U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The Iowa design exception illustrates the use of tradeoffs as a mitigation measure. Instead of simply using full lane widths and writing a design exception for shoulder width, the Iowa DOT went through a thoughtful design process that led to mitigating the narrow shoulders by using a small amount of width from several lanes on the freeway to provide a wider and more useable shoulder.
Even though the slightly narrower lanes were considered appropriate for this location, this treatment may not be appropriate for every location with limited cross-sectional width. It is important to analyze each location and its particular conditions individually.
Issues such as highway type, truck volumes, horizontal alignment, terrain, speed, and cross-sectional width available would all be variables that influence lane width. In this case study, even the method of pavement rehabilitation was a site-specific issue that influenced the lane and shoulder widths.
Good sources of information related to lane and shoulder widths on urban freeways include HSIS Summary Report, Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways (https://www.fhwa.dot.gov/publications/research/safety/05001/05001.pdf) and NCHRP Report 369, Use of Shoulders and Narrow Lanes to Increase Freeway Capacity.
The process that was followed for the Iowa design exception also makes this a particularly good model:
The Iowa DOT evaluated concepts that would meet design criteria. The designers identified the impacts and costs of meeting criteria before developing design exception alternatives. This included a benefit/cost analysis of widening the freeway to the outside. A clear understanding of both the social and economic costs was developed.
The Iowa DOT worked cooperatively with the FHWA Division Office. A number of ideas and alternatives were shared and discussed between the two agencies to determine the optimal lane and shoulder widths for this particular location.
Research was consulted to better understand the potential impacts of narrowing the traveled lanes.
The crash history of the location was evaluated, with particular emphasis on crashes that would be sensitive to shoulder width.
Several mitigation measures were evaluated and implemented.
The design exception was clearly documented. Initial review was inherent in the decisionmaking process that took place between staff working on the issue from both the DOT and the FHWA. Final review and approval was by the Director of the DOT’s Office of Design and the FHWA Division Director.