4 horizontal alignment - land transport new zealand€¦ · · 2009-12-10section 4: horizontal...

STATE HIGHWAY GEOMETRIC DESIGN MANUALSECTION 4: HORIZONTAL ALIGNMENT

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Draft: S4-Horizontal Alignment_Draft Amendment - 4 May 2005.wpd

4 Horizontal Alignment

4.1 GeneralA horizontal road alignment is a usually series of straights(tangents) and circular curves. Transition curves are oftenused to join straight sections smoothly into circular curvesections.

A curve should normally be used whenever there is a changeof direction in a road alignment and must be of sufficientlength to avoid the appearance of a kink in the roadalignment.

Small changes in alignment are not usually noticed by driversand in some cases it might not be necessary to provide a curvebetween adjacent straight tangent sections of road, providedthey do not produce a kinked road alignment.

Horizontal road alignments without significant straightsections are described as curvilinear. A curvilinearalignment normally has:

• long, large radius circular curves, with or withoutspiral transitions, and

• occasionally, other types of curves which conform topolynomial mathematical relationships.

Curvilinear alignment is most suited to dual carriagewayroads but can also be successfully used on two-lane roads inflat and undulating terrain, providing overtaking provisionsare not impaired. The horizontal curves of a curvilinearalignment are generally of large radius and:

• do not normally restrict overtaking opportunities,• help reduce headlight glare, and• give drivers a better perception of the speed of

approach of opposing vehicles.

4.2 Vehicle Stability4.2.1 GeneralVehicles become unstable by either sliding or overturning, orboth. Cars rarely overturn without sliding first.

When a vehicle slides sideways it is susceptible to tripping byan uneven road surface, potholes or kerbs. Under theseconditions both cars and trucks can overturn at low speeds, ie.sliding sideways at approximately 10 km/h.

Trucks, particularly those with high or moving loads, aresusceptible to overturning at all speeds. This problem ishowever greatest on small radii curves, eg. the radii typicallyencountered at intersections.

Figure 4.1 shows the lateral forces which act on a vehicle asit travels around a curve.

Source: Vicroads

Figure 4.1: Lateral Forces Acting on a Heavy Vehicle

4.2.2 SlidingThe lateral friction developed at the tyre/road interface as avehicle travels around a curve is directly related to the squareof its speed. As speed is increased the force required tomaintain a circular path eventually exceeds the force whichcan be developed by friction and superelevation. At this pointthe vehicle will start to slide in a straight line tangential to theroad alignment. This is illustrated in Figure 4.2.

Source: Vicroads

Figure 4.2: Lateral Sliding Illustration

4.2.3 OverturningOverturning is not normally a problem for cars and other lightvehicles. It can however be a significant problem for trucks,particularly those with a high centre of gravity.

When a vehicle travels around a curve an overturning momentis formed by the forces acting on it. Figure 4.3 shows theforces which act on a cornering truck.


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Source: Vicroads

Figure 4.3: Forces acting on a Cornering Truck

Overturning will occur when the vectorial sum of the twoforces extends beyond the limits of the wheels.

Other features which also contribute to reduce the stability oftrucks on curves are:

C Adverse superelevation which reduces the horizontaldistance between the centre of gravity and the hingepoint.

C The dynamic affects associated with wheel bounce oncurves.

C The rigidity of the fifth wheel linkage between theprime mover and the trailer on articulated vehicles.

C The changes in geometry which occur on low radiuscurves.

The truck side friction factors shown in Figure 2.7 shouldprovide safe operating conditions for the majority of trucks.However, trucks can roll over at friction factor levels belowthose listed because at low speeds overturning can also beinitiated by:

C Tripping: A vehicle sliding sideways can overturn atspeeds below 10 km/h when tripped by a kerb orpothole. Road surfaces must be kept in good conditionwhere critical turning movements occur to help avoidthis problem.

C Loading: Small lateral offsets of the centre of gravityof a truck’s load can significantly reduce its lateralstability. Uneven longitudinal loading can also reducethe vehicle’s stability.

C Load Shift: A moving load such as liquid in tankers oranimals on the top deck of stock transporter.

C Dynamic Forces: These are associated with tyre andsuspension bounce and are related to the speed of thevehicle and the condition of the pavement.

C Aquaplaning: This can lead to loss of control androllover.

C Braking: As the brakes are applied the frictionavailable in the radial direction decreases. If the wheelslock, lateral stability and steering is lost.

C Rearward Amplification: This is a "whiplash" effectand is defined as the ratio of the maximum lateralacceleration at the rear axle over the lateral accelerationon the prime mover.

C Speed: Critical lateral accelerations (or friction forces)are speed dependent.

4.3 Sight Distance Requirements4.3.1 Stopping Sight DistanceWhere a lateral obstruction off the pavement such as a bridgepier, building, cut slope, or natural growth restricts driversight distance, the minimum radius for an adjacent horizontalcurve is determined by the design speed stopping sightdistance. Table 2.12 shows stopping sight distances on alevel grade.

Figure 4.4 shows the relationship between sight distance,horizontal curve radius and lateral clearance to the obstructionThis relationship is valid when the sight distance is notgreater than the length of the curve and assumes that thedriver's eye and the sighted object are above the centre of theinside lane, ie. there is no or very little vertical curvature.

When the sight distance is greater than the horizontal curvelength a graphical solution is appropriate.NOTE:

All technical reductions in design speed caused bypartial or momentary horizontal sight distancerestrictions must be approved - refer to Section 2.7.1:Design Speed - General for details of the approvalprocess.

4.3.2 Truck Stopping DistancesRoad curvature adversely affects the stopping distances of allvehicles, trucks more so than other vehicles. Until reliableresearch figures are available, the following guidelines aresuggested for use on curves of less than 400 m radius:

• If the design vehicle is a rigid truck, stopping distancesshould be increased by 10%.

• If the design vehicle is an articulated truck stoppingsight distances should be increased by 20%.

This allows for the tendency of these vehicles to jack-knifeunder heavy braking while travelling on a horizontal curve.

4.3.2 Benching for Visibility on HorizontalCurves

Benching is the widening of a cutting on the inside of a curveto obtain the specified sight distance and usually takes theform of a flat table or bench over which a driver can see anapproaching vehicle or an object on the road.

In plan view, the benching is fixed by the envelope formed bythe lines of sight. The driver and the object are assumed to bein the centre of the inner lane and the sight distance ismeasured around the centre line of the lane, ie. the path thevehicle would follow while braking. Benching adequate forinner lane traffic more than meets requirements for the outerlane.

The horizontal and vertical limits of sight benching in cuttingson horizontal curves, or a combination of horizontal andvertical curves, should be determined graphically.


