CE 453 Highway Design Iowa State University Highway Design Criteria Overview

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CE 453 Highway DesignIowa State
UniversityHighway Design Criteria Overview

• April 24 2006
• David R. Dougherty P.E.

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Some Basics

• Highway Design is a Science and an Art
• Need to understand the context our designs are
  within and how criteria translates into driver
  expectancy/behavior and vehicle
  performance/behavior.
• Balance safety cost mobility community values
  environmental politics liability sustainable
  development etc.
• Our job is to protect the health safety and
  welfare of the public
• A skilled highway designer or design team
  understands the human characteristics of drivers
  bicyclists and pedestrians as they relate to the
  driving task. A skilled designer also has
  knowledge of the physical and operational
  characteristics of vehicles. Finally and
  perhaps more importantly the skilled highway
  designer is able to translate this knowledge to
  the actual design of the highway. A Guide for
  Achieving Flexibility in Highway Design May
  2004 by AASHTO

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Design CriteriaWhere It All Starts

• Traffic Volumes
• Level of Service Goals
• Design Vehicle
• Functional Classification
• Freeway Arterial Local etc.
• Urban or Rural
• Terrain
• Design Speed
• Logical with respect to the topography
  anticipated operating speed adjacent land use
  and functional classification. Rule of thumb 5
  mph over posted speed.

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Highway Design Criteria

• 13 Basic Design Criteria Based on AASHTO
• Additional criteria derived from basic criteria
• AASHTO provides guidance range of values
• Local jurisdictions might be more stringent or
  might use lower range values (DOT County City)
• Local jurisdictions establish standards based on
  AASHTO guidance
• Know Your Clients Criteria and Preferences

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Golden Rules

• A. Define the Facility Type
• B. Define the Design Speed
• Green Book Organized by Functional Classification
  (Page references in this presentation to 2001
  AASHTO Green Book)

• Green Book
• Highway Functions
• Design and Controls Criteria
• Elements of Design
• Cross Sections Elements
• Local Roads and Streets
• Collector Roads and Streets
• Rural and Urban Arterials
• Freeways
• Intersections
• Grade Separations and Interchanges

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13 ASSHTO Criteria

• Design Speed
• Lane Widths
• Shoulder Widths
• Bridge Widths
• Structural Capacity
• Vertical Clearance
• Grades

• Cross Slope
• Superelevation
• Horizontal Alignment
• Vertical Alignment
• Stopping Sight Distance
• Horizontal Clearance

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Lane and Shoulder Widths

• Examples
• Freeways w/ high percent trucks wider
  shoulders.
• Freeways w/ 3 or more lanes per direction wider
  inside shoulders.
• Curb offsets can vary 1 to 2.5 feet.
• Low Speed Low Volume roadways allow narrower
  lanes.
• Interchange ramps typically wider because of
  turning roadway nature but shoulders are
  narrower.

• Factors
• Facility Type
• Average Daily Traffic
• Number of Lanes
• Auxiliary Lanes/Ramps
• Design Speed
• Traffic Mix
• Geometry Turning Roadways
• Provisions for Passing Stalled Vehicles
• Drainage Needs
• Sight Distance
• Local Practice

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Bridge Widths

• Factors
• Facility Type
• Bridge Length
• Average Daily Traffic
• Geometric Considerations
• Local Practice

• Examples
• Typically match roadway approach width.
• Long bridges might have less than full width
  shoulders.
• Farm to Market large equipment.
• Curved Flyovers for interchanges stopping sight
  distance.

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Structural Capacity

• HS-20 unless dealing with existing structures on
  local roads.
• Check local practice
• Handled by your bridge engineer

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Vertical Clearance

• Factors
• Facility Type
• What is under
• Structure type might influence clearance
• Measure at edge of travel way or usable shoulder
  edge at critical clearance point

• Examples
• 16 feet is typical.
• Most local agencies add 6 for future overlays or
  future bridge widening.
• Higher clearances for fragile structures (sign
  bridge ped bridge).
• Possible lower clearances for local roadways.
• 23 feet for clearance over railroads. Rail bridge
  type also influences clearance.
• Utility company criteria.

