rpdm chapter10

August 2001

Road Planning and Design Manual Chapter 10: Alignment Design

10Chapter 10

AlignmentDesign

Table of Contents

10.1 Introduction 10-1

10.2 Design Principles 10-110.2.1 Introduction 10-1

10.2.2 Driver Perception of Roadway 10-2

10.2.3 Holistic Design Approach 10-3

10.2.4 Aesthetics of Road Design 10-3

10.2.5 Designing for Sun Position 10-5

10.3 Alignment Coordination 10-510.3.1 General 10-5

10.3.2 Principles of Co-ordination 10-8

10.3.3 Specific Co-ordination Issues 10-8

10.4 Fitting the Road to the Terrain 10-910.4.1 General 10-9

10.4.2 Approaches to Design 10-15

10.5 Curvilinear Design in Flat Terrain 10-19

References 10-22

Relationship to Other Chapters 10-22

August 2001

Chapter 10: Alignment Design Road Planning and Design Manual

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Chapter 10 Amendments - August 2001

Revision Register

Issue/ Reference Description of Revision Authorised DateRev No. Section by

1 First Issue Steering MayCommittee 2000

2 10.2.4 Paragraph added at the end of section

10.3.3 Cross reference to Figure 10.7 added Steering AugFigure 10.8 - title added Committee 2001

10.4.1 Corrections to references

New Relationship to Other Chapters

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10.1 Introduction

The overall quality and appearance of a road willbe determined by the quality of the alignmentdesign (horizontal and vertical) and itsrelationship to the surrounding environment.

“An important concept in highway design is thatevery project is unique. The setting and characterof the area, the values of the community, the needsof the highway users, and the challenges andopportunities are unique factors that designersmust consider with each highway project.Whether the design to be developed is for amodest safety improvement or 10 miles of new-location rural freeway, there are no patentedsolutions. For each potential project, designers arefaced with the task of balancing the need of thehighway improvement with the need to safelyintegrate the design into the surrounding naturaland human environments.” (Flexibility inHighway Design (US DOT, 1997).

A holistic approach to the design is requiredwhere the various elements of the road design areconsidered as a whole in the context of theenvironment through which the road passes andthe expectations of the community affected by theroad. To achieve the best result, designers mustconsider the engineering requirements of the roadin question together with the characteristics of thearea in which the road is located. The designermust consider the character of the area in thecontext of its physical location and give properweight to:

• Physical characteristics of the area

• Use of the corridor (destination spots,pedestrians, cyclists, rural industry, urbanresidential etc)

• The type and scale of the road to be designed

• Vegetation in the corridor (density, type)

• Views available, scenic values

• Historic features (including both indigenousand introduced cultural heritage)

• Features to preserve (including naturalenvironment)

Designers need to employ imagination, ingenuityand flexibility to get the best result.

It is important that all of the necessaryinformation on both physical constraints andcommunity expectations is obtained early in theprocess. It is also important that the specialexpertise of landscape architects, environmentalspecialists and others is employed at thebeginning before controlling decisions are madeand changes can still be adopted.

This chapter is devoted to the design of thehorizontal and vertical alignments as acoordinated whole in harmony with the landscape,the environment and the community in general.

10.2 Design Principles

10.2.1 Introduction

Design principles have to consider the entirerange of factors impinging on the design -engineering, landscape, environmental,community expectations. This manual deals indetail with the engineering requirements of roaddesign, taking into account the other factors to beaddressed. For a complete treatment of the otherissues, designers should refer to the respectivemanuals developed for the disciplines involved:

• Road Landscape Manual

• Erosion and Sediment Control Manual

• Road Project Environmental ManagementProcesses Manual

• Road Traffic Noise Management Code ofPractice

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Chapter 10Alignment Design

In engineering terms, road alignments have toservice traffic in terms of providing a route thatmeets constraints imposed by vehicle dynamics,occupant comfort, and topography. This meansthat most road alignments are in fact complexthree-dimensional splines that do not have asimple mathematical definition. This problem hashistorically been addressed by reducing the three-dimensional alignment to two two-dimensionalalignments. In each case, the alignments are madeup of geometric elements that are convenient tocalculate and construct yet still ensure that vehicledynamics constraints are met. Even with theadvent of computers, this approach has stillproven to be the most convenient for both designand construction. To use complex three-dimensional models would introduce unnecessarycomplexity into the process but it is necessary toensure that the two alignments are properlycoordinated and complement each other.

