Improving the Safety of Left-Turn Operations at Signalized Intersections for High-Risk Groups
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Summary
This study investigates the safety factors influencing left-turn operations at signalized intersections, specifically focusing on high-risk driver groups and intersection control modes. The research was motivated by the disproportionate number of crashes involving left-turning vehicles in Idaho, which account for approximately 42% of intersection crashes despite representing a small fraction of traffic volume. The primary objective was to determine the relationship between left-turn crash frequency and severity, the mode of left-turn control (protected, permissive, or protected-permissive), and the specific signal display used (doghouse versus flashing yellow arrow). The researchers analyzed 1,062 left-turn-related crashes at signalized intersections in Idaho occurring between 2006 and 2015. To identify contributing factors, they conducted a comparative analysis of intersection characteristics, examining design and control elements at sites with frequent crashes versus those with few crashes. A subset of 265 signalized intersections was selected for in-depth analysis to evaluate the impact of left-turn operation modes, signal sequences (lead versus lag), and signal displays. Crash rates were calculated by dividing average left-turn daily volumes by conflicting through-movement daily volumes to account for traffic exposure. The results indicated that protected-only left-turn operations significantly reduced both total crashes and fatal or severe-injury crashes compared to permissive-only or protected-permissive operations. Permissive-only operations exhibited the highest crash rates. However, no statistically significant difference was found between the two signal display options for protected-permissive operations: the traditional five-head doghouse display and the flashing yellow arrow performed similarly regarding crash frequency and severity. Additionally, lead left-turn operations showed slightly better safety performance than lag operations, though this difference was not statistically significant. Demographic analysis revealed that drivers under 19 years old were the most vulnerable group, followed by mature drivers over 75 years old. Driver impairment was a minor factor, contributing to only 2.54% of crashes, while inclement weather accounted for approximately 8% of incidents. Most crashes occurred between 9:00 AM and 9:00 PM under clear or cloudy conditions. The study concludes that targeted interventions can improve safety for specific demographics. The authors recommend emphasizing left-turn permissive operations in teen driver education programs, as young drivers are the highest-risk group. For mature drivers, implementing protected-only left-turn operations in neighborhoods with high concentrations of older residents could eliminate a significant portion of their crash involvement. Furthermore, adopting weather-responsive traffic signal systems that eliminate permissive left-turn phases during inclement weather could reduce crashes by up to 8%. The findings suggest that while signal display type (doghouse vs. flashing yellow arrow) does not significantly impact safety, the operational mode (protected vs. permissive) is a critical determinant of crash risk.
Key finding
Intersections utilizing protected-only left-turn operations experienced significantly fewer left-turn related crashes than those using permissive-only operations.
Methodology
dataset
Sample size: 1062
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 (6 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 | — | — | 24 | 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