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Anything is Fragile if You Have a Big Enough
Hammer
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Grades

• Factors
• Facility Type Speed
• Terrain
• Drainage Curb and Gutter or Shoulder
• Length of Grade
• ADA
• Intersections

• Examples
• Short segments may have steeper grades.
• Most local agencies require 0.4 to 0.5 minimum
  grades for curb and gutter AASHTO says 0.3.
• Stopping at intersections and the through
  roadway

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Cross Slope

• Factors
• Facility Type
• Number of Lanes
• Drainage
• Local Practice

• Examples
• 2 is typical minimum AASHTO allows 1.5.
• 1.5 minimum in median turn lanes. Some agencies
  allow 1 in left turn lane areas.
• 3 lanes and wider outside lanes typically 3.
• Shoulders typically 4.

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Superelevation

• Factors
• Centripetal Acceleration e f or both
• Facility Type
• Design Speed
• Region/Area
• Snow/Ice
• Terrain
• Rural or Urban
• Slow moving vehicles
• Local Practice
• AASHTO now says spirals not required. Iowa is
  keeping them
• Design values for f include substantial margin of
  safety

• Examples
• Max super tied to region. Lower for areas with
  ice and snow.
• Low Speed Urban Acceptable of more Gs.
  Typically do not want to super in urban setting.
• Watch out for trapping drainage in transition
  areas.
• Length of transition depends on maximum rate of
  change of grade of outside edge depends on
  number of lanes and rotation point.
• Location of transition relative to PC/PT varies
  by jurisdiction
• New AASHTO tables for 2004

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Horizontal Alignment

• Factors
• Facility Type
• Design Speed
• Minimum Length of Curve
• Sight Distance
• Coordinate Vertical and Horizontal Alignments

• Examples
• Dont hide changes in horizontal behind crest
  verticals Pg 283 AASHTO .
• Intersection sight distances hard to judge
  distance on curve.
• No broken back curves (two consecutive curves in
  same direction separated by short tangent.
• Reverse curves consider super transition.
• Rules for compound curves Pg 205 AASHTO.

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Vertical Alignment

• Factors
• Design Speed
• Driven by stopping sight distance for crests.
• Object height previous was 6 now 2. Iowa
  still prefers 6.
• Sag curves driven by headlight sight distance
  but also consider
• G force
• Drainage

• Examples
• Dont hide changes ahead behind crest curves.
• Watch out for long flat sag and crest curves with
  curb and gutter remember the minimum grade
  requirements for drainage.
• Consider passing sight distance and decision
  sight distance.

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Stopping Sight Distance

• Factors
• Design Speed
• Height of Eye 3.5
• Height of Object 2
• Reaction Time 2.5 seconds
• Deceleration Rate 11.2 ft/s2
• Passing Sight Distance
• Decision Sight Distance
• Based on Passenger Car
• Adjustments for Grade

• Examples
• Intersection sight triangles
• Intersection maneuvers
• Decision sight distance situations requiring
  avoidance maneuvers. Reaction maneuver time
  varies from 3 seconds to 14.5 seconds.
• Horizontal curves with sight obstructions.

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Horizontal Clearance

• Factors
• Facility Type
• Design Speed
• Average Daily Traffic
• Offsets to objects from edge of travel lane
• Clear Zone Unobstructed relatively flat area
  provided beyond the edge of the traveled way for
  the recovery of errant vehicles.
• AASHTO Roadside Design Guide
• Local Practice

• Examples
• Shoulders and Auxiliary Lanes count as part of
  clear zone.
• Side Slopes
• 41 Recoverable
• 31 Traversable
• Steeper than 31 Non-traversable outside clear
  zone or shield
• Beak-away objects meet horizontal clearance
  requirements but can be within clear zone.
• No curbs or sloped curbs 45 mph.

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Other Criteria

• Level of Service Traffic Operations
• Turn Lanes and Tapers
• Design Vehicle for Turning Roadways
• Construction Staging (How many lanes need to stay
  open)
• Border Area for urban projects room between curb
  and right-of-way for utilities landscaping
  sidewalk snow storage etc.
• Pedestrian mobility
• ADA requirements

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Exceptions to the Rules

• 3R Projects Resurfacing Restoration and
  Rehabilitation
• Context Sensitive Design
• Design Exceptions
• Crash and Safety Studies Cost/Benefit
• Florida Handout Help navigate the Green Book
  (Based on 2001). See your local jurisdiction for
  design exception procedures.

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Urban ReconstructionGotcha

• Utility Coordination
• Right-of-Way Agreements
• Permits
• Maintenance of Traffic

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Life