Evaluation of Geometric Design Needs of Freeway Systems Based on Traffic and Geometric Data

Teng, Hualiang (Harry) · 2013 · ROSA P / Nevada. Dept. of Transportation

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Summary

This study, conducted for the Nevada Department of Transportation, addresses the safety performance of freeway systems in Las Vegas, Nevada, where increased traffic volume competes for limited space, leading to reduced safety. The research specifically investigates the geometric design factors influencing crash occurrence and severity on two primary segment types: EN-EX (entry-exit, or weaving segments) and EX-EN (exit-entry) segments. The motivation stems from data indicating that freeway crashes in Nevada are more severe than those on arterials, necessitating a detailed evaluation of geometric elements such as lane counts, curve radii, shoulder and median widths, and segment lengths to identify design issues and propose countermeasures. The methodology involved collecting traffic and geometric data for freeway segments in Las Vegas, focusing on EN-EX and EX-EN configurations while excluding less common EN-EN and EX-EX types. The researchers developed statistical models to analyze crash rates and crash severity for each segment type. The analysis incorporated variables including Annual Average Daily Traffic (AADT), number of through lanes, curve radius, shoulder and median widths, and segment length. The study aimed to calibrate regression models to quantify the relationship between these geometric and operational factors and safety outcomes, building upon previous literature that had largely focused on EN-EX segments or limited geometric variables. The results revealed distinct impacts of geometric elements on safety for each segment type. For EN-EX segments, the number of through lanes, curve radius, shoulder width, and median width significantly impacted average crash rates. Specifically, larger curve radii, wider shoulders, and wider medians reduced average crash rates. However, increasing the number of through lanes increased both average crash rates and high-severity crashes. Long segments reduced average crash rates but did not affect severity. For EX-EN segments, large curve radii, wide shoulders, and wide medians reduced both average crash rates and high-severity crashes. The impact of the number of through lanes varied by freeway: on I-15 and I-215, more lanes increased crash rates and severity, whereas on US 95, more lanes reduced high-severity crashes. High traffic volume increased average crash rates on I-215 but showed no significant impact on I-15 or US 95. The study concludes with specific recommendations for mitigating safety issues. For EN-EX segments, congestion often arises from weaving movements or off-ramp queues backing up onto the freeway. Recommended countermeasures include spatially separating traffic streams, installing ramp metering for heavy on-ramp traffic, adjusting off-ramp signals to clear queues, and installing warning signs on long segments to alert drivers of downstream congestion. For EX-EN segments, congestion typically results from heavy merging traffic or off-ramp backups. The authors suggest installing ramp metering, spatially separating merging traffic from through traffic during extreme volumes, and using warning signs on long segments to improve driver awareness and reduce crash frequency and severity.

Key finding

Wider shoulders and medians reduced both average crash rates and high-severity crashes on both EN-EX and EX-EN segments, while increasing the number of through lanes generally increased crash rates and severity, though the effect on through lanes varied by specific freeway route.

Methodology

dataset

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• incidence prevalence
• intersection design
• road geometry
• roadway lighting effects

Information type

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• Empirical Findings: crash risk outcomes
• Methodological Resource: dataset resource