Clear Zone - A Synthesis of Practice and an Evaluation of the Benefits of Meeting the 10 ft Clear Zone Goal on Urban Streets

Project status


Start date: 06/01/06
End date: 11/30/08


Principal investigator:

Co-principal investigators:

Other authors: Christian Sax

Student researcher:

  • Christian Sax

About the research

In urban communities, there are often limited amounts of right-of-way available for establishing a large setback distance from the curb for fixed objects. Urban communities must constantly weigh the cost of purchasing additional right-of-way for clear zones against the risk of fixed object crashes. From 2004 to 2006, this type of crash on curbed roads represented 15% of all fatal crashes and 3% of all crashes in the state of Iowa. Many states have kept the current minimum AASHTO recommendations as their minimum clear zone standards; however, other states have decided that these recommendations are insufficient and have increased the required minimum clear zone distance to better suit the judgment of local designers.

This report presents research on the effects of the clear zone on urban curbed streets. The research was conducted in two phases. The first phase involved a synthesis of practice that included a literature review and a survey of practices in jurisdictions that have developmental and historical patterns similar to those of Iowa. The second phase involved investigating the benefits of a 10 ft clear zone, which included examining urban corridors in Iowa that meet or do not meet the 10 ft clear zone goal. The results of this study indicate that a consistent fixed object offset results in a reduction in the number of fixed object crashes, a 5 ft clear zone is most effective when the goal is to minimize the number of fixed object crashes, and a 3 ft clear zone is most effective when the goal is to minimize the cost of fixed object crashes.


Report: (6.8 mb pdf) November 2008

Tech transfer summary: Establishing Effective Clear Zones on Urban Streets (186 kb pdf) Nov 2008



  • Iowa Department of Transportation
  • Iowa Highway Research Board
  • Midwest Transportation Consortium