Optimum Urban Clear-Zone Distance

In urban communities, there are often limited amounts of right-of-way available for establishing a setback distance from the curb for fixed objects. Urban road designers must weigh the cost of purchasing additional right-of-way for clear-zones against the risk of fixed object crashes.

From 2004 to 2006, fixed object crashes comprised fifteen percent of all fatal urban crashes and three percent of all crashes in the state of Iowa. Many states use AASHTO recommendations as minimum clear-zone standards, while other states have increased the required minimum clear-zone distance but little research exists to support the specification of these minimums.

This paper summarizes a study on the effects of the clear-zone on safety performance of urban curbed streets. The study included synthesizing selected state practices and investigated the benefits of various clear-zone widths based on thirteen urban corridors in Iowa. The results suggest that a four to five foot clear-zone could be effective in reducing 90 percent of urban fixed object crashes and recommends that additional research be conducted to account for variations in speed, traffic, and other corridor characteristics.

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