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Sight Distance and Roadway Condition

Sight Distance and Roadway Condition. Dan Walsh, P.E. Summary. Sight distance and variable speed limit signs Roadway conditions Wet Weather Accident Reduction Program. Interstate 35. I-35 constructed in 1955 No change in vertical profile 60 mph design speed

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Sight Distance and Roadway Condition

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  1. Sight Distance and Roadway Condition Dan Walsh, P.E.

  2. Summary • Sight distance and variable speed limit signs • Roadway conditions • Wet Weather Accident Reduction Program

  3. Interstate 35 • I-35 constructed in 1955 • No change in vertical profile • 60 mph design speed • Design speed is maximum safe speed • 70 mph posted speed limit • 74 mph 85th percentile speed

  4. Sight Distance • Roadway ahead visible to driver • Long enough to allow a vehicle traveling at design speed to stop

  5. Available sight distance 767 feet

  6. Stopping Sight Distance • Distance traversed from object detection to stop • Should be less than available sight distance • 900 feet at 60 mph • 1,293 feet at 70 mph • Exceeds available sight distance of 732 to 767 feet

  7. Variable Speed Limit Signs • Alert drivers to reduce speeds in hazardous conditions • Increase safety • Washington (Snoqualmie Pass) • New Jersey Turnpike • More cost-effective than reconstruction

  8. Macrotexture Pavement Depth • Affects water drainage • No national standards • TxDOT research recommended 0.035 inch • Accident site • 0.017 to 0.018 inch in right lane • 0.024 to 0.027 inch in left lane

  9. Rut Depth • Water accumulation • TxDOT recommends resurfacing when ruts greater than 0.5 inch • Shallow rut depth problem • Right wheel path: 0.25 to 0.35 inch • Left wheel path: 0.40 to 0.42 inch • Differing rut depths can lead to rotational forces on vehicle

  10. Roadway Friction • FHWA does not specify minimum • Texas Pavement Management Information System • Accident site coefficient of friction 0.12 in 2000 • Half of interstates in Texas have coefficients of friction 0.26 or below • Coefficients of friction on ice range from 0.12 to 0.25

  11. Texas Wet Weather Accident Reduction Program • Identify locations overrepresented in wet weather accidents • 5 accidents within 1/10 mile segment • Short distance for interstate highways

  12. Texas Wet Weather Accident Reduction Program • Weighted toward identifying locations in high population areas • I-35 accident location • 16 percent of accidents in wet weather • 2.35 percent wet weather exposure • Poor geometry, rutting, low macrotexture, and low friction

  13. Wet Weather Accident Reduction Program • Limited guidance from FHWA • Each State has its own method • Illinois DOT relies on friction

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