Roadside

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Roadside

• CE 453 Lecture 21

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Sideslopes Foreslope (Backslope) Design

• Considerations Stability and Vehicle Recovery
• if slope gt 31 use barrier (if in clear zone)
• Hinge Point minimize to avoid airborne vehicle
• Foreslope flat enough to allow recovery (41 or
  less desired)
• Toe of Slope round to avoid plowing
• Ditch Bottom
• Backslope (31 or less desired)
• AASHTO RDG has preferred cross section for two
  types of channels

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Recoverable slopes

• motorists are able to stop/slow and return to the
  roadway safely
• 1 to 4 or flatter

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Recoverable Slope (unless you hit the sign)
ltwww.geocities.com/Colosseum/Slope/7683/site/highw
ay/10-1.jpggt, February 28, 2002
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Non-recoverable, traversable slopes

• motorists unable to stop or return to roadway
  safely
• between 13 and 14
• recovery area must be to the toe of the slope and
  a clear runout area at base with 16 or flatter
  slope

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Critical slopes

• vehicle likely to overturn

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Non-Recoverable Slope
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ionspec
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Roadside Components

• Urban Areas
• Curb
• Horizontal Clearance
• Buffer to Land Use
• Walkways
• Rural Areas
• Shoulder
• Drainage Channel
• Traffic Barrier

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Roadside Design Guide

• American Association of State Highway and
  Transportation Officials (2002)
• Guide to address design issues off the traveled
  way (traffic lanes)

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Roadside

• Clear Zone (def.) The total roadside border
  area, starting at the edge of the traveled way,
  available for safe use by errant vehicles
• Concept area should be as wide, flat, and
  rounded as practical and as free as practical
  from physical obstructions (want a driver who
  leaves roadway to regain control and return to
  the roadway)

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Roadside

• AASHTO RDG
• 30 feet (9 m) 80 to 85 recovery
• Clear zone recommended f(design speed, ADT,
  roadside slope, curvature)
• Compromise between absolute safety and
  engineering, environmental, and economic
  constraints (always)

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Roadside Design Options for Objects

• Remove (REDESIGN)
• Relocate
• Reduce impact severity (breakaway)
• Redirect by shielding
• Delineate
• But what is an object? signs, poles, slopes
• Could be guardrail

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Clear Zone

• Unobstructed, relatively flat area beyond the
  edge of the traveled way
• Provides area for vehicles to recover
• Width depends on
• Volume
• Design speed
• Slope of embankment
• Measured from edge of pavement
  to nearest
• Obstacle (tree, telephone pole, etc)
• Steep slope, non-traversable ditch
• Cliff
• Body of water

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Clear Zone
Source Garber and Hoel
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http//epdfiles.engr.wisc.edu/pdf_web_files/tic/ot
her/SAFERcontent_96.pdf
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VEGETATION
after
before
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Iowas Roadway-Related Fatal Crashes

• 52 of Iowas fatalities are related to Lane
  Departure
• 39 of Iowas fatal crashes are
  single-vehicleRun-Off-the-Road (ROR) crashes

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What would clear zone be for same scenario with
cut slope instead?
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What would clear zone be for same scenario with
cut slope instead? 23 feet vs. 30 feet Why???
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Traffic Barriers

• Purpose is to minimize severity of potential
  accidents when vehicles leave the roadway
• Prevents vehicles from striking an object such as
  tree or bridge abutment
• Are obstacles themselves (need to consider use)

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http//www.southernguardrail.com/p_highway_gr_ar_0
1.html
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http//www.southernguardrail.com/p_highway_gr_ar_0
1.html
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CONCLUSIONS

• Clear zone varies ROT is 30 ft
• Best solution nothing in clear zone
• Remove it
• Move it
• Reduce energy
• Protect it
• Mark it