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3/9/2009 7:07:37 http://www2.mainroads.wa.gov.au/Internet/Standards/RTEms/geometric_design/roadways/alignment.asp

Road Alignment

Document No: Revision: Date amended:

The information below is intended to reflect the preferred practice of Main Roads WesternAustralia ("Main Roads"). Main Roads reserves the right to update this information at any timewithout notice. If you have any questions or comments please contact Stephen Curgenvenbye-mail or on (08) 9323 4415.

To the extent permitted by law, Main Roads, its employees, agents, authors and contributorsare not liable for any loss resulting from any action taken or reliance made by you on theinformation herein displayed.

Revision Register

Ed/VersionNumber

ClauseNumber

Description ofRevision

Date

ISSUE 1 ALL GUIDELINE DEVELOPED AND APPROVED 14/03/02

ISSUE 2 6.6 FIGURES 6.3a & 6.3b AMENDED 12/02/04

ISSUE 3 6.4 FIGURE 6.1 - COMPOUND CURVE PLANTRANSITION SHIFT FORMULA CORRECTED

14/02/05

ISSUE 4 ALL GUIDELINE REVISED AND APPROVED 16/08/05

6. CHAPTER 6 OF 10. THE ROAD ALIGNMENT

6.1. GENERAL6.2. TANGENTS6.3. CIRCULAR CURVES

6.3.1. GENERAL6.3.2. RADIUS TO MEET SIGHT DISTANCE REQUIREMENTS6.3.3. LENGTH OF HORIZONTAL CURVES6.3.4. SMALL DEFLECTION ANGLES

6.4. CURVES WITH ADVERSE CROSSFALL6.5. TYPES OF HORIZONTAL CURVES

6.5.1. COMPOUND CURVES6.5.2. BROKEN BACK CURVES6.5.3. ADJACENT HORIZONTAL CURVES SEPARATED BY A SHORT TANGENT

6.5.3.1. ADJACENT HORIZONTAL CURVES SEPARATED BY A SHORT TANGENT ...6.5.3.2. ADJACENT HORIZONTAL CURVES SEPARATED BY A SHORT TANGENT ...6.5.3.3. ADJACENT HORIZONTAL CURVES SEPARATED BY A SHORT TANGENT ...

6.5.4. ADJACENT HORIZONTAL CURVES SEPARATED BY A LONG TANGENT AND...6.5.5. REVERSE CURVES

6.5.5.1. REVERSE CURVES WITH PLAN TRANSITIONS AND A SHORT TANGENT6.5.5.2. REVERSE CURVES WITHOUT PLAN TRANSITIONS AND A SHORT TANG...6.5.5.3. REVERSE CURVES WITH LONG SEPARATING TANGENT6.5.5.4. REVERSE CURVES WITHOUT A SEPARATING TANGENT - (COMPOUND ...

6.6. PLAN AND SUPERELEVATION TRANSITIONS6.6.1. CALCULATING SUPERELEVATION DEVELOPMENT LENGTH AND PLAN TRA...

6.7. SELECTION OF DESIGN SUPERELEVATION RATE6.7.1. GENERAL6.7.2. MINIMUM SUPERELEVATION6.7.3. MAXIMUM SUPERELEVATION6.7.4. SUPERELEVATION ON BRIDGES6.7.5. SUPERELEVATION AT INTERSECTIONS6.7.6. DEVELOPMENT OF SUPERELEVATION TO AVOID DRAINAGE PROBLEMS.

Table Of Content:

6.Chapter 6 of 10. THE ROAD ALIGNMENT

6.1.GeneralThe road alignment, more than any other single design feature, affects the driver's perception andconsequently the operating speed that is adopted. For this reason, whenever curves are used tochange the direction of travel, the radii and superelevation must be designed to permit operatingspeeds commensurate with those expected on adjoining tangents or along a section of the road.Generally, the adopted alignment should be as direct as possible, with curve radii as large aspracticable.

