Safety Effectiveness of Inside Shoulder Widths on Freeways in Georgia

Details

• Creators:
  Liu, Jun ; Su, Yanfang ; Xu, Ningzhe ; Barnett, Timothy E. ; Jones, Steven ; Yang, Jidong J.
• Corporate Creators:
  University of Georgia Research Foundation, Inc. ; Board of Trustees of The University of Alabama
• Corporate Contributors:
  Georgia. Dept. of Transportation. Office of Performance-Based Management and Research ; United States. Department of Transportation. Federal Highway Administration
• Subject/TRT Terms:
  Crash Injuries Crash Modification Factors Data Analysis Freeways Inside Shoulder Width Performance Road Shoulders Safety
• Publication/ Report Number:
  FHWA-GA-25-2322
• Resource Type:
  Tech Report
• Geographical Coverage:
  United States ; Georgia
• Edition:
  Final Report (November 2023–October 2025)
• Corporate Publisher:
  Georgia. Department of Transportation. Office of Performance-Based Management & Research
• Abstract:
  This report presents a comprehensive analysis of the safety effectiveness of inside (left) shoulder widths on Georgia freeways, conducted as part of Georgia Department of Transportation (GDOT) Research Project 23-22. The study integrates six years (2018–2023) of traffic crash data, traffic volume data (annual average daily traffic [AADT] and truck percentages), and roadway inventory data to quantify the relationship between left shoulder width and crash outcomes across diverse freeway configurations. Covering over 1,800 freeway segments (2,400+ directional route miles) and 255,000+ georeferenced crashes, the research categorizes freeways into three types: statewide freeways without barrier, statewide freeways with barrier, and Atlanta urban freeways, further stratified by lane count (two lanes vs. three-plus lanes), traffic volume, and truck percentage. Safety performance functions (SPFs) and crash modification factors (CMFs) are developed using negative binomial regression to model crash frequencies for total crashes (KABCO), fatal/injury crashes (KAB), median-related crashes, and median-related KAB crashes, with a baseline of left shoulder width ≥10 ft. Key findings indicate that wider shoulders (6–8 ft) consistently reduce crash frequency, particularly for severe and median-related crashes, whereas narrow shoulders (<4 ft) correlate with elevated crash frequency – especially at high AADT (>60,000 vpd) and high truck percentages (>10 percent). Atlanta urban freeways with high truck percentages (>15 percent) require 8–10 ft shoulders to reduce crash frequency. The study aimed to provide evidence-based insights on when narrower shoulder widths may still achieve acceptable safety outcomes. These findings are intended to guide context-sensitive design decisions, especially when they differ from current GDOT and AASHTO/FHWA standards. Where recommendations deviate from prior research, such as the Highway Safety Manual (HSM) or NCHRP Web-Only Document 306, they should be carefully validated and critically examined.
• Format:
  PDF
• Funding:
  RP 23-22
• Collection(s):
  US Transportation Collection
• Main Document Checksum:
  urn:sha-512:186ca353e9a16e71a0e80d2413c104dac8f8e6a896ca8e9ff4f70e69b4867982c07c9aef2cd1e76b9f868d0717e60efbfa5fafeaf9a7dfbae3a24f6ee03c168a
• Download URL:
  https://rosap.ntl.bts.gov/view/dot/87358/dot_87358_DS1.pdf
• File Type:
  [PDF - 11.31 MB ]