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Chapter 3: Elements of DesignChapter 3: Elements of Design3.3.8 Transition Design Controls 3.3.8 Transition Design Controls

(p.3-59 – 3-84)(p.3-59 – 3-84)

Be able to discuss two considerations for the design of transition Be able to discuss two considerations for the design of transition sectionssectionsBe able to determine the length of superelevation runoffBe able to determine the length of superelevation runoffBe able to use relative gradient correctly to achieve full Be able to use relative gradient correctly to achieve full superelevationsuperelevationKnow how to determine the length of tangent runoutKnow how to determine the length of tangent runoutKnow typical method to lay down superelevation runoffKnow typical method to lay down superelevation runoffKnow when to select one of the four methods to attain Know when to select one of the four methods to attain superelevationsuperelevation

Objectives:

Please note we skip p.3-68 – 3-74 but with Civil 3D adding a spiral is very simple.

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General considerations (p.3-59)General considerations (p.3-59)

Two considerations for the design of Two considerations for the design of transition sections:transition sections:1.1. Transitions in the roadway cross slope Transitions in the roadway cross slope

(superelevation transition)(superelevation transition)

2.2. Possible transition curves incorporated in Possible transition curves incorporated in the horizontal alignment (alignment the horizontal alignment (alignment transition)transition)

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1. Superelevation Transition1. Superelevation Transition(p.3-59)(p.3-59)

Superelevation transition = Superelevation transition = superelevation runoff + superelevation runoff + tangent tangent runoutrunout– Superelevation runoff: Length Superelevation runoff: Length

of roadway needed to of roadway needed to accomplish a change in accomplish a change in outside-lane cross slope from outside-lane cross slope from zero (flat or adverse crown) to zero (flat or adverse crown) to full superelevation, or vice full superelevation, or vice versa.versa.

– Tangent runout: Length of Tangent runout: Length of roadway needed to roadway needed to accomplish a change in accomplish a change in outside-lane cross slope from outside-lane cross slope from the normal cross slope rate to the normal cross slope rate to zero (adverse crown), or vice zero (adverse crown), or vice versa.versa.

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2. Alignment Transition2. Alignment Transition(tangent-to-curve or Tangent-Spiral-curve)(tangent-to-curve or Tangent-Spiral-curve)

A spiral or compound transition A spiral or compound transition curve may be used to introduce curve may be used to introduce the main circular curve in a natural the main circular curve in a natural manner.manner.An alignment transition introduces An alignment transition introduces the lateral acceleration associated the lateral acceleration associated with the curve in a gentle manner.with the curve in a gentle manner.Transition is smooth but there is Transition is smooth but there is no definitive evidence that no definitive evidence that transition curves are essential to transition curves are essential to the safe operation of the roadway the safe operation of the roadway and, as a result, they are not used and, as a result, they are not used by many agencies.by many agencies.Many use a tangent-to-curve Many use a tangent-to-curve transition. (A typical arrangement transition. (A typical arrangement in the US. In Japan, for instance, in the US. In Japan, for instance, most of the alignment transitions most of the alignment transitions are designed with spiral curves.)are designed with spiral curves.) Tangent-to-spiral-to curve

Tangent-to-curve

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Typical Alignment Transition Typical Alignment Transition (p.3-59)(p.3-59)

In one widely used In one widely used empirical expression, the empirical expression, the runoff length is runoff length is determined as a function determined as a function of the slope of the outside of the slope of the outside edge of the traveled way edge of the traveled way relative to the centerline relative to the centerline profile. profile. Called “relative Called “relative gradientgradient””

Table 3-15. Maximum Relative Gradients

Table 3-15

Can you picture this concept?

Fig. 3-16

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Tangent-to-Curve TransitionTangent-to-Curve Transition(p.3-60)(p.3-60)

Min. length of superelevation Min. length of superelevation runoff: The relative gradient runoff: The relative gradient must not exceeds the values must not exceeds the values in Table 3-15. in Table 3-15. Range: .78% for 15 mph Range: .78% for 15 mph design speed to 0.35 for 80 design speed to 0.35 for 80 mph design speed (see Table mph design speed (see Table 3-15).3-15).