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Source: Austroads Rural Road Design Guide

Figure 4.4: Horizontal Stopping Sight Distance on a Horizontal Curve


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4.4 Curve Widening4.4.1 GeneralTraffic lanes may need to be widened on horizontal curves tomaintain lateral clearances between vehicles equal to thoseavailable on straight sections of road. This is commonlyknown as 'curve widening' and it is required for one or moreof the following reasons:

• Vehicles travelling on a curve occupy a greater widthof pavement than they do on a straight - at low speedsthe rear wheels track inside the front wheels and athigh speeds the rear wheels track outside the frontwheels.

• Vehicles tend to deviate more from the centreline ofthe traffic lane on a curve than on a straight.

• To maintain clearances between vehicles to those onstraight sections of road.

Other factors, such as overhang of the front of the vehicle,wheelbase and track width, also contribute to the need forcurve widening.

4.4.2 Traffic Lane Widening RequirementsApproximate values for the total amount of curve wideningrequired by a single unit design truck on a circular arc sectionof a two-lane two-way road are shown in Table 4.1.

The amount of widening required, on a per lane basis, can becalculated using the method detailed in Figure 4.5.

CurveRadius

(m)

Total amount of traffic lane widening required (inmetres) on horizontal curves where the normal width of

two traffic lanes is:

6.0 m 6.5 m 7.0 m 7.5 m

30 - 50 2.0 1.5 1.5 1.0

50 - 100 1.5 1.0 1.0 0.5

100 - 250 1.0 1.0 0.5 –

250 - 750 1.0 0.5 – –

Over 750 0.5 – – –

Source: Austroads

Table 4.1: Recommended Values for the Widening ofTwo-lane Pavements on Horizontal Curves

Source: R.T.A.

Figure 4.5: Lane Widening on Horizontal Curves

4.2.3 ApplicationCurve widening should not be applied until 200 mm, or more,per lane is necessary.

Traffic lane widths for curves of 30 m, or less, radii should bedetermined by the tracking requirements of the appropriatedesign vehicle(s), using turning path templates orcomputerised vehicle path plotting programs.

On transitioned curves traffic lane widening is normallyapplied in a uniform manner along the plan transition. Halfthe total widening required should be applied on each side ofthe curve centreline.

On circular curves traffic lane widening should be applied ina uniform manner over the length of road used to developsuperelevation. The total widening required should benormally be applied on the inside of the curve, to give theeffect of a plan transition.


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L �

V 2

36

4.5 Horizontal Curves 4.5.1 GeneralThe design standards for horizontal curves on state highwaysare derived from the basic design criteria described in Section2.8: Horizontal Alignment and are shown in Tables 4.2 and4.3. They show the minimum radius horizontal curve that canbe used for a given design speed when the maximum valuesof superelevation and side friction allowable at that designspeed are applied. The unit chord, which relatesuperelevation development at the appropriate pavementrotation rate to curve radii, is also shown.

Although any radii greater than the minimum shown for thedesign speed selected can be used, the range of curve radiisuitable for a given design speed is normally a function ofspeed environment. Good design practice avoids the use ofminimum radii and pavement rotation rates in excess of 2.5%per second, except in extreme cases. Superelevation and sidefriction requirements are also reduced at radii greater than theminimum, by Transit’s convention, to less than theirmaximum values. This practice results in road designs whichhave some additional margin of safety built in to them.

When minimum radii are avoided, ie. the values towards theleft hand end of the speed environment graph segments ofFigure 2.5 (also 4.14), it will be found that the design speedsof all curves are within 10 km/h, or thereabouts, of adjoiningcurves.

The curve notation used in this Manual, and formulae whichcan be used for manual calculation of critical curve detailswhile developing horizontal alignment schemes are shown inFigures 4.6, 4.7 and 4.8.

4.5.2 Maximum Tangent Deflection Anglebefore a Horizontal Curve must be used

A horizontal curve must be used when the deflection anglebetween intersecting straight tangent sections of road exceedthose shown in Table 4.2.

Road Type Two-lane road Multi-lane road

Dual Carriageway: Motorway

Expressway0

0° 15´0

0° 15´

2 Lane Arterial 1° 00´ 0° 30´

2 Lane Collector 1° 00´ 0° 30´

Minor road 1° 30´ N/A

Table 4.2: Maximum Tangent Deflection Anglebefore a Horizontal Curve must be used

NOTE: A succession of shorttangents and small deflectionangles cannot be used toavoid the need for a horizontalcurve.

4.5.3 Minimum Length of a Horizontal CurveThe minimum length of a horizontal curve is determinedmainly by aesthetic considerations and is therefore verysubjective. The radii to needed to give a curve of sufficientlength to avoid an unsightly kink in the horizontal alignmentin flat terrain is given in Table 3.1 while Table 4.3 indicatesthe minimum length of curve requires for some typical designsituations.

Road TypeMinimum

Length of Curve(m)

Dual Carriageways:Motorway Expressway

250200

Arterials 150

Others No limitations

Table 4.3: Minimum Length of Horizontal Curveneeded to avoid an Unsightly Kink

Two convenient methods for determining the length ofhorizontal curves, which satisfy most aesthetic requirements,are:

(i) The circular arc portion of a transition curveshould be approximately 1.5 to 3 three times thetransition spiral length.

(ii) The length of a plain circular curve should beabout the distance travelled by a vehicle duringone second for each 10 km/h of curve designspeed.

The latter can be calculated by the following formula:

Where:L = length of curve (m)V = design speed (km/h).

4.5.4 Circular CurvesPlain circular arc horizontal curves may be used in horizontalalignments if the plan transition shift distance P, as calculatedby the following formula, is less than 250 mm.

Where:

P = plan transition shift (mm)SL = plan transition length (m)

= (UC 2 x 35.81)R

R = circular arc radius (m)UC = unit chord (m), from Tables 2.9 and

2.10.

P �

SL 2

(0.024 x R)


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Figure 4.7: Plan Transition Curve Details

PI = tangent intersection pointTC = tangent point on back tangentCT = tangent point on ahead tangentCC = curve centre pointI = tangent deflection angleR = circular arc radiusTL = tangent length

= R x Tan I2

L = circular arc length= R x I (radians)

ET = external distance

= R Sec I2

� 1

MO = mid ordinate distance

= R 1 � Cos I2

LC = long chord length

= 2R � Sin I2

TS = tangent point on back tangentXC = X coordinate, refer unit chord spiral table

= LC x Cos k

LT = long tangent length= XC - YC Cot θ

YC = Y coordinate, refer unit chord spiral table= LC x Sin k

ST = short tangent length= YC cosec θ

SC = common tangent point of central circular arcand back spiral

k = long chord deflection angleθ = spiral angleSL = plan transition lengthLC = long chordR = central circular arc radiusP = shift distance

= (approx.)SL 2

24R

K = shift point distance

= (approx.)SL2

Figure 4.6: Circular Curve Details


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PI = tangent intersection pointI = tangent deflection angleTS (ST) = tangent point back (ahead)TTA (TTB) = tangent length ahead (back)SLA, (SLB) = plan transition length ahead

(back)θA (θB) = plan transition ahead (back)

deflection anglePA (PB) = shift distance ahead (back)

= (approx.)SL 2

24R

KA (KB) = tangent shift point distanceahead (back)

= (approx.)SL2

SC (CS) = common tangent point ofcentral circular arc and back(ahead) spiral

CI = tangent intersection point of central circular arc

R = central circular arc radius

For curves with equal transitions:

TTA = TTB = R � P Tan I2

� K

L = R x I (radians) � SL

ET =

For curves with unequal transitions:

TTA = R � PA Tan I2

� KB � (PA � PB) x Cosec I

TTB = R � PB Tan I2

� KB � (PB � PA) x Cosec I

L = R x I (radians) �

SLB � SLA

2

NOTE: Should other curve formula be required eg. the exactformula for P, K, SL etc, consult survey and otherrecognised road design textbooks.

