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NCHRP Project 17-35: Evaluation of Safety Strategies at Signalized Intersections

 

 

Evaluations of Low Cost Safety Improvements Pooled Fund Study

 

PPT version for Printing

Advanced Street Name Signs

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.

Dr. James Jenness, Westat


Overview

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

Background

Provide advance guide signs and street name signs

  • Ranked 14th
  • NCHRP Report 500 Series Older Driver Guidebook
  • Older drivers are more involved in intersection crashes than other drivers
  • Older drivers exhibit excessive vehicle–braking at/before intersections
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  • Tried, low cost, short–term implementation
  • Target crashes
    • All intersection related
    • Rear–end
    • Late lane changes
    • Night
    • Older drivers
  • Consistent signing across jurisdictions also important
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  • Differing methods to provide street names in advance of intersection:
    • Green and white Advance Street Name signs
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    • MUTCD W3–3 advance traffic control signs with W16–8 supplemental warning plaques

Data Collection

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Status of Data Collection

  • Massachusetts
    • Blanketed strategy across four districts in 2004
    • 200 sites included in the study
      • Over 400 installed
      • Lack of ADT data limited sample
    • Limited crash data 2002–2005
    • 500 intersection–years of before crash data
    • 250 intersection–years of after crash data
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  • North Carolina
    • Blanketed strategy in Cary, NC in 2001
    • 100 sites included in the study
    • Crash data from 1997–2004
    • 400 intersection–years of before crash data
    • 300 intersection–years of after crash data
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Study Design

  • Required Sample Size
    • Minimum: 803 intersection–years per period
      • Detect a 5 percent reduction in total crashes with 95 percent confidence
    • Desirable: 2,031 intersection–years per period
      • Detect a 10 percent reduction in older driver crashes with 95 percent confidence
  • Assumptions
    • 7.0 crashes per intersection per year in the before period, of which 9 percent involve older drivers
      • Crash rate based on preliminary data from MA sites, older–driver share based on Toronto data
    • Before and after periods are of equal length
    • Equal number of intersection–years for treatment and reference sites

Remaining Data Needs

  • Minimum: Need 450 more after intersection–years
  • Desirable: Need 1,130 before intersection– years and 1680 after intersection–years for older driver
  • Need additional installations

Evaluation Considerations

  • Do not have large enough sample size
  • Comparing two methods of signing
  • Limited ADT data in Massachusetts

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