Difference between revisions of "Roadways: Design Speed"

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'''Horizontal and Vertical Alignments'''
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'''Design Speed'''
  
 
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'''Primary Guidance'''
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=Primary Guidance=
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*When possible, avoid locating bridges on curved horizontal alignments, since curved bridges are more expensive than straight bridges .
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*High-speed is considered to be a design speed greater than 45 mph and low-speed less than or equal to 45 mph
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*Cross slopes should be constant across a bridge.
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*For all public facilities including interstates, design speed should be based on roadway functional classification  and context
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*Bridges should not be located in a sump (low point) in the roadway profile.
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*For Interstates
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*For additional information, refer to Horizontal Alignments and [[Roadways:_Vertical_Alignment| Vertical Alignments]] in the Roadway section.
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**Urban or Mountainous shall meet or exceed 50 mph
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**Rural Non-Mountainous shall meet 70 mph
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*New Roadways will be the anticipated posted speed
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*Design speed is NOT automatically set at 5mph or 10 mph over the posted speed
 
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'''Discussion'''
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=Discussion=
  
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Curved Horizontal Alignments
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Select a design speed appropriate for the roadway and use that speed to determine the various design geometric features.  The selected design speed should realistically represent actual or anticipated operating speeds and conditions on the roadway being designed.
  
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If possible, avoid locating bridges on curved horizontal alignments, since curved structures are more expensive to fabricate and are more difficult to construct than straight structures.  If a bridge falls within a curved roadway alignment, see if a tangent (straight) section can be introduced into the curve which encompasses the limits of the bridge.  If the radius of the curve is large enough, consider constructing the bridge with straight girders and deck, with variable width shoulders.  To determine if this is feasible, determine the ordinate between the inside curve gutter line and the chord created by connecting the location of the gutter line at the center line of bearing at each abutment.  If this value is less than 1’-0”, then the structure should be laid out with straight girders and deck.        
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==New Construction Projects==
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The anticipated posted speed  will be used for the design speed.  A design speed set higher than the anticipated posted speed will lead to a roadway that provides more comfort for drivers to travel at a higher speed than may be desired.  The higher design speed will also lead to flatter horizontal and vertical curves as well as larger typical sections.  These conservative design elements lead to higher roadway construction costs, long-term maintenance costs, and higher than anticipated speeds.
  
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Cross Slopes
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==Reconstruction Projects==
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Refer to the as-built plans to determine the existing facility design speed.  If as-builts do not exist or cannot be found, the designer can use multiple methods to determine the design speed.
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*Layout a preliminary alignment for the existing facility and determine the design speed in which the existing facility meets based on the latest design criteria.
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*A prevailing speed study can be done to determine the prevailing or 85th percentile speed.  This speed can then be used for the design speed of the facility.
  
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The preferred minimum cross slope on a bridge deck is 2%, which will facilitate drainage from the travel lanes and helps to avoid an icy bridge deck.  Constant cross slopes are preferred across the entire bridge length to reduce complex deck construction operations.  It is possible to accommodate transitional cross slopes in a bridge deck, however the transition from normal crown (2% in each direction from the crown) to plane incline requires a longitudinal joint at the crown/pivot point and complicated camber in the girders to account for the unusual deflections that will occur.  It is also possible to have transitioning superelevation across a bridge, which would occur if it was in a reverse curve.  This should be avoided when possible since it will result in a flat area on the deck when the superelevation is 0%, which does not facilitate the drainage of water from the travel lanes and can lead to safety and maintenance issues.  If it is not possible to adjust the alignment so the bridge is not within the area of transitioning superelevation, scuppers should be placed on either side the flat area to capture drainage from the bridge deck before it reaches this point.
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==Considerations==
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The design speed should NOT be set lower than the posted or prevailing speed (based on a speed study).  If there is a need for the project to lower prevailing speeds on the facility, simply lowering the project design speed will not reach this goal.  If the design speed is lowered for the project and there are no geometric modifications considered, the existing facility geometric elements will still meet the existing facility design speed, and the driver will not be compelled to lower driving speeds.
  
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Sumps in Roadway Profile
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==Interstate==
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For the highest type interstates in Maryland, the posted speed can be set up to 70mph.  Care should be taken on all existing and proposed interstate designs that are anticipated to be posted at 70mph to set the design speed at a minimum of the anticipated posted speed.
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Bridges should not be located in a sump (low point) in the roadway profile.  Sumps on bridge decks will result in an area where water will collect and can lead to safety and maintenance issues. Even when the sump is moved off the bridge, the roadway elevations should be checked to ensure the bridge deck is not essentially flat as you approach the sump.  If it is not possible to avoid a sump on the bridge, scuppers should be placed at the sump and on either side of the sump to ensure sufficient drainage.
 
  
 
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=See Also=
 
=See Also=
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*[[Practical Design Implementation Guidance|Practical Design Implementation Guidance]]
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[[Category:Practical Design]][[Category:Practical Design Guidance]]

Latest revision as of 20:05, 5 December 2016

Design Speed

Primary Guidance

  • High-speed is considered to be a design speed greater than 45 mph and low-speed less than or equal to 45 mph
  • For all public facilities including interstates, design speed should be based on roadway functional classification and context
  • For Interstates
    • Urban or Mountainous shall meet or exceed 50 mph
    • Rural Non-Mountainous shall meet 70 mph
  • New Roadways will be the anticipated posted speed
  • Design speed is NOT automatically set at 5mph or 10 mph over the posted speed

Discussion

Select a design speed appropriate for the roadway and use that speed to determine the various design geometric features. The selected design speed should realistically represent actual or anticipated operating speeds and conditions on the roadway being designed.

New Construction Projects

The anticipated posted speed will be used for the design speed. A design speed set higher than the anticipated posted speed will lead to a roadway that provides more comfort for drivers to travel at a higher speed than may be desired. The higher design speed will also lead to flatter horizontal and vertical curves as well as larger typical sections. These conservative design elements lead to higher roadway construction costs, long-term maintenance costs, and higher than anticipated speeds.

Reconstruction Projects

Refer to the as-built plans to determine the existing facility design speed. If as-builts do not exist or cannot be found, the designer can use multiple methods to determine the design speed.

  • Layout a preliminary alignment for the existing facility and determine the design speed in which the existing facility meets based on the latest design criteria.
  • A prevailing speed study can be done to determine the prevailing or 85th percentile speed. This speed can then be used for the design speed of the facility.

Considerations

The design speed should NOT be set lower than the posted or prevailing speed (based on a speed study). If there is a need for the project to lower prevailing speeds on the facility, simply lowering the project design speed will not reach this goal. If the design speed is lowered for the project and there are no geometric modifications considered, the existing facility geometric elements will still meet the existing facility design speed, and the driver will not be compelled to lower driving speeds.

Interstate

For the highest type interstates in Maryland, the posted speed can be set up to 70mph. Care should be taken on all existing and proposed interstate designs that are anticipated to be posted at 70mph to set the design speed at a minimum of the anticipated posted speed.

See Also

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