As with other elements of design, the horizontal alignment should provide for safe and continuousoperation at a uniform operating speed. Sudden reductions in standard, such as isolated curves ofsmall radius, particularly at the end of long tangents, introduce an element of surprise to the driverand should be avoided.

Where physical restrictions on curve radii cannot be overcome and it becomes necessary tointroduce curvature of lower standard, the design speed of successive geometric elements should

67-08-62G 4 16-Aug-2005
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6.2.Tangents

6.3.CIRCULAR CURVES

6.3.1.General

6.3.2.Radius to meet Sight Distance Requirements

6.3.3.Length of Horizontal Curves

not change by more than 10 km/h. On two-way roads both directions of travel need to beconsidered.

Where the road design is required to connect into existing alignment(s), the tie-ins shall match theexisting road alignment centreline and shall be designed with minimum disturbance to the existingroad. If the geometry is not satisfactory it may be necessary to extend the design section slightlyand sacrifice a portion of the existing pavement.

The design shall provide for continuity between new and existing geometry.

The road alignment should be designed in accordance with this guideline and the following referencedocuments:

Horizontal Curve Table (2005)

Austroads - A Guide to the Geometric Design of Rural Roads (2003)

Austroads - Guide to the Geometric Design of Major Urban Roads (2002)

AASHTO - A Policy on Geometric Design of Highways and Streets (2004)

NAASRA - Design Guide for Grade Separated Interchanges (1984)

NAASRA - Guide Policy for Geometric Design of Freeways and Expressways (1976)

In the event of any inconsistency between these documents, the inconsistency must be resolved bygiving precedence to the document in the order they are listed above.

The tangent is one of the major elements of an alignment. It provides clear orientation, but at thesame time can be visually uninteresting unless aimed at a landmark. Being totally predictable, witha view that appears static, it can cause driver monotony and encourage the undesirable combinationof fatigue and excessive speed. At night, opposing headlights can also be a problem.

Tangents of suitable length are desirable on two lane roads to facilitate overtaking manoeuvres andshould be provided as frequently as the terrain permits. Excessively long tangents should beavoided as they may encourage drivers to travel in excess of the safe operating speed.

In flat terrain, long tangents may have to be accepted. If curves are deliberately introduced into thedesign to break the monotony, they should have long arc lengths to avoid the appearance of kinks.Unless the change in alignment is considerable, oncoming headlights will remain a nuisance todrivers.

The length of the curve should be taken as the length of the central ci rcular curve plus the sum of

the plan transition lengths.

The appropriate minimum length of curved roadway is a function of aesthetics and is thereforesubjective, and should generally satisfy the following conditions:

1. A minimum length of curve equivalent to three times the length of the plan transition.

2. The distance travelled by a vehicle during one second for each 10 km/h of design speed. Thisis calculated by the following formula:

Lh

= V2/36

Where:

Lh

= Length of horizontal curve (m).

For information on radius to meet sight distance requirements refer to:Main Roads Sight Distance Guidelines Chapter 5, Section 5.4.Austroads - A Guide to the Geometric Design of Rural Roads (2003) - Section 9.11 andAustroads - Guide to the Geometric Design of Major Urban Roads (2002) - Section 8.5

The second major element of an alignment is the curve. A wide range of curve radii may be chosenfor an operating speed. The choice of the radius depends on the def lection angle, superelevation,sight distance, site limitations, and cost of providing a better facility and overall design philosophyand intent. In developing the road alignment, the designer should generally select the largest curveradii attainable. Isolated small radii curves in an otherwise free flowing alignment, and small radiicurves at the end of long straights, on steep down grades and over crests are unsafe and must beavoided.

The Horizontal Curve Table - Main Roads WA (2005) should be used as a guide to the selection of asuitable curve radius. These Tables list the appropriate superelevation and transition lengths fordifferent design speeds and curve radii.


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6.3.4.Small Deflection Angles

6.4.Curves with Adverse Crossfall

6.5.Types of Horizontal Curves

6.5.1.Compound Curves

6.5.2.Broken Back Curves

Adverse crossfall occurs when the road pavement slopes down from the inside of a curve to theoutside of the curve, which is contrary to normal practice.