Use Eq. 3-23 to determine Use Eq. 3-23 to determine the length of superelevation the length of superelevation runoff.runoff.

(3-23)

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Adjustment for Multilane Highways, bAdjustment for Multilane Highways, bww

To avoid excessive To avoid excessive lengths for multi-lanes, lengths for multi-lanes, minimum minimum superelevation runoff superelevation runoff lengths be adjusted lengths be adjusted downward, e.g. 4-lane downward, e.g. 4-lane highways. Do not highways. Do not multiply by 2, but use multiply by 2, but use Table 3-16.Table 3-16.

Table 3-16

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Design speed affects the value of relative gradient (how so?). 1 & 2 in Table 3-17b are: 1. two-lane two-way rotated along the centerline

2. one side (2-lanes) of a multi-lane highway

Table 3-17b

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Min. Length of Tangent Runout Min. Length of Tangent Runout (p.3-66)(p.3-66)

The length of tangent runout is The length of tangent runout is determined by the amount of determined by the amount of adverse cross slope to be adverse cross slope to be removed and the rate at which removed and the rate at which it is removed.it is removed.

To effect a smooth edge of To effect a smooth edge of pavement profile, the rate of pavement profile, the rate of removal should equal the removal should equal the relative gradient used to define relative gradient used to define the superelevation runoff the superelevation runoff length.length.

The tangent runout lengths for The tangent runout lengths for a 2.0% normal crown a 2.0% normal crown determined with Eq. 3-24 are determined with Eq. 3-24 are listed in Table 3-17b in the listed in Table 3-17b in the 2.0% row. (Because e2.0% row. (Because eNCNC = e = edd, , LLtt = L = Lrr for 2.0%) for 2.0%)

(3-24)

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Location with respect to end of curve: Location with respect to end of curve: Alignment transition (p.3-66~)Alignment transition (p.3-66~)

Where do we place the Where do we place the superelevation runoff?superelevation runoff?

– Observations indicate that a spiral Observations indicate that a spiral path results from a driver’s natural path results from a driver’s natural steering behavior during curve entry steering behavior during curve entry or exit. This natural spiral usually or exit. This natural spiral usually begins on the tangent and ends begins on the tangent and ends beyond the beginning of the circular beyond the beginning of the circular curve (PC).curve (PC).

– Most agencies use 2/3 on the Most agencies use 2/3 on the tangent and 1/3 on the curve. tangent and 1/3 on the curve. However, research showed larger However, research showed larger values on the tangent are preferred. values on the tangent are preferred. The values in Table 3-18 are The values in Table 3-18 are desired.desired.

– But use of a single value in the But use of a single value in the range of 0.6 to 0.9 for all speeds and range of 0.6 to 0.9 for all speeds and rotated widths is considered rotated widths is considered acceptable. The values in Table 3-acceptable. The values in Table 3-18 are desirable where possible.18 are desirable where possible.

PC

Table 3-18

Fig. 3-16

ExamplesExamples

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Photos from the Public Roads website (FHWA)

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Method of Attaining Superelevation (p.3-76)Method of Attaining Superelevation (p.3-76)

Four methodsFour methodsA.A. Revolving a traveled way with normal cross slopes Revolving a traveled way with normal cross slopes

about the centerline profileabout the centerline profile

B.B. Revolving a traveled way with normal cross slopes Revolving a traveled way with normal cross slopes about the inside-edge profileabout the inside-edge profile

C.C. Revolving a traveled way with normal cross slopes Revolving a traveled way with normal cross slopes about the outside-edge profileabout the outside-edge profile

D.D. Revolving a straight cross-slope traveled way about Revolving a straight cross-slope traveled way about the outside-edge profile (turning roadways or one the outside-edge profile (turning roadways or one direction of a divided highway)direction of a divided highway)

No general recommendation for the adoption of any particular axis of rotation can be made. To obtain the most pleasing and functional results, each superelevation transition section should be considered individually.