Figure 4.8: Transition Curve Details


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4.5.5 Transition Curves(a) General

A plan transition helps to produce a smooth, pleasingalignment when joining a straight or tangent section ofa road to a circular curve, or when joining two adjacentcurves. The resultant compound curve normallycontains a plan transition section either side of thecentral circular arc section and is commonly known asa 'transition curve'.

he plan transition shifts the extended circular curve arcaway from the extended straight tangent line and:

• provides a convenient length of road over whichsuperelevation and/or widening is applied,

• can improve the appearance of a road,particularly on a bridge where a rigid handrailfollows the exact road alignment,

• can improve the appearance of the road where acurve is visible in plan at the end of a longstraight,

• provides a length of road over which steeringadjustment can be made, particularly in reversecurve situations, and

• provides a length of road over which speedadjustments may be made between curves ofdifferent radii.

A plan transition:

(i) is not necessary when the shift distance is lessthan 250 mm because the contribution by the plantransition to the positioning of vehicles on a curveand to curve appearance is negligible.

(ii) is not necessary where the shift distance is lessthan approximately half of the extra wideningneeded to accommodate the off-trackingrequirements of heavy vehicles on the curve,provided that all the widening is applied on theinside of the curve and over the length that wouldnormally be occupied by the plan transition.

(iii) may be used for appearance purposes when it is ofsufficient length to give a shift distance of 250 mmor more.

In predominantly flat terrain and in high speedenvironments where curves are visible from longdistances the appearance of a road can sometimesbe improved by increasing the length of the plantransition to give a shift of about 500 mm.

(iv) may not be appropriate in low speedenvironments where drivers regulate their travelspeed from their judgement of the apparentcurvature of the road ahead.

Some variation of curve approach speeds canoccur in these situations. Plan transitionsintroduced merely for appearance reasons mayaffect a driver's perception of the road’s curvatureand are probably best avoided.

(v) should not be used on small radii curves in lowspeed environments where pavement widening toaccommodate the tracking widths required byheavy vehicles is necessary.

In these cases all pavement widening should beapplied on the inside of the curve and introducedover the length that would normally be occupiedby the plan transition. A centreline marked on thecentre of the widened pavement effectively formsa transition curve while a circular arc form isretained on the outside of the curve, to help driversmake a better judgement of the apparent curvatureof the road ahead.

(b) Transition Form

A wide range of curve forms can be successfully usedfor a plan transition but the most common are theClothoid, Lemniscate and Cubic spirals, because theyprovide a uniform rate of change of curve radius.

(c) Transition : Circular Arc : Transition Ratio

When all, or most, of a transitioned horizontal curve isvisible in plan to an approaching driver its transitionlength : circular arc length : transition length ratioshould be in the range 1 : 1.5 : 1 to 1 : 3 : 1, anddesirably 1 : 2 : 1. Greater ratios are acceptable wherethe whole curve is not visible in plan to approachingdrivers.

There is little difference between the various spirals andas an approximation the selection of transition curveproportions can be based on the fact that in theLemniscate the angle equivalent to k is equal to whenθ

3θ is small and in the range of normal transition curvedesign. For larger tangent deflection angles this is notso and k because is less than .θ

3

It can be shown that the spiral angle whenθ �

I2 (x�1)

the transition : circular arc : transition ratio is 1 : x : 1.This means that for ratios:

1 : 1.5 : 1 ork �

I15

θ �

I5

1 : 2 : 1 ork �

I18

θ �

I6

1 : 2½ : 1 ork �

I21

θ �

I7

1 : 3 : 1 ork �

I24

θ �

I8

(d) Transition Length

The plan transition length is normally the distanceneeded to develop the full superelevation required onthe circular arc section of the curve at a reasonablepavement rotation rate, from the point where thepavement crossfall is level, ie. 0% superelevation.


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Superelevation is normally applied at a rate of pavementrotation not exceeding 2.5% per second of travel time atdesign speed, for vehicle occupant comfort reasons. In moredifficult situations, ie. in low speed environments of 70 km/hand less, this may not always be achievable and pavementrotation rates of up to 3.5% per second of travel time may beused without undue vehicle occupant discomfort.

On sections of two-way roads with wider than normalpavements, eg. passing lanes, the relative grade of thetwo pavement edges must be kept within reasonablelimits, for appearance and comfort reasons. Thisbecomes the control for transition length and theappropriate design standards should be used in thesesituations.

The standard superelevation and side frictionrequirements for horizontal curves are always metwhen the unit chord design method is used. The unitchord (UC) can be calculated from the followingformula:

Where:

R = circular arc radius (m)L = plan transition length (m)

NOTE: The calculated unit chord must equal, orexceed, the minimum value given in Table2.nn for the relevant design speed androad type.

4.5.6 Unit Chord SpiralTable 4.4 contains details of the unit chord spiral. Theseinclude unit dimensions for:

• Spiral length SL• Phi C k (degrees, minutes,

seconds)• Long chord LC• X Curve coordinate XC (distance from TS to

SC along curvetangent)

• Y Curve coordinate YC (offset from tangent toSC

from curve tangent)• Curve radius R• Spiral angle θ (degrees, minutes,

seconds)• Shift P (offset from tangent to

circular arc)• Shift distance K (distant from TS to

circular arc)

A plan transition for any circular curve can be determined bymultiplying the unit chord spiral values obtained from Table4.4 by the appropriate design speed unit chord from Table 2.9or 2.10.

The unit chord is in effect a scaling factor. A greater or lesserlength of the scaled spiral is used depending on the circulararc radius it must match.

