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| Publications | NCHRP Project 17-25: Crash Reduction Factors forTraffic Engineering and Its Improvements



Evaluations of Low Cost Safety Improvements Pooled Fund Study


PPT version for Printing

Offset Left–Turn Lanes

Header image – Picture shows series of three scenarios: a vehicle on a meandering road, safety personnel at work, and a car that is very badly damaged after it appears to have collided into a telephone pole.

Nancy Lefler, VHB


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


  • 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)
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  • 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
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Data Collection

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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
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  • 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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