Establishing Equilibrium Skid Resistance

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Establishing Equilibrium Skid Resistance

• David Cook, Surfacing Engineer
• New Zealand Transport Agency
• Chris Kennedy, Director of Civil Engineering
  Services
• WDM Limited
• Darren Newland, Project Manager
• WDM Limited

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Why measure skid resistance

• Worldwide increasing skid resistance reduces
  crash rate.
• New Zealand Cenek Davies
• Wet increase SCRIM Coefficient by 0.1 reduce
  crash rate by around 30
• Dry increase SCRIM Coefficient by 0.1 reduce
  crash rate by around 15
• Safety is one of the four key NZTA outcomes
• (The difference in skid resistance between wet
  and dry roads can be used to give an indication
  of the effect of increasing skid resistance.)

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What is skid resistance

• Every vehicle is different, trucks, cars,
  handling characteristics.
• Skid resistance is the ratio of the horizontal to
  vertical forces on the particular vehicle.

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Some general factors for skid resistance

• SCRIM, GripTester, Pendulum all measure low speed
  skid resistance.
• ROAR may be towed at higher speeds and
  approximate the full friction curve. Dynamic
  Friction Tester has faster slip speed.
• I believe the most accurate modeling of vehicles
  is provided by the Pavement Friction Tester.
• NZTA has chosen to standardise network surveys
  with SCRIM because
• Robust
• Slip speed close to the speed for peak friction
• Suitable for 22,000 lane km annual survey
• There is a good correlation with slow speed data
  from skid cars.

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Total skid resistance

• Total skid resistance, for wet roads, is a
  function of the Low Speed skid resistance and the
  loss at higher speeds due to the time taken to
  drain water from under the tyre road surface.
• This loss may be up to 80 or more.
• Hysteresis for those who want to ask?

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Skid resistance varies with time

• Initially skid resistance of crushed aggregates
  is quite high, and then reduces to an equilibrium
  value.
• Skid resistance varies with
• Heavier traffic
• Traffic numbers
• Traffic stress
• Weather
• Short term changes and smoothed values
  approximates sinusoidal curve.

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What measure of skid resistance

• For crash investigations use the skid resistance
  at the time of the crash for the path of the
  vehicle and the weather conditions.
• For maintenance decisions we need a single value
  to enable prioritization over the whole of the
  network.
• Equilibrium SCRIM Coefficient (ESC)
• The ESC methodology enables skid resistance data
  to be normalised for both within year variations
  (Mean Summer SCRIM Coefficient, MSSC) and between
  year variations ESC.

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Why does skid resistance vary

• Over long dry periods there is a buildup of
  contamination, oil, grease, tyre rubber, silt,
  clay and cattle effluent.
• During rain and under vehicle tyres this
  contamination is washed and scrubbed away.
• In addition when roads are dry most of the
  detritus is ground to a fine flour that acts like
  a polishing agent under the action of tyres.
  When this is washed away the larger grit provides
  a medium that roughens the stone chips.

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Mean Summer SCRIM Coefficient (MSSC)

• Seasonal control sites are tested three time each
  summer and the mean value is MSSC for the site.
• MSSC averaged over several sites in a seasonal
  control area to give the mean area MSSC
  correction factor.

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Equilibrium SCRIM Coefficient (ESC)

• For mature surfacings the MSSC values may vary
  from year to year.
• Under or over investment in surfacing for skid
  resistance
• Comparing last four years an ESC Correction
  Factor may be calculated to normalise for between
  year variations.

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Regression Analysis
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Conclusions

• When considering priority for skid resistance
  treatments it is important to correct for both
  within year variations and between year
  variations.
• There is some justification for applying
  regression analysis to the seasonal correction
  methodology but more research is needed to
  consider whether the amount of data required will
  justify this further development.

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Footnote

• Extreme dry and hot weather 2007/08 summer.
• TM 5004, link in paper.