An example of how to use the unit chord spiral table tocalculate horizontal curve details is:

Design speed: 80 km/hTangent deflection angle: 30° 10' 00''Transition : circular arc : transition ratio: 1 : 2 : 1

Spiral Angle: 30° 10�

6� 5°01�40��

From Table 2.9:Min. unit chord for 80 km/h = 18.6 m

From Table 4.4:Spiral angle = 5° 00' 00" (nearest match)Radius = 14.3239Shift = 0.0182Shift distance = 1.2497Spiral length = 2.50

Therefore:Curve radius R = 14.3239 x 18.6 = 266.4 mTransition length SL = 2.50 x 18.6 = 46.5 mShift P = 0.0182 x 18.6 = 0.3385 mShift distance K = 1.2497 x 18.6 = 23.2 m

The tangent length, external distance and circular curve arclength are calculated from the formula shown in Figure 4.nn,ie:TT � 14.3239 � 0.0182 x Tan 30°10�

2� 1.2497 x 18.6

� 95.1 m

ET � 14.3239 � 0.0182 x Sec 30°10�

2� 14.3239 x 18.6

� 9.9 m

L � 14.3239 x 30°10 (radians) � 2.5 x 18.6

� 93.8 m

Alternatively, where an approximate radius, say 300 m, isknown for the same deflection angle, then:

Unit chord UC = 30014.3239

= , which equates to a20.9 mdesign speed ofapproximately 83 km/h.

UC �

R x L35.81


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Table 4.4: Dimensions of Unit Chord Spiral and Central Circular Arc


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Table 4.4 (cont.): Dimensions of Unit Chord Spiral and Central Circular Arc


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Figure 4.9: Axis of Rotation for SuperelevationDevelopment on Two-lane Two-way Roads

Figure 4.10: Axis of Rotation for SuperelevationDevelopment on Ramps and Motorway /Expressway Connectors

Figure 4.11: Axis of Rotation for SuperelevationDevelopment on Dual Carriageway Roadswhere the median is wider than 4.0 m

4.6 Superelevation4.6.1 Standards for SuperelevationThe maximum superelevation rates for use on state highwaysare shown in Tables 2.9 and 2.10.The use of normal crossfall , ie. -3% superelevation, may onlybe considered when curve radii exceed the values given inTable 2.7.Figures 4.15, 4.16 and 4.17 must be used to determine thedesign superelevation required all state highway horizontalcurves.

4.6.2 Axis of Rotation(a) General

Crossfall is used on straight sections of road to shedwater. This crossfall, which is commonly known as'normal crossfall', is usually away from the centrelineon two-lane two-way roads and away from the medianon dual carriageway roads. Normal crossfall generallychanges to superelevation when the horizontal alignmentbecomes a curve, except where the curve radius is verylarge and adverse or normal crossfall can be used,ie. -3% superelevation.The point about which the pavement is rotated todevelop superelevation, ie. the 'axis of rotation',depends upon the type of road, the road cross section,terrain and the location of the road.

(b) Two-lane Two-Way RoadsOn two-lane, two-way roads the axis of rotation forsuperelevation development is usually the roadcentreline, as shown in Figure 4.9 below.

Each half of the pavement, including shoulders, isrotated about the road centreline and the resultantprofiles of the edges of the carriageways are in oppositedirections to each other. This can lead to the creation offlat sections of pavement and drainage problemsparticularly in flat terrain where curves are preceded bylong relatively level tangents. In these situations theaxis of rotation for superelevation development shouldbe shifted to the edge of the traffic lane on the inside ofthe curve, to improve drainage as well as driverperception of the curve.

(c) Interchange Ramps and Motorway / Expressway toMotorway / Expressway ConnectionsThe axis of rotation may be about either edge of thetravelled way or, the centreline, if multilane. However,the lower side of the travelled way is normally the bestlocation. See Figure 4.10.

Appearance and drainage considerations should alwaysbe taken into account in the selection of the axis ofrotation for superelevation development on ramps andmotorway / expressway connectors.

(d) Dual Carriageway Roads(i) General

Aesthetics, grade distortion, superelevationtransitions, drainage and driver perception must allbe considered when selecting the location of theaxis of rotation for dual carriageway roads. In flatcountry it may be desirable to adopt the left (orouter) edge of the carriageway as the axis ofrotation or to use independently gradedcarriageways

(ii) Motorways and Expressways

Where the median width is 4.0 m or less, the axisof rotation should be the centreline of the road.

For staged construction projects where the initialmedian width is greater than 4.0 m, and theultimate median width is to be 4.0 m or less, theaxis of rotation should be at the road centreline,except where the resulting initial median slopewould be steeper than 1:10. In the latter case, theaxis of rotation should be at the ultimate medianedges of the travelled way.

Where the ultimate median width is greater than3.8 m, the axis of rotation should be at the ultimatemedian edges of the travelled way.

To avoid a 'sawtooth' profile on bridges withdecked medians, the axis of rotation, if not on thecentreline, should be shifted to it.


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(iii) Other State HighwaysThe location of the axis of rotation should beconsidered on an individual project basis and themost appropriate solution for the specific designconditions selected.

For roads with painted medians the axis of rotationwill normally be the road centreline. In all othersituations the location of the axis of rotationshould be determined by using the selectioncriteria given in Section 4.6.2 (d) (i) above.

4.6.3 Application of Superelevation(a) General

The transition from normal crossfall to the maximumsuperelevation for a horizontal curve on a two-lane two-way road generally consists of a crown runoff and asuperelevation runoff, as shown on Figure 4.12 (a).

This type of superelevation transition is also applicableto wide one-way pavements on dual carriageway roadswhere a longitudinal crown is needed for drainagereasons.

The more normal situation on dual carriageway roads isa one-way pavement which has a constant crossfall fromthe median towards the shoulder. This pavement isrotated at a constant rate to the maximum superelevationrequired by the horizontal curve, as indicated on Figures4.12 (a), (b) and (c).

(b) Superelevation Runoff

'Superelevation runoff' is the length of road required torotate the road pavement from level, or 0% crossfall, tothe maximum superelevation rate required.

On circular curves the superelevation runoff shouldoccur 60 % on the tangent and 40% within the curveitself. This results in 60% of the full superelevation rateat the beginning of the curve.

On transition curves the superelevation runoff shouldnormally occur over the plan transition length.NOTE:

The use of the unit chord in transition curve designwill ensure the correct relationship between transitioncurve spiral length, circular arc radius andsuperelevation runoff.

(c) Crown (Tangent) Runoff

'Crown runoff' is the length of road required to rotateone half of a crowned road pavement from normalcamber to level, or 0% crossfall, at the same pavementrotation rate used to apply superelevation on theassociated horizontal curve, see Figure 4.12 (a), (b) and(c).

'Tangent runoff' is the length of road required to rotatea constant crossfall road pavement from normal camberto level, or 0% crossfall, at the same pavement rotationrate used to apply superelevation on the associatedhorizontal curve, see Figure 4.13 (a), (b) and (c).

(d) Superelevation Development Length

(i) The criteria used to determine the length of roadover which superelevation is developed are:• design speed, and• the rate of pavement rotation, ie. a vehicle

occupant comfort control, or• the relative grade between grade between

the inner and outer edges of the travelledway, ie. an appearance control.

The superelevation development length must belong enough to ensure a satisfactory riding qualityand also give a visually pleasing road alignment.

(ii) On two-lane two way roads superelevationdevelopment length is determined by rate ofpavement rotation. This rate should not normallyexceed 2.5% per second of travel time at thedesign speed.