Although adverse crossfall on curves should be avoided, situations may arise where the adoption ofadverse crossfall may be necessary.

Reference should be made to the Main Roads Horizontal Curve Tables (2005) andAustroads - A Guide to the Geometric Design of Rural Roads (2003) - Section 9.8 andAustroads - Guide to the Geometric Design of Major Urban Roads (2002) - Section 9.5. (Noteconsideration should be given to the text in Section 9.5 using 50% of the normal f value for theabsolute minimum radius for adverse crossfall when calculating from first principals. The values inTable 9.6 are desirable minimum radii.).

In general, consecutive horizontal curves should satisfy the following condition:

The design speed of the flatter curve should be not more than 10 km/h greater than thedesign speed of the sharper curve.

V = Design speed (km/h).

This is the formulae used to calculated the rounded values of minimum curve length inAustroads - Guide to the Geometric Design of Major Urban Roads (2002) - Table 9.9.

3. A minimum length of curve equal to the stopping sight distance for the curve design speed.

In some situations the use of larger curve lengths to enhance appearance may be desirable.Refer to Austroads - A Guide to the Geometric Design of Rural Roads (2003) - Section 9.9 andAustroads - Guide to the Geometric Design of Major Urban Roads (2002) - Section 9.6.

For details of the desirable minimum curve length for small deflection angles for appearance andmaximum deflection angle for which a curve is required refer to Table 6.1.

Deflection Angle Min Curve Length

5o 150

4o 180

3o 210

2o 240

1o 270

0o

30' to 1o

30' 300

< 0o

30' No Curve Required

Table 6.1 Desirable Minimum Curve Lengths for Small Deflection Angles

Such curves would normally have a radius large enough that it would not be necessary to providesuperelevation.

Refer AASHTO - A Policy on Geometric Design of Highways and Streets (2004), page 229.

Broken back curves are adjacent curves in the same direction separated by a tangent (or straight)length of less than 1.0V. Broken back curves should be avoided. Where they are unavoidable, thealignment should be connected by a curve-to-curve plan transition.

For application of the plan transition and superelevation development Refer to Figure 6.1.

Compound curves are curves of two different radii turning in the same direction with a commontangent point. Compound curves which have radii < 1 000m should be avoided and a single curveused if economically possible.

The ratio of the flatter radius to the sharper radius should not exceed 1.5:1, Max ratio of 2:1 maybe used at intersections and ramps. (Reference: A Policy on Geometric Design of Highways andStreets", AASHTO, 2004, p201.)

Refer to: Austroads - A Guide to the Geometric Design of Rural Roads (2003) - Section 9.3.1.2.Austroads - Guide to the Geometric Design of Major Urban Roads (2002) - Section 9.2.1.


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6.5.3.Adjacent Horizontal Curves Separated by a Short Tangent

6.5.3.1.Adjacent Horizontal Curves Separated by a Short Tangent with PlanTransitions

Where both curves require a plan transition hold the lowest value of the superelevation through thatcurve's plan transition and the tangent and develop the greater superelevation in the plan transitionof the curve that requires the larger superelevation. Refer Figure 6.2a.

Notes:

1. The superelevation development length is then placed coincidentwith the plan transition length.

Figure 6.1 - Broken Back Curve (Curve to Curve Plan Transition)

These are horizontal curves in the same direction separated by a tangent (or straight) length ofgreater than 1.0V and less than 3.0V - 4.0V (where V is the operating speed in km/h). Such curvecombinations should be avoided. It is virtually impossible to produce a pleasing grading of theedge profiles.

If due to constraints this combination of curves is unavoidable one of the following cases detailed inSections 6.5.3.1 to 6.5.3.3 should be applied.

The length of superelevation runoff/plan transitions should be calculated assuming the tangentsection between curves returns to a crown cross-section. Refer Section 6.6.

