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Superelevation Guidlines

Jun 03, 2018



  • 8/12/2019 Superelevation Guidlines


    WSDOT Design Manual M 22-01.05 Page 1250-1June 2009

    Chapter 1250 Superelevation

    1250.01 General1250.02 References1250.03 De nitions1250.04 Superelevation Rate Selection1250.05 Existing Curves1250.06 Turning Movements at Intersections1250.07 Runoff for Highway Curves1250.08 Runoff for Ramp Curves1250.09 Documentation

    1250.01 GeneralTo maintain the design speed, highway and ramp curves are usually superelevatedto overcome part of the centrifugal force that acts on a vehicle.

    For additional information, see the following chapters:

    Chapter Subject 1130 Roadway widths and cross slopes for modi ed design level

    1140 Minimum lane and shoulder widths for full design level 1360 Lane and shoulder widths for ramps

    1250.02 References

    (1) Design Guidance

    Standard Plans for Road, Bridge, and Municipal Construction ( Standard Plans ),M 21-01, WSDOT

    Standard Speci cations for Road, Bridge, and Municipal Construction( Standard Speci cation s ), M 41-10, WSDOT

    (2) Supporting Information

    A Policy on Geometric Design of Highways and Streets (Green Book),AASHTO, 2004

    1250.03 De nitionslane A strip of roadway used for a single line of vehicles.

    lane width The lateral design width for a single lane, striped as shown in the

    Standard Plans an d the Standard Speci cation s.median The portion of a highway separating the traveled ways for traf c inopposite directions.

    roadway The portion of a highway, including shoulders, for vehicular use.

  • 8/12/2019 Superelevation Guidlines


    Page 1250-2 WSDOT Design Manual M 22-01.05June 2009

    Superelevation Chapter 1250

    superelevation (super) The rotation of the roadway cross section in such a manneras to overcome part of the centrifugal force that acts on a vehicle traversing a curve.

    superelevation runoff The length of highway needed to accomplish the changein cross slope from a section with adverse crown removed (level) to a fullysuperelevated section, or vice versa.

    superelevation transition length The length of highway needed to change the crossslope from normal crown or normal pavement slope to full superelevation.

    tangent runout The length of highway needed to change the cross slope fromnormal crown to a section with adverse crown removed (level).

    traveled way The portion of the roadway intended for the movement of vehicles,exclusive of shoulders and lanes for parking, turning, and storage for turning.

    turning roadway A curve on an open highway, a ramp, or the connecting portion ofroadway between two intersecting legs of an intersection.

    1250.04 Superelevation Rate Selection

    The maximum superelevation rate allowed is 10%.Depending on design speed, construct large-radius curves with a n ormal crownsection. The minimum radii for normal crown sections are shown i n Exhibit 1250- 1 .Superelevate curves with smaller radii as follows:

    Exhibit 1250-4a (emax =10%) is desirable for all open highways, ramps, andlong-term detours, especially when associated with a main line detour.

    Exhibit 1250-4b (emax=8%) may be used for freeways in urban design areasand areas where the emax=6% rate is allowed but emax =8% is preferred.

    Exhibit 1250-4c (emax =6%) may be used, with justi cation, for nonfreewaysin urban design areas, in mountainous areas, and for short-term detours, whichare generally implemente d and removed in one construction season.

    Exhibit 1250-5 may be u sed for turning roadways at intersections, for urbanmanaged access highways with a design speed of 40 mph or less, andwith

    justi cationfor ramps in urban areas with a design speed of 40 mph or less.

    When selecting superelevation for a curve, consider the existing curves on thecorridor. To maintain ro ute cont inuity and driver expectance on open highways, selectthe chart (see Exhibits 1250-4 a, 4 b , or 4c) that best matches the superelevation on theexisting curves.

    In locations that experience regular accumulations of snow and ice, limitsuperelevation from the selected chart to 6% or less. In these areas, provide

    justi cation for superelevation rates greater than 6%. Vehicles moving at slow speeds

    or stopped on curves with supers greater than 6% tend to slide inward on the radius(downslope).