(iii) However, on two-lane two-way roads inconstrained design conditions, eg. low speedenvironments, mountainous terrain, etc, and wheredesign speeds are # 70 km/h, pavement rotationrates up to 3.5% per second at the design speedmay be used. In some specially approved casesthis rate may also sometimes be used at an 80 km/hdesign speed.

(iv) Divided roads ae usually designed to higherstandards and have wider pavements than two-lanetwo-way roads. Superelevation on these roads isdeveloped over the longer of the lengthsdetermined by the following criteria:• an enhanced vehicle occupant comfort

criteria which limits the maximumpavement rotation rate to 2% per second atthe design speed,

• an appearance criteria which limits therelative grade between edges of thetravelled way to a value consideredappropriate for the design speed.

(v) The minimum superelevation development lengthto satisfy the rate of rotation criteria for any road isdetermined by the following expression:

Le �

( e1 � e2 ) x Vd

kWhere:Le = superelevation development

length (m)e1, e2 = crossfall or superelevation at the

ends of the superelevationdevelopment length (%)

k = 7.2 for a rotation rate of 2.0%per second,9.0 for a rotation rate of 2.5%per second, and12.6 for a rotation rate of 3.5%per second

Vd = design speed (km/h)


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(vi) The minimum superelevation development lengthto satisfy the road relative grade criteria for adivided road is determined by the followingexpression:

Where:

Le = superelevation developmentlength (m)

W = width of the travelled waye1, e2 = crossfall or superelevation at the

ends of the superelevationdevelopment length (%)

Gd = relative grade from Table 4.5.

Design Speed(km/h)

Relative Grade between the inner and outeredges of the traffic lane(s)

(%)

1 lane 1 2 lanes 2 More than 2 lanes 3

30 0.75 1.50 1.25

40 0.70 1.39 1.16

50 0.64 1.28 1.07

60 0.59 1.18 0.98

70 0.54 1.08 0.90

80 0.50 1.00 0.83

90 0.46 0.92 0.77

100 0.43 0.86 0.72

110 0.40 0.80 0.67

120 0.38 0.75 0.63

130 0.36 0.71 0.59

140 0.34 0.67 0.56

1 One lane one-way roads where the axis of rotation is oneedge of the traffic lane.

2 4 lane divided roads where the axis of rotation is one edgeof the one-way road travelled way, and passing lanesections of two- lane two-way roads where the axis ofrotation is the road centreline.

3 6 or more lane divided roads where the axis of rotation isone edge of the one-way road travelled way.

Table 4.5: Relative Grade betweenEdges of the Travelled Way

(e) Shoulder Transitions

Shoulders are normally part of the carriageway plane androtate with it. Shoulder profiles should be made smoothand compatible with the adjacent pavement.

Shoulders on superelevated curves must have the samesuperelevation as the traffic lanes.

The pavement outside the shoulder should sloped awayfrom the road and sealed, to minimise the likelihood ofdrivers losing control on the curve and infiltration into thepavement layers.

Unsealed shoulders on straight sections of two-lane two-way roads must have a crossfall awayfrom the pavement, to minimise infiltration into theunsealed shoulder.

(f) Superelevation Application Problem Areas(i) General

Profiles for both edges of the travelled way andboth shoulders should always be plotted and anyirregularities resulting from interactions betweenthe superelevation transition and vertical alignmentof the roadway should be eliminated byintroducing smooth curves. Flat areas ofpavement, which are undesirable for stormwaterdrainage reasons, will also be revealed by thisprocess and they can be remedied at the designstage.In restrictive situations, such as two lane roads inmountainous terrain, interchange ramps, collectorroads, frontage roads, etc., where curve radii aresmall, curve length and the tangents betweencurves are short, standard superelevation ratesand/or development lengths may not be attainable.In such situations the highest permissiblesuperelevation rates and the shortest developmentlengths should be used.In situations where it is considered desirable toexceed the maximum permissible superelevationand pavement rotation rates specific approval to doso must be sought from the Strategy and StandardsManager on a case by case basis.

(ii) BridgesA superelevation transition on a bridge has almostthe same effect as a vertical curve in the samesituation and almost always results in anunsightly appearance of the bridge and the bridgerailing. If possible, horizontal curves should beginand end a sufficient distance from the bridge sothat no part of the superelevation transition extendsonto the bridge. Alignment and safetyconsiderations are, however, of paramountimportance and they must not be sacrificed to meetthe above criteria. Refer to Section 3.8 for moredetails on the effect of combined horizontal andvertical curvature.

(iii) Adverse CrossfallAdverse crossfall should normally be avoidedexcept on curves with radii sufficiently large to beregarded as straight sections of road. Such curvesneed not be superelevated but superelevation at thenormal crossfall rate used on straights should beapplied.In rural situations all curves less than 3000 mradius should be superelevated. However, toimprove pavement drainage on very flatlongitudinal grades, or in the design of temporaryroadways and connections, consideration may begiven to the use of up to 3% adverse crossfall.


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Figure 4.12: Superelevation Development on Two-lane Two-way Roads, Interchange Ramps andMotorway / Expressway to Motorway / Expressway Connections


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Figure 4.13: Superelevation Development on Dual Carriageway Roads


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In urban situations where drivers are moreadaptable to changes in radius, superelevation andtransverse friction, the use of adverse crossfall onsmall radii curves is tolerable.

The minimum radii that may be used with anadverse crossfall of 3%, for various design speeds,are listed in Table 2.7.Notes:1. Radii larger than those in Table 2.7 should

be used wherever possible.2. The radii recommended in Table 2.7 are

based on curves which have been found tobe generally appropriate in level terrain.

3. Adverse crossfall must not be used incombination with any other factor whichcould increase the force on the tyre orreduce the friction forces which can bedeveloped at the tyre road interface, eg.adverse crossfall should not be used in thefollowing circumstances:• on the approaches to intersections

because of the friction forcesrequired for braking,

• in areas subject to aquaplaning oricing, and

• on downhill grades.3. For aesthetic reasons, short arc lengths

should not be used with adverse crossfall.

(iv) Intersections

Where a side road joins a main road on the outsideof a horizontal curve, a compromise is oftennecessary between adequate superelevation on themain road and safe conditions for vehicles turningagainst the adverse crossfall. The situation is worseif the horizontal curve is located on a steep gradeand the intersection must be modified to ensure safeturning conditions.

Generally, if the side road is important or the curvehas a steep longitudinal grade, ie. over 5%, thesuperelevation must not exceed 5% and shouldpreferably be limited to 3%.

The problem does not exist where the junction is onthe inside of a horizontal curve as thesuperelevation then favours the turning movements.

(v) Steep Downhill Grades

The use of maximum superelevation rates on steepgrades may unacceptably increase the longitudinalgrade on the outer lanes. Usually, superelevation isthe only geometric element which can be varied inthese situations and it will sometimes be necessaryto either reduce the superelevation or extend thelength of the superelevation development.