The length of the plan transition determined above is that required for the horizontal design for thiscurve radius/speed combination.

Figure 6.2a - Adjacent Horizontal Curves Separated by a Short Tangent with Plan Transitions


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6.5.3.3.Adjacent Horizontal Curves Separated by a Short Tangent without PlanTransitions

6.5.4.Adjacent Horizontal Curves Separated by a Long Tangent and PlanTransitions

When neither curve requires a plan transition hold the lowest value of the superelevation throughthe tangent and apply the additional superelevation to the curve that requires it using the designpractice as described in Section 6.6.Refer Figure 6.2c.

To achieve the desirable design of adjacent curves in the same direction the distance between thesuperelevation development lengths should represent a minimum of 4.0 seconds of travel time atthe operating speed.

Figure 6.2b - Adjacent Horizontal Curves Separated by a Short Tangent with One Plan Transition

Figure 6.2c - Adjacent Horizontal Curves Separated by a Short Tangent without Plan Transitions


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6.5.5.2.Reverse Curves without Plan Transitions and a Short Tangent

6.5.5.3.Reverse Curves with Long Separating Tangent

6.5.5.4.Reverse Curves without a Separating Tangent - (Compound orContiguous Reverse Curves)

For reverse curves without plan transitions and a short tangent, the superelevation developmentfrom one curve to the next should be applied to the separating short tangent and the superelevationrunoff should extend into the circular curve as recommended in Austroads - A Guide to theGeometric Design of Rural Roads (2003) - Table 9.7. The minimum length of short tangent should bethe length of both curve superelevation runoff lengths less the amount extended into the circularcurve. The length of short tangent may exceed the total of the curve superelevation runoff lengthsbut should not be too long as to cause surface drainage problems. All tangent lengths should also bechecked to ensure proper drainage.

Notes:

1. Rounding vertical curves (not shown above) should be applied at all changesof grade of edge profiles.

Figure 6.3a - Reverse Curves with Plan Transitions and a Short Tangent

For reverse curves with a long separating tangent the road section should be returned to a crowncross section. Refer Figure 6.3b.

Notes:

1. Rounding vertical curves (not shown above) should be applied at all changes of gradeof edge profiles.

2. Superelevation runoff length and plan transition length are common.

Figure 6.3b - Reverse Curves with Long Separating Tangent

Reverse curves without a separating tangent should be avoided.

If they cannot be avoided the superelevation runoff length/plan transition length for each curveshould be calculated assuming the tangent section is to return to a crown cross-section. (ReferSection 6.6)If the shift for either curve is less than the required 0.200m and no plan transition is required, thenthe application of the reverse curves is to be avoided and the alignment changed to suit the shorttangent without transitions case in Figure 6.3a.


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6.6.Plan and Superelevation Transitions

Main Roads only permits contiguous reverse horizontal curves when both curves have plantransitions/superelevation runoff lengths that meet at a common tangent point. (Refer Figure 6.3c).

The length of the superelevation runoff/plan transition may be increased to fit the topography butshould not be too long as to cause surface drainage problems.

Superelevation development length shall be profiled to avoid the creation of flat pavement area anddrainage problems.

Refer to Austroads - Guide to the Geometric Design of Rural Roads (2003) Section 9.3.1.1 andAustroads - Guide to the Geometric Design of Major Urban Roads (2002) Section 9.2.3.

Notes:

1. There may be situations where adjacent reverse curves do not readily conform to theabove cases. In such situations, amend the alignment to suit the short tangent case.(Fig 6.3a).

2. Superelevation run-off length and plan transition length are the same.

3. Rounding vertical curves (not shown above) should be applied at all changes of gradeof edge profiles.

Figure 6.3c - Reverse Curves without a Separating Tangent -(Compound or Contiguous Reverse Curves)

Refer to Austroads - Guide to the Geometric Design of Rural Roads (2003) - Section 9 and MainRoads Horizontal Curve Tables (2005).

Before a curve radius is finally selected it is generally necessary to give consideration to thetransitions. Two types of transitions are generally required.

