Optimum Urban Clear Zone Distance

Urban communities often have 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 made up 15% of all fatal urban crashes and 3% of all crashes in the state of Iowa. Many states use AASHTO recommendations as minimum clear zone standards, whereas other states have increased the required minimum clear zone distance, but little research exists to support the specification of these minimums. The effects of the clear zone on safety performance of urban curbed streets were studied. The study included synthesizing selected state practices and investigating the benefits of various clear zone widths on the basis of 13 urban corridors in Iowa. The results suggest that a 4- to 5-ft clear zone could be effective in reducing 90% of urban fixed-object crashes and recommend that additional research be conducted to account for variations in speed, traffic, and other corridor characteristics.