Superelevation should however be increased ondownhill grades to counter the effect of thecombination of grade and curvature on the stabilityof motor vehicles - particularly articulated trucks.

In comparison to the downward forces that act ona vehicle on a level grade, the downward force onthe front axle is increased and decreased on the rearaxle when the vehicle travels on a downhill grade.

This load shift effectively reduces the lateral forcewhich can be supplied by the rear axle so that whenthe vehicle is travelling close to the maximum safespeed for a superelevated curve on a downhillgrade, and its brakes are applied, the back wheelsare likely to lose traction and this can lead to thevehicle spinning.

To minimise this problem, higher superelevationrates should to be used on roads with downhillgrades of 3% or more. The increase insuperelevation is obtained using the followingequation:

Superelevation increase (%) �

(g � e)6

Where:g = downhill grade (%)e = curve superelevation (%)

NOTE: Fractions of any significance should berounded up.

(vi) Compound Curves

A compound curve is a curve formed by joiningtwo or more contiguous unidirectional curves ofdifferent radius. Compound curves are generallyundesirable because:

• drivers have difficulty estimating thecurvature of the road and there is a dangerthat they may not notice a reduction inradius,

• vehicle tracking problems can, and do, occurwhere the change of radius is not obvious todrivers,

• braking on curves can be hazardous forarticulated trucks and this problem is madeworse on downhill grades, and

• the visual appearance of a ‘broken-back’alignment can be created when anintermediate curve has a radius significantlylarger than the adjacent curves.

When a compound curve must be used the smallercurve radius should not be less than b of the largercurve radius. This relationship between curve radiialso applies to similar curves.


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(vii) Reverse Curves1. General

Reverse curves can introduce problems inachieving a suitable superelevation developmentpavement rotation rate, unless the curves havespiral transitions and the tangent points areadequately separated. Wherever possible a straightor spiral should be provided between reverse curvesto allow for:

• The time taken to turn the steering wheel. Times of two to four seconds have beenrecorded for this driver action and in thisperiod a vehicle travels along a spiral pathfor a distance of between 0.8 V and 1.1 Vmetres, where V is in km/h.This enables the length of straight or spiralbetween the reverse curves to be reduced to0.6 V metres. In steep to mountainous areaswhere it is not practical to provide thisseparation between the curves, sufficientwidth should be provided to enable driversto negotiate a spiral path within the width ofthe traffic lane or sealed pavement.

• The development of superelevation.

In mountainous country it is often necessary tohave coincident tangent points but it is generallypreferable to reduce the curve radii so that theminimum tangent length can be provided betweencurves.

2. Important Checks

Two important points must be checked in allreverse curves situations:

(i) Surface drainage at the curve reversal point. As a general rule, the longitudinal roadgrade at this point should be between 1%and 3%.

(ii) When fully circular curves are used theirradii must be at least equal to, and desirablywell above, the radius required for zerocrossfall, ie. e = 0, at the horizontalalignment’s design speed.

3. Truck Instability

Reverse curves can, and often do, create instabilityproblems for articulated trucks and, for this reason,they should only be used as a last resort. Theinstability is created by:

• the high yaw forces which are developed atthe curve tangent points which can causesteering problems for drivers of trucks,particularly articulated trucks, and

• the loss of superelevation on the approach tothe curve tangent points. Drivers tend tonegotiate curves at speeds close to the limitsof stability and any reduction in roadstandard, such as a reduction in crossfall,can lead to loss of control.

4. Circular Curves separated by a ShortTangent

The length of straight tangent between reversecircular curves should be sufficient to allow fullsuperelevation to be developed at the start of thecurve. In practice this is often not possible and ashorter distance has to be used. As a general rule,the minimum tangent length should not be less than0.6V metres when V is in km/h.5. Transition Curves

Where minimum radius curves are used, fullsuperelevation should be achieved at the start of thecircular arc sections of those curves.

Where large radii circular curves are used, somesuperelevation may be developed within the curves,provided b of the full superelevation rate isachieved at curve tangent points.

6. Transition Curves separated by a ShortTangent

A straight between adjacent curve spirals usuallyallows each traffic lane to be rotated independentlyavoiding the development of a flat pavementsurface across both traffic lanes. This type ofdesign is appropriate for use in flat terrain wherepavement drainage is often a problem.

(viii) Similar Curves

Two horizontal curves in the same direction,sometimes joined by a short straight, can form anunsightly alignment which is commonly known asa 'broken back' alignment.

Similar curves should not normally be usedbecause:

• it is very difficult to produce a visuallypleasing vertical alignment for the edges ofthe road pavement,

• is often impossible to provide the correctamount of superelevation throughout bothcurves, and

• truck drivers can experience problemsrelated to sight distance and brakingconditions on the approaches to similarcurve sections of road.

NOTE: The appearance and safety problems canusually be avoided by replacing similarcurves with a single curve.

Refer to Section 3.6.5 for more details on problemsrelated to similar curve situations.


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(ix) Pavement Surface Drainage1. General

For effective road drainage the minimum gradesare:• 0.3% for concrete kerb and channel, and• 1% for open ditches.

Where these grades can be achieved there areusually no problems in draining the edge of theroad pavement but there may be pavement surfacedrainage problems. Checks must therefore be madeto ensure there are no potential aquaplaning andspray problems due to surface water build up.

On flat grades pavement surface drainage problemsare likely to occur on chip sealed surfaces wherethe crossfall is less than 2.5%, and on smoothasphalt surfaces where it is less than 2%.

Pavement surface drainage problems may alsooccur:

• on steep grades because of the increase inlength of the drainage flow path through thecombination of crossfall and grade,

• at entry and exit lanes where merging andbraking manoeuvres are common, and

• on the approaches to intersections and lowradius curves where a lot of heavy brakingoccurs.

In view of the potential drainage problems insuperelevation development areas, the length ofsurface drainage flows and water depths in theseareas must be checked on all new projects as wellas all rehabilitation projects involving geometricimprovements. The solution to surface drainageproblems will usually involve a change to thehorizontal alignment, to the grading or to both.

For more details on pavement surface drainagerefer to the Transit New Zealand publicationHighway Surface Drainage.

2. Surface Drainage Problem Treatments

• Realign the road. The superelevationdevelopment can sometimes be moved to alocation which provides improved surfacedrainage.

• Re-grade the road.• Shorten the superelevation development.• Move the design line (grading) away from

the traffic lanes.• Provide one or more longitudinal (parallel)

crowns. Skew crowns are not recommendedbecause they can create stability problemsfor trucks and, more importantly, they arevery difficult to construct with sufficientaccuracy to correct the surface drainageproblem.

• Provide a grated drain across the roadpavement.

3. Flat Grades

The most dangerous areas on flat grades are thoselocated within the traffic lanes of wide pavementswhere relatively deep puddles can develop. Flatspots located at the outer edge of the shoulder areless of a problem because water can usually drainaway over the pavement edge.