    Round the selected superelevation rate to the nearest full percent.

  • 8/12/2019 Superelevation Guidlines


    WSDOT Design Manual M 22-01.05 Page 1250-3June 2009

    Chapter 1250 Superelevation

    Design Speed (mph) Minimum Radius for NormalCrown Section (ft)

    15 945

    20 1,680

    25 2,430

    30 3,325

    35 4,360

    40 5,545

    45 6,860

    50 8,315

    55 9,920

    60 11,675

    65 13,130

    70 14,675

    75 16,32580 18,065

    Minimum Radius for Normal Crown SectionExhibit 1250-1

    1250.05 Existing CurvesEvaluate the supe relevation on an existin g curve to determine its adequacy.Use the equation in Exhibit 1250-2 to det ermine the minimum radius for a givensuperelevation and design speed.

    f eV R


    Where: R = The minimum allowable radius of the curve (ft)V = Design speed (mph)e = Superelevation rate (%)

    f = Side friction factor from Exhibit 1250-3

    Minimum Radius for Existing CurvesExhibit 1250-2

    For Preservation projects where the existing pavement is to remain in place, the

    superelevation on existing curves may be evaluated with a ball banking analysis.Address superelevation when the existing radius is less than the minimum radiuscalculated using the equation or when the maximum speed determined by a ball

    banking analysis is less than the design speed. Wh en modifying the sup erelevationof an existing curve, provide superelevation as giv en in 1250.04 .

  • 8/12/2019 Superelevation Guidlines


    Page 1250-4 WSDOT Design Manual M 22-01.05June 2009

    Superelevation Chapter 1250

    Design Speed (mph) Side Friction Factor (f)

    15 32

    20 27

    25 23

    30 2035 18

    40 16

    45 15

    50 14

    55 13

    60 12

    65 11

    70 10

    75 9

    80 8

    Side Friction Factor Exhibit 1250-3

    1250.06 Turning Movements at IntersectionsCurves associated with the turning movements at intersections are superelevatedusing the rates for low-speed urban roadway curves. Use supereleva tion rates as hig has practicable, consistent with curve length and climatic conditions. Exhibit 1250-5 shows the minimum superelevation for the given design speed and radius. Whenusing high superelevation rates on short curves, provide smooth transitions with

    merging ramps or roadways.

    1250.07 Runoff for Highway CurvesProvide tran sitions for all superelevated highway curves as speci ed in Exhibits1250-6a thr oug h 6e. Which transition to use depends on the location of the pivot

    point, the direction of the curve, and the roadway cross slope. The length of therunoff is based on a maximum allowable difference between the grade at the pivot

    point and the grade at the outer edge of traveled way for one 12-foot lane.

    Pay close attention to the pro le of the edge of traveled way created by thesuperelevation runoff; do not let it appear distorted. The combination ofsuperelevation transition and grade may result in a hump and/or dip in the pro le of

    the edge of traveled way. When this happens, the transition may be lengthened toeliminate the hump and/or dip. If the hump and/or dip cannot be eliminated this way,

    pay special attention to drainage in the low areas to prevent ponding. Locate the pivot point at the centerline of the roadway to help minimize humps and dips at the edge ofthe traveled lane and reduce the superelevation runoff length.

  • 8/12/2019 Superelevation Guidlines


    WSDOT Design Manual M 22-01.05 Page 1250-5June 2009

    Chapter 1250 Superelevation

    When reverse curves are necessary, provide suf cient tangent length for completesuperelevation runoff for both curves (that is, from full superelevation of the rstcurve to level to full superelevation of the second curve). If tangent length islonger than this but not suf cient to provide full super transitions (that is, from fullsuperelevation of the rst curve to normal crown to full superelevation of the secondcurve), increase the superelevation runoff lengths until they abut. This provides onecontinuous tr ansition, wi thout a n ormal crown section, similar to Designs C 2 andD2 in Exhibi ts 1250-6c and 6d, except that full super will be attained rath

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