Low Cost Safety Improvements Pooled Fund Study

Offset Left-Turn Lanes

Nancy Lefler, VHB

Overview

• Background
• Status of Data Collection
• Remaining Data Needs
• Evaluation Considerations

Background

Ranked 12th NCHRP Report 500 Series Older Driver Guidebook Left-turn maneuvers Improved sight distance Beneficial to left-turning vehicles Particularly older drivers (Stapin et al., 1997)
Tried Moderate to high-cost Depends on available right-of-way and median width Moderate implementation 2-4 years depending on right-of-way and median width Potential difficulties Initial driver confusion Advance signs and pavement markings Key to success Identify appropriate intersections

Data Collection

Status of Data Collection

• Nebraska
  • Blanketed strategy in Lincoln
  • 184 strategy sites included in study
  • Reference sites from Omaha
  • Crash data obtained from NE DOR

• Iowa
  • 12 strategy sites included in the study
  • Reference sites from recent intersection study
• Florida
  • Only 7 sites so far – need at least 12
  • Currently searching for additional sites
  • Crash data obtained from FLDOT

Remaining Data Needs

• Nebraska
  • Geometric data for study sites
• Iowa
  • Iowa collecting roadway, traffic, and crash data for treatment and reference sites
• Florida
  • Additional study sites
  • Similar reference sites (signalized with permitted)
  • Intersection data for strategy and reference sites

Evaluation Considerations

• Aggregate analysis
  • Crash-based evaluation
  • Empirical Bayes before-after study design
  • Evaluate each State individually and combined
    • Individual analysis depends on data availability
• Disaggregate analysis
  • Depends on variation in sample
  • Depends on sample size

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