Flat spot locations can be predicted using contourdiagrams and they can also be locateddiagrammatically using relative grade diagrams.

4. Steep Grades

On steep grades the main drainage problem is thelength of the surface water flow paths at the pointsof zero crossfall in superelevation developmentareas.

Whenever a superelevation development is locatedon a grade steeper than 5% the length of the surfacedrainage flow path(s) needs to be checked.


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4.7 Two-way Two-lane Road Design4.7.1 General(a) Speed Environment

The speed environment of a section of an existing two-way two-lane road with reasonably consistent geometricstandard which runs through terrain having relativelyconsistent characteristics can be determined by:

• speed studies, or• estimated from a consideration of its topography

and the range of existing horizontal curve radii.

A length of road which is to be realigned or upgraded hasthe speed environment of the section of road withinwhich it lies. The speed environment does not changerapidly and it takes some distance before driversrecognise a change because of a difference in terrain,geometric standard or both of these parameters.

The speed environment for a new road may be estimatedfrom Table 4.6 once the terrain type and the range ofhorizontal curve radii likely to be used have beendetermined.

(b) High Speed Environments

In high speed environments drivers will adopt relativelyuniform travel speeds. Design speeds should therefore beat least equal to the speed environment, and may evenexceed it. A constant design speed, which is in keepingwith the terrain type should be used for all aspects oftwo-way two-lane road design in these situations.

(c) Intermediate and Low Speed Environments

In intermediate and low speed environments drivers tendto vary their speeds in a direct relationship to thecharacteristics of the road and its surroundings. In theseenvironments there is a need for some iteration in theselection of horizontal curve elements because individualcurve geometry is determined by, and also helps todetermine, the speed environment.

The 85th percentile speed must be used in intermediateand low speed environments to co-ordinate the geometricfeatures of a two-way two-lane road alignment. Arecommended design procedure is described in Section4.7.2 below.

4.7.2 Two-lane Road Design ProcedureThe seven step iteration procedure for two-lane road designshown in Figure 4.14 will ensure a safe and consistent roadalignment. The procedure is summarised as follows:

Step 1. Select Nominal Speed EnvironmentDetermine the speed environment for the length of roadunder consideration. The speed environment is regardedas constant for a section of road with reasonablyconsistent geometric standards and terrain characteristicsso, the speed environment for the length of road underconsideration is therefore equal to that of the section ofroad within which it is located.

Source: Austroads Rural Road Design

Figure 4.14: Two-lane Roads - Horizontal Alignment Design Procedure


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NOTES:

1. The speed environment for a section of road canbe determined by:• speed studies carried out on the longer

straight and/or very large radii sections ofthe existing road, or

• estimated from an assessment of theoverall geometric standard of the horizontalalignment and the terrain type by usingTable 4.6.

2. It takes some time for drivers to recognise andreact to a change in the speed environmentbecause of a change in terrain characteristics orthe overall geometric standard of the road.

The length of road needed before driversrecognise an increase in speed environment isapproximately:• 3 km in a 110 km/h speed environment,• 1 km in a 90 km/h speed environment, and• 250 m in a 70 km/h speed environment.A decrease in speed environment is usually farmore easily recognised and the length of roadneeded for this is approximately:• < 3 km in a 110 km/h speed environment,• 500 m to 1 km in a 90 km/h speed

environment, and • 250 m in a 70 km/h speed environment.

Step 2. Determine Trial Alignment(s)Prepare trial alignments, using radii within the rangegiven in Table 4.6, for the speed environment selected inStep 1 above. Good design practice will avoid the use ofminimum radii.

Trial grade lines should also be devised at this stage.

Step 3. Check Speed EnvironmentCheck the speed environment of trial alignments usingTable 4.6. This will only differ from the nominal speedenvironment initially chosen if significantly differenthorizontal curve radii have been used.

Occasional very large radii curves should be ignoredwhen assessing the speed environment. They should beconsidered as being equivalent to a short straight sectionwithin the overall speed environment.

Step 4. Determine the Design Speed(s)In high speed environments a constant design speedshould be used. This must be equal to the speedenvironment and may even exceed it.

In low and intermediate speed environments drivers willtend to vary their speeds on horizontal curves in a directrelationship to the perceived curve radii. Design speedsmay therefore vary but great care must be taken to checkthat their design parameters, ie. design speed, side frictionand superelevation, are reasonably consistent and liewithin acceptable variation limits.

Speed environment value as a function of terrain type and overallgeometric for single carriageway two-lane rural roads. a

ApproximateRange ofHorizontal

Curve Radius

(m) b

Terrain Type / Speed Environment (km/h) c

Flat Undulating Hilly Mountainous

< 75 75 70

75 - 300 90 d 85 e

150 500 100 95

> 300 - 500 115 d 110 e

> 600 - 700 120 e

a Can also be used for divided roads when the geometric standards are constrained, ie. design speeds < 120 km/h.

b These values represent the general overall horizontalgeometric standard of the section of road under consideration.

c The maximum speed regarded as acceptable by most drivers inthe particular environment and also the 85th percentile speedon the unconstrained sections of road, eg. the longer straightsand curves with radii well above those listed.

d An overall horizontal geometric standard of < R300 m (flat) or <R l00 m (undulating) will not be normal and speedenvironments below about 115 km/h and 90 km/h respectivelyshould not be used. Where low design speed curves arenecessary refer to Section 2.7.5 (v).

e Use the speed environment of the next less severe terrain typewhen a more liberal geometry is used in undulating andmountainous terrain.

Source: Austroads Rural Road Design

Table 4.6: Terrain Type / Horizontal Curve RadiusRange / Speed Environment Relationshipfor Two-lane Roads

Special care must also be taken when determining designspeeds for curves at the ends of straighter sections of roadbecause high speeds are likely in these areas.

The design speed of a curve can be considered equal tothe speed environment when the length of precedingstraight, or very large radius curve, is at least:

• 250m in a 70 km/h speed environment,• 1 km in a 90 km/h speed environment, and• 3 km in a 110 km/h speed environment.

The appropriate design speed, or 85th percentile operatingspeed, for a horizontal curve in a given speedenvironment can be determined from Figure 4.15.

The superelevation required for horizontal curves on two-lane two-way state highways can be determined fromFigures 4.16 and 4.17.


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Figure 4.14: Horizontal Curve Radius / Design Speed / Speed Environment Relationship


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Figure 4.16: Low Speed Two-lane Two-way State Highways where the Design Speed # 70 km/h

Design Speed / Horizontal Curve Radius / Superelevation Relationship


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Figure 4.17: Standard Two-lane Two-way State Highways



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Step 5. Check Alignment Consistency and SightDistance, and

Step 6. Modify Alignment if necessaryDrivers do not expect, or readily accept, having to travelat low speeds in a high speed environment and anygeometric element which allows a higher speed to beachieved, such as a long straight or very large radiuscurve, will be used to advantage. This type of featuretends to raise the overall speed environment of a sectionof road and means that the design speeds of adjacentgeometric elements must be very carefully checked. Inmany cases design speeds will have to be increased tomatch the higher speed environment.

