.  Crushed brick has been used to fill the space between the median barriers, and there is a new overhead structure in the distance, further illustrating the improved appearance of the new freeway.
 
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Page last modified on April 1, 2019
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