A plan transition maybe required to give a gradual change in curvature from zero on thetangent to a value corresponding to the circular curve radius. The curve used by Main Roadsfor this purpose is the clothoid spiral.

A superelevation transition is required to give a gradual change in cross section shape froma crowned or one-way crossfall section on the tangent to the superelevation section on thecurve.

The practice adopted by Main Roads is similar to that adopted by Austroads. The difference is MainRoads applies the superelevation transition on the inside edge over the full length of thesuperelevation development length.

This is illustrated in Figure 6.4. This practice enables the pavement to be rotated about eitherpavement edge in a superelevation transit ion in order to prevent flat areas and associateddrainage problems.

On freeways for aesthetic reasons, the ratio of the plan transition length to arc length should begenerally such that the circular arc occupies at least half of the total length of the curve. (GuidePolicy for Geometric Design of Freeways and Expressways - NAASRA 1976)


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6.6.1.Calculating Superelevation Development Length and Plan TransitionLength

Typical Development of Superelevation for Curves with Plan Transitions - Overview

Typical Development of Superelevation for Curves with Plan Transitions - Profile

Note: Rounding vertical curves should be applied at all changes of grade of edge profiles.

Legend

PI Point of intersection of the main tangents.

TSTangent to Spiral - common point of tangent and spiral and startof superelevation development.

SCSpiral to Curve - common point of plan transition and circularcurve.

CSCurve to Spiral - common point of circular curve and plantransition.

STSpiral to Tangent - common point of plan transition and tangent

at end of superelevation development.

Lp Length of plan transition, TS to SC (m) and CS to ST (m).

Lt Length of tangent runout (m).

Ls Length of superelevation runoff (m).

Le Length of superelevation development (m).

n Normal pavement crossfall (%).

e Pavement superelevation (%).

Main Roads uses the same terminology used by Austroads with the exception of the following: -

Superelevation Transition Length = is the superelevation development length including therounding vertical curves.


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Figure 6.4 Typical Development of Superelevation for Curves withPlan Transitions on Two-Way Roads

Rounding VerticalCurve = Ease.

For appearance purposes Main Roads has adopted the rounding curves lengths as shown in Table 1.

NUMBER OF LANES IN ONE DIRECTION LENGTH OF ROUNDING (m)

1 (3.5 m) 20

2 (7.0 m) 30

3 (10.5 m) 40

TABLE 1 - Superelevation Development Length Rounding Curve Lengths

Note these rounding curve lengths are not applicable in situations where stopping sight distance toa zero object height is required (i.e. at intersections) and therefore they shall be amendedaccordingly.

Step 1

Calculate the Length of Superelevation Development Using R a t e o f R o t a t i o n

(Refer Austroads 'Guide to the Geometric Design of Major Urban Roads', 2002)

Calculate the length of superelevation development using the following formulae;

Le

= (e1 - e2) V/0.126 for vehicle operating speed < 80 km/h

Le

= (e1 - e2) V/0.09 for vehicle operating speed => 80 km/h

Where, Le

= superelevation development length (m)

e1, e2 = crossfall or superelevation at ends of development length (m/m)V = vehicle operating speed (km/h)

Step 2

Calculate the Length of Superelevation Development Using R e l a t i v e G r a d e

Select a value of Gr from either Austroads - A Guide to the Geometric Design of Rural Roads (2003)- Table 9.4 or Austroads - Guide to the Geometric Design of Major Urban Roads (2002) - Table 9.3This value for Gr is substituted in the formula below to calculate the length of superelevationdevelopment (Le) required to satisfy the relative grade criterion.

Where, Le = superelevation development length (m)Wr = width from axis of rotation to outside edge of traffic lanes (m)e1, e2 = crossfall or superelevation at ends of transition length (m/m)Gr = relative grade (%)

The greater value of the lengths calculated in Steps 1 and Step 2 is then adopted for Le.