Conversely, any geometric element that requires drivers toreduce their travel speed more than 10 to 15 km/h belowthe speed environment is likely to create a hazardoussituation because most drivers do not significantly reducetheir travel speed until they can actually see the hazard,even when the appropriate advance warning signs areerected.

Normally, design speeds should not differ by more thanabout 10 km/h, and never more than 15 km/h, betweensuccessive geometric elements. However, where theterrain or the general standard of the road dictates areduction in the speed environment the geometric elementsconnecting the two speed environments must be carefullydesigned to provide a safe transition between them.

A safe reduction in operating speeds can be achieved insome circumstances by using a sequence of horizontalcurves whose design speeds do not vary by more than 10km/h. This is may be done by using the design, orpredicted 85th percentile, speed of each horizontal curve asthe speed environment for the succeeding curve and thenderiving that curve’s design speed from Figure 4.14.

Where a local constraint is not sufficiently large to definea new speed environment for a section of road, and itcannot be bypassed, a design speed reduction in excess of10 to 15 km/h will occasionally have to be accepted andthe appropriate warning devices should be installed as anintegral part of the new roadworks.

Both horizontal and vertical sight distances must beconsistent with design speeds and side friction demandsshould not vary excessively on successive horizontalcurves.

Stopping sight distance requirements based on a reactiontime of 2.5 seconds are used for most design conditions.However, in difficult terrain and other constrainedconditions where design speeds of 70 km/h and less areused, shorter sight distances based on a 1.5 secondreaction time may be used because drivers are consideredto be in a more alerted state in these conditions.

Table 2.12 lists the stopping sight distances required ona level grade. Adjustments must be made for the actualgradients used in each design.

NOTE:Horizontal sight distance rather than side frictionand superelevation may fix the minimum radiusfor the design speed where side clearances arelimited.

Where it is not feasible to achieve a design fullycompatible with the speed environment of the section ofroad, because of topographical, environmental and/oreconomic reasons, careful thought must be given to theinstallation of traffic management measures which willreliably alert drivers to unexpected changes in operatingconditions on that section of road. These measures cannotbe considered good design practice and should only benecessary on very rare occasions.

NOTE:Two-lane two-way roads must be checked forconsistency in both directions of travel. This isparticularly important where there are speedchange situations similar to those describedabove.

Step 7. Satisfactory Alignment / Final DesignThe process described in Steps 1 to 6 above will ensurethe geometric elements of trial alignments have designspeeds equivalent to operating speeds. The design speedmay vary along the section of road under consideration butthe procedure will ensure that, between successivegeometric elements, design speed differences lie withinacceptable and safe operational limits.The trial alignments should be analysed and the best onerefined into a final design by ensuring that:

• horizontal and vertical curve designs are consistentwith the predicted 85th percentile operating speedsat all locations,

• a safe join-in to the existing road at each end of thenew alignment must be provided in all cases andthe extent of the work may need to be extended toachieve this in some situations,

• horizontal/vertical alignments are co-ordinated withthe design speed of the vertical alignment being atleast equal to its associated horizontal alignmentdesign speed, and preferably 10 or more km/hhigher,

• earthworks are minimised, and• all other controlling criteria are satisfied, eg. special

consideration given to the location of intersectionsand points of access to ensure that minimum sightdistances and critical crossfall controls are met.

NOTES:1. Horizontal alignment design is not often a

straightforward matter and the process describedabove can not always be followed. Designstandards can prove impossible to achieve andcompromises may have to be made. In thesesituations a broad understanding of basic trafficengineering theory as well as the principles andassumptions used in the development of roadingstandards is essential to enable safe and realisticdesign compromises to be made.

2. In ALL circumstances any variation of statehighway design standards must be approved inwriting by the State Highway Policy DivisionDesign and Traffic Manager before they can beimplemented.


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4.8 Dual Carriageway Road Design4.8.1 GeneralAll dual carriageway roads are considered to be high standardroads by the majority of drivers. Therefore, to achieve safe,consistent operating conditions a constant speed must be usedas the main design control for all geometric elements.

The design speed for a dual carriageway state highway mustbe at least:

• numerically equal to the proposed speed limit for thatsection of road, and

• at least numerically equal, and generally exceed, thespeed environment for the section of road within whichit is located.

The design speed should not normally vary along the entirelength of a dual carriageway road, unless there is some definitechange in terrain type which alters the speed environment fora significant section of the road.

4.8.2 Design SpeedThe minimum design speeds for dual carriageway statehighways are shown in Table 4.7.

Environment Rural Urban

Terrain Flat Rolling Mountainous All

Design Speed (km/h) Motorway

Expressway120+110

110+100

10080

70 - 8060

Table 4.7: Minimum Design Speeds forDual Carriageway State Highways

4.8.3 Horizontal AlignmentHorizontal alignments should be developed using a series ofstraights and circular curves and/or transition curves.NOTE:

The length of a transition curve spiral must be sufficientto give a shift distance of 250 to 500 mm generally, andat least 500 mm in flat terrain.

As a general rule, curves must be used whenever a dualcarriageway road changes direction. However, very smallchanges in alignment without horizontal curves are not noticedby drivers and in some cases it might not be necessary toprovide curves between adjacent straight sections. Themaximum allowable deflection angle between adjacent straightsections without the need for a horizontal curve is:

• 0° 00' for motorways,• 0° 15' for expressways.

NOTE:It is not permissible to use a succession of smalldeflection angles to avoid the need for a horizontal curveon any dual carriageway road.

Horizontal curves must also be of sufficient length to avoid theappearance of a kink. The minimum curve length to avoid anunsightly kink is:

• 250 m for motorways, and• 200 m for expressways.

4.8.4 Superelevation(a) General

Dual carriageway roads generally have wide pavements.It is therefore desirable to minimise superelevation foraesthetic reasons, and also because of the longsuperelevation transition lengths that are often necessary.

(b) Minimum Superelevation

It is desirable to superelevate all dual carriageway curvesto a rate equal to that of the crossfall used on the straightsections. This is usually 3%.

However, if the radius of a curve is greater than that givenin Table 2.7 it can be regarded as straight and normalcrossfall, or -3% reverse superelevation, may be used.

(c) Maximum Superelevation

The maximum superelevation rates for dual carriagewaystate highways, in relation to design speed, terrain typeand environment are given in Table 4.8.

Design Speed(km/h)

Maximum Superelevatione (%)

Rural Environment TerrainUrban

EnvironmentFlatRolling

andMountainous

60 - - 8

70 - - 8

80 - 8 8

90 - 8 -

100 6 8 -

110 6 - -

$ 120 6 - -

Table 4.8: Maximum Superelevation Rates for DualCarriageway State Highways

The superelevation required for horizontal curves on dualcarriageway state highways can be determined from Figure4.18.


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Figure 4.17: Dual Carriageway State Highways



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