Difference between revisions of "Roadways: Facility Selection"

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=See Also=
=See Also=
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'''Horizontal and Vertical Alignments'''
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'''Primary Guidance'''
*When possible, avoid locating bridges on curved horizontal alignments, since curved bridges are more expensive than straight bridges .
*Cross slopes should be constant across a bridge.
*Bridges should not be located in a sump (low point) in the roadway profile.
*For additional information, refer to Horizontal Alignments and [[Roadways:_Vertical_Alignment| Vertical Alignments]] in the Roadway section.
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Curved Horizontal Alignments
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.       
Cross Slopes
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. 
Sumps in Roadway Profile
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.
[[Category:Practical Design Guidance]]
[[Category:Practical Design Guidance]]

Revision as of 15:38, 26 October 2016

Facility Selection

Primary Guidance

  • The facility type must reflect the corridor function(s) in an appropriate design year not to exceed the 20 year forecast.
  • Practitioners are encouraged to start by evaluating less costly solutions such as operational strategies, network solutions or transportation system management techniques before selecting significant facility upgrades that entail major capital expenditures.


Design Year

  • Major Capital Projects require the use of a Metropolitan Planning Organization (MPO) Travel Demand Model, or the Maryland Statewide Transportation Model (MSTM) both of which are based on a 20 year time horizon.
  • System Preservation Projects may necessitate the use of a design year that supports more immediate traffic operation and/or safety needs, not to exceed 20 years.

Functional Classification

The facility must represent the appropriate balance between access and mobility for its intended purpose. The functional classification system developed by the Federal Highway Administration (FHWA) reflects the degree of local access and regional connectivity a roadway provides. The level of access control should reflect the official functional classification as well as the actual role that the roadway serves, which may vary along a roadway corridor. While there may be substantial flexibility in defining the level of access control for new roadways, the options may be substantially more complex or limited on projects that are modifying existing roadways.

Roadway Users

MDOT values the needs of all of its roadway users, including cars, trucks, bicyclists, and pedestrians. All user needs both along a roadway and across it should be considered.

  • Truck and freight needs should consider existing truck volumes, identification of primary and secondary truck routes, and local and industry activity.
  • Transit needs should consider coordination with transit operators when bus and/or rail service exists along a roadway.
  • Pedestrian needs should consider locally adopted plans, existing pedestrian activity, proximity to pedestrian traffic generators, and network connectivity.
  • Bicycle needs should consider locally adopted plans, existing bicycle activity, proximity to bicycle traffic generators, and network connectivity.

Roadway Capacity and Operations

Level of Service indicates general traffic capacity needs:

  • When there is a design year Level of Service C(need graphic), or better, no additional capacity is needed.
  • When there is a design year Level of Service D, or worse, additional capacity may be needed.

Adding capacity that creates failing conditions upstream, or downstream, of proposed improvements should be avoided.

Other performance measures may be needed to develop solutions that address specific project needs.

  • Additional measures may pinpoint specific capacity and operational needs more accurately. Additional measures should be considered in certain urban areas and constrained areas where level of service improvements may be cost prohibitive, physically infeasible, and/or impractical due to other project limitations.
  • Performance measures such as vehicular queueing, network delay, latent demand, travel time, person throughput, and Planning Time Index can also be used to guide roadway capacity decisions.

See Also

See Also