Step 3

Calculate the Superelevation Runoff length and Tangent Runout Length

The superelevation runoff length is calculated using the formula:-L

s= L

e- L

e[e1/(e1 - e2)]

Lt

= Le

- Ls

Where, L

s= superelevation runoff length (m)

Le

= superelevation development length (m)

e1 = normal crossfall at start of development length (%)e2 = full superelevation at end of development length (%)L

t= tangent runout (m)

AASHTO - A Policy on the Geometric Design of Highways and Streets (2004) pages 175 to 192investigates the lengths of plan transition curves. It states that a value of 0.200m is consistent withthe minimum lateral shift that occurs as a result of the natural steering behaviour of most driversand likewise a value of 1.0m is generally the maximum lateral shift that occurs. (Refer page 188).

AASHTO also recommends on page 190"For the most part the calculated values for length of spiral and length of runoff do not dif fermaterially."Also "The length of runoff is applicable to all superelevated curves, and it is recommended that this


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value should be used for minimum lengths of spiral. In this manner, the length of spiral should beset equal to the length of superelevation runoff."

Therefore Main Roads has adopted superelevation runoff length to be equal to the plan transitionlength and any shift less than 0.200m can be contained within the width of the normal lane.

Step 4

Calculate the Shift for the Superelevation Runoff Length/Plan Transition Length

(i) The superelevation runoff length is assumed to be equal to the plan transition length.L

s = L

p= plan transition length.

(ii) Calculate the shift using the formula: -

P=L2/24R

Where, P = shift (m)L = plan transition length/superelevation runoff length (m)R = radius of central curve (m)

If the shift is less than 0.200m then no plan transition is required.If the shift is equal to or greater than 0.200m then a plan transition is to be applied.

Main Roads method of rounding the Superelevation Development Length is to round to the nearest1 metre and for the superelevation runoff length, round to the nearest 1 metres. The uniformapplication of the super development tends to locate the level cross section at the tangent pointwhen plan transitions are used.

The superelevation runoff length should be coincident with the plan transition.

When the plan transition is omitted, the superelevation development length is positioned

with the larger portion of the superelevation runoff length on the approach tangent. - As specifiedin 'Austroads - Rural Road Design' A Guide to the Geometric Design of Rural Roads (2003) Table 9.7

For a typical development of superelevation for curves Without plan transitions refer to Figure 6.5.

Typical Development of Superelevation for Curves Without Plan Transitions - Overview

Typical Development of Superelevation for Curves Without Plan Transitions - Profile


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6.7.Selection of Design Superelevation Rate

6.7.1.GeneralThe objective of the designer is to select values of curve radius and superelevation (e) such thatthe individual curve is safe for the operating speed of the road section.

The "Horizontal Curve Tables" should be used as a guide in the selection of suitable radii/superelevation. As a general rule these values may be used in most situations. However inconstrained, low speed urban situations, the designer may need to return to first principles.

The design speed and superelevation is generally applied as recommended in 'Austroads - RuralRoad Design' A Guide to the Geometric Design of Rural Roads (2003).'Austroads - Rural Road Design' A Guide to the Geometric Design of Rural Roads (2003). uses alinear distribution method to distribute 'e' and 'f' such that there is a consistent relationshipbetween them for superelevations up to 6% (i .e. e

max= 6%). Above 6%, Austroads uses the

maximum amount of 'f' (fmax

) to calculate the required superelevation. The superelevation that is

applied (e) is limited to a maximum of:

6% for high speed roads (>100 km/hr)

7% for intermediate speed roads (>70 to


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6.7.3.Maximum Superelevation

6.7.4.Superelevation on Bridges

6.7.5.Superelevation at Intersections

6.7.6.Development of Superelevation to Avoid Drainage Problems.

Main Roads general maximum superelevation is limited to 6%. Shaded areas within the 'HorizontalCurve Tables' is for superelevations on turning roads and loop ramps. The absolute maximumsuperelevation for turning roads and loop ramps is 10%.