1313

Method A (p.3-77)Method A (p.3-77)

Most popular. One-half of the change in elevation is made at each edge. Less distortion at the edges.

Fig. 3-16

1414

Method BMethod B

One-half of the change in elevation is made by raising the actual centerline profile with respect to the inside-edge profile and the other half by raising the outside-edge profile an equal amount with respect to the actual centerline profile.

Fig. 3-16

1515

Method CMethod C

This method is similar to Method B except that the elevation change is accomplished below the outside-edge profile instead of above the inside-edge profile.

Fig. 3-16

1616

Method DMethod D

This method is often used for two-lane one-way roadways where the axis of rotation coincides with the edge of the traveled way adjacent to the highway median.

Fig. 3-16

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Sample problemSample problemGiven:

Design speed: 70 mph

Superelevation: 6%

2-lane 2-way highway

Lane width = 12 ft

Cross slope = 1.5%

From Table 3-17b the length of superelevation runoff is 180 ft. Now what is the length of tangent runout and the station of beginning and ending of superelevation runoff and tangent runout when the station of PC is STA 100 + 50.00?

PC: STA 100+50

Rise at the outer edge is: 12ft x 0.06 = 0.72ft

Relative gradient is: 0.72ft / 180ft = 0.004=0.4%

Tangent runout is: (12ft x 0.015)/0.004 = 45 ft

or Lt = 0.015/0.06 x 180 = 45 ft)

180ft45ft

45ftAbout 30% of superelevation runoff is in the curve (see Slide #10) = 180*0.3 = About 54 ft in the curve.

About 70% is in the tangent = 126 ft.

STA 101+4STA 99+24

STA 98+79 STA 99+69(See Slide #10 or Tab 3-18 for percent splits.)

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Axis of Rotation (p.3-80)Axis of Rotation (p.3-80)

In the design of divided highways, streets, and parkways, the In the design of divided highways, streets, and parkways, the inclusion of a median in the cross section influences the inclusion of a median in the cross section influences the superelevation transition design. The most appropriate location for superelevation transition design. The most appropriate location for this axis depends on the width of the median and its cross section.this axis depends on the width of the median and its cross section.

Case ICase I The whole of the The whole of the traveled way, traveled way, including the including the median, is median, is superelevatedsuperelevated

Limited to narrow medians up to Limited to narrow medians up to 15 ft or less.15 ft or less.

The length of runoff should be The length of runoff should be based on the total rotated width based on the total rotated width (including the median).(including the median).

However, because narrow However, because narrow medians have very little effect on medians have very little effect on the runoff length, median widths of the runoff length, median widths of up to 10 ft may be ignored when up to 10 ft may be ignored when determining the runoff length. (A)determining the runoff length. (A)

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Axis of Rotation (cont)Axis of Rotation (cont)

Case IICase II The median is held The median is held in a horizontal plane in a horizontal plane and the two traveled and the two traveled ways are rotated ways are rotated separately around separately around the median edges.the median edges.

Most appropriate for medians with Most appropriate for medians with widths between 15 to 60 ft.widths between 15 to 60 ft.

Superelevation can be attained Superelevation can be attained using any of the methods B, C, or using any of the methods B, C, or D, with profile reference being the D, with profile reference being the same for both traveled ways.same for both traveled ways.

Case IIICase III The two traveled The two traveled ways are separately ways are separately treated for runoff treated for runoff with a resultant with a resultant variable difference variable difference in elevation at the in elevation at the median edges.median edges.

Can be used with wide medians Can be used with wide medians 60 ft or more60 ft or more

With a wide median, it is possible With a wide median, it is possible to design the profiles and to design the profiles and superelevation transition separately superelevation transition separately for the two roadways.for the two roadways.

Accordingly, superelevation can Accordingly, superelevation can be attained by the method be attained by the method otherwise considered appropriate otherwise considered appropriate (A, B, C, or D).(A, B, C, or D).