Clear zone : a synthesis of practice and an evaluation of the benefits of meeting the 10-ft clear zone goal on urban streets.
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Summary
This study addresses the challenge of establishing appropriate clear zone standards for fixed objects on urban curbed streets, where limited right-of-way often conflicts with safety goals. The research was motivated by the significant safety risk posed by fixed object crashes, which accounted for 15% of all fatal crashes and 3% of all crashes in Iowa between 2004 and 2006. While many states adhere to minimum AASHTO recommendations, others have increased required setbacks, creating a need to evaluate the specific benefits of meeting a 10-foot clear zone goal in urban environments. The research was conducted in two phases. The first phase involved a synthesis of practice, comprising a literature review and a survey of jurisdictions with developmental and historical patterns similar to Iowa to understand current state standards and enforcement practices. The second phase evaluated the benefits of a 10-foot clear zone by examining urban corridors in Des Moines and Waterloo, Iowa. Researchers collected physical characteristics of these corridors, including fixed object setbacks, using handheld GPS and laser distance meters. The data were analyzed at three scales: segments, blocks, and 15-meter sections. The analysis compared corridors that met or did not meet the 10-foot goal, assessing variables such as minimum, average, and 15th percentile setbacks, speed limits, average daily traffic (ADT), and fixed object density. The study also performed an economic evaluation to determine the incremental benefits of various setback distances. The findings indicate that maintaining a consistent fixed object offset reduces the number of fixed object crashes. Specifically, the study determined that a 5-foot clear zone is the most effective distance for minimizing the total number of fixed object crashes. Conversely, a 3-foot clear zone is the most effective when the objective is to minimize the total cost associated with these crashes. The analysis further explored how crash frequency and costs varied across different speed limits (30, 35, and 40 mph) and traffic volumes (1,500–6,000 ADT and over 6,000 ADT), providing detailed cumulative percent crash and cost data for various setback scenarios. The significance of this research lies in its ability to inform transportation agencies and urban planners about the trade-offs between right-of-way acquisition costs and crash risk reduction. By identifying that smaller clear zones (3–5 feet) offer substantial safety and economic benefits compared to larger setbacks, the study provides evidence-based guidance for jurisdictions facing space constraints. The results suggest that strict adherence to larger clear zone goals may not always yield proportional safety benefits, allowing local designers to make more informed decisions regarding fixed object placement and urban street design.
Key finding
A 5-foot clear zone is most effective for minimizing the number of fixed object crashes, whereas a 3-foot clear zone is most effective for minimizing the cost of fixed object crashes.
Methodology
field_study
Provenance
The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_rosap on 2026-05-23 (7 acquisition events logged).
| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | rosap | — | — | 2 | 2026-05-23 |
| archive | success | — | — | — | 1 | 2026-05-23 |
| extract | success | cached | — | — | 2 | 2026-06-10 |
| clean | success | — | — | — | 1 | 2026-06-01 |
| chunk | success | — | — | — | 1 | 2026-06-01 |
| embed | success | — | — | — | 1 | 2026-06-02 |
| enrich | success | — | — | — | 1 | 2026-05-23 |
| promote | success | — | — | — | 1 | 2026-05-23 |
| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 3 | 2026-06-10 |
| tag | success | vector_similarity | — | — | 25 | 2026-06-11 |
| verify | success | — | — | — | 2 | 2026-06-10 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.
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- Empirical Findings: crash risk outcomes