Heavily laden or slow moving vehicles limit the maximum acceptable value of superelevation. Thedesigner should take into account the types of vehicles using the road and limit the acceptablemaximum superelevation to ensure their safety.For the stability of turning vehicles at intersections, changes of grade greater than 4% through the

turning movement or superelevation exceeding 4% at the intersection is not acceptable. - Refer toSection 6.7.5 Superelevation at Intersections

Bridge designers prefer that bridges are located on tangent alignments with straight grades. If thisis not possible , the next best situation is a bridge on constant curvature. Superelevation transitionson bridges should be avoided.

Where a side road junction is on the outside of a curve, a compromise may be necessary betweenadequate superelevation on the through road and safe conditions for vehicles turning against theadverse crossfall. The situation worsens if the curve is located on a steep grade. If the intersectioncannot be relocated, the superelevation may need to be modified to ensure safe turning conditions.

Generally, if the through road has a steep longitudinal grade over 3%, the superelevation/crossfallon the through road should not exceed 4% and should preferably be limited to 3%. The sameproblem does not exist where the junction is on the inside of the curve as the superelevation thenfavours the turning movements. Junctions on the inside of curves are not desirable due to sightdistance constraints.

The maximum effective adverse crossfall for turning movements at intersections is 5%.At intersections with higher speed turning movements (i.e. traffic signal controlled intersections)the maximum safe effective adverse crossfall may need to be less than the maximum.

The effective adverse crossfall can be determined using vector diagrams.

Figure 6.6 Vector Diagram of Maximum Effective Crossfall

The changes of grade required on the edges to provide superelevation transitions are often of thesame order as the changes of grade in the main profile. This is particularly so in flat terrain.

It is also necessary to avoid situations where a level cross section within a superelevation transitioncoincides with a level longitudinal grade on any part of the cross section. This situation creates aflat area where water ponding will occur with consequent hazardous driving conditions.


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To provide a smooth appearance to the edge profiles and avoid creating a flat area, it is necessaryto coordinate the design of the main profile with the design of the edge profiles through thesuperelevation transitions.

The 'flat area' problem can be avoided by ensuring that either crossfall or longitudinal grade ismaintained through the superelevation transitions. This can be achieved by rotating the pavementabout one edge or the other rather than around the centreline. A minimum grade of 0.3% should beapplied to any kerbed edges and 0.2% to any unkerbed edges through a superelevation transition.Refer Chapter 7 Section 7.3.2.

The superelevation transition length design should be checked for flat areas by contouring thepavement at 20mm intervals.

The changes of grade on the pavement edges should be provided with rounding vertical curvescentred on the nominal ends of the superelevation development length as shown in Figures 6.7 to6.14 inclusive.

For appearance purposes Main Roads has adopted the rounding curves lengths as shown in Chapter6 Section 6.6.1 - Table 1.

Note for Figure 6.7 to Figure 6.14 the illustration is for a two-way road with a centre line designreference line.

Consideration should also be given to the Coordination of Vertical Curves with SuperelevationRounding Curves. Refer Chapter 7 Section 7.4.5.

General rules for coordination of Vertical Curves with Superelevation Transition Rounding Curves:

1. The main profile Vertical Curve should not overlap the rounding Vertical Curve.

2. A main profile Vertical Curve may be compounded with a rounding Vertical Curve turning the

same direction.

3. Compound reverse Vertical Curves are not allowed.

4. On a superelevation transition the whole pavement shall have a longitudinal fall in only onedirection.

It should be noted that the treatment of the superelevation transitions with the vertical alignment inFigure 6.7 to Figure 6.14 are shown as specific cases. Other combinations may be used to providecorrect road drainage and appearance.

Figure 6.7 Crest VC on flat grades


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Figure 6.8 Crest VC on steeper grades

Figure 6.9 Rising or falling grade with Crest VC


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Figure 6.10 Sag VC on flat grades

Figure 6.11 Sag VC on steeper grades

Figure 6.12 Rising or falling grade with Sag VC


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Figure 6.13 Rising or falling flat grade

Figure 6.14 Rising or falling steep grade

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