Details of the specific engineering requirementsfor horizontal and vertical alignment are providedin Chapters 11 and 12 of this manual. The intentof this Chapter is to put those requirements incontext and emphasise the importance of thecomplementarity of the design of those elements.

10.2.2 Driver Perception ofRoadway

Drivers use the visual information of the roadwaygeometry to develop their perception of the roadconditions and thereby control the speed andposition of their vehicles on the roadway. Theyneed sufficient information to allow them to takethe necessary decisions to safely negotiate eachgeometric feature.

The amount of visual information perceived bydrivers is limited, however, by the following:

1. Drivers travelling for relatively long periods oftime at high speed become ‘velocitised’ (losingperspective of speed of travel and physicalsurroundings).

2. Inadequate attention because of driver ‘daydreaming’ rather than concentrating entirely onthe driving task. In this state, a driver can often

become more alert at a particular point in timeand not be able to recall the last few minutes ofthe trip.

3. Driver fatigue.

4. Driver use of drugs and alcohol.

5. Driver distraction.

6. Driving at night/dawn/dusk.

7. Driving in rain.

8. Driving in fog.

All of the above factors (except dot point 4) canaffect most drivers at particular times. This resultsin instances where drivers have reducedperception of the roadway. For this reason, it isvery important to provide drivers with as manyclues as possible as to what lies ahead. This willminimise any safety problems especially duringtimes of reduced driver perception. Roadsideconditions must not be ambiguous or misleadingas this provides an inadequate level of driverperception.

The provision of as many clues as possible as towhat lies ahead is especially critical for geometricelements comprising a large decrease in speed i.e.high speed minor road ending at a ruralintersection. Methods to achieve this arediscussed in Section 8.6.

Particular aspects of geometric design whichprovide good/poor driver perception are discussedbelow.

• Co-ordinating horizontal and verticalalignments as discussed in Section 10.3improves driver perception of the roadway.

• The smooth curving face of a cutting canprovide good driver perception of thecorresponding curvature in a road.

Conversely, a fill cannot provide good driverperception if the view ahead is open to the skywhere there may be no visual background offixed objects.

Even on flat terrain, a horizontal curve canprovide poor driver perception if there is no

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visual background of fixed objects.

• Vegetation or other obstructions on the insideof sharper curves can provide inadequate driverperception of the tightness of the curve.

• A driver will not expect a curve if this sole cuecomes from a line of service poles or treeswhich continue straight on, or from a deceptivegap in a line of trees on the outside of the curve.

• A crest should not obscure a potential hazardsuch as a narrow bridge, a railway at-gradecrossing, an at-grade intersection or ahorizontal curve which requires a significantreduction of speed. Even small crests canobscure important features. The visual cluespresented to the driver by the view of the roadsurface are especially important on sections ofsharp curvature.

10.2.3 Holistic Design Approach

Designers must take a holistic approach to thetotal road design and consider the effects of eachelement on the other elements of the design. Thehorizontal and vertical alignments must beconsidered together and the effect of the crosssection must also be considered at this time. Theapproach to alignment coordination is discussedin 10.3.

An important issue is the size of the roadcompared to its surroundings (its scale). The crosssection has the greatest effect on this - the widerthe road, the greater the scale. In someenvironments, a large scale may fit well with thesurrounds and be compatible with them. However,it is often the case that roads will have a scale toolarge for the environment and the design shouldattempt to mitigate this effect.

Some design techniques that will tend to reducethe perceived width and therefore the perceivedscale include:

• Median planting

• Reducing the extent of shoulder seal

• Independent grading

• Landscaping

Using a holistic approach will mean that designerswill critically examine the application of minimumrequirements in the design. Combinations of theminimum requirements of complementaryelements will rarely produce a satisfactory resultand in many cases will produce an unsafe result. Abetter result will usually be achieved by adoptingbetter than the minimum and this can often bedone at the same or lower cost.

10.2.4 Aesthetics of RoadDesign

It is important to realise that no road can beregarded as aesthetically successful if it is notintegrated with the natural and man made featuresthat surround it. Cosmetic treatments in the formof shrubs and trees cannot overcome the aestheticdeficiencies of poor location and design.

In addition to providing a pleasant visualexperience to drivers (and possibly because of it),accident rates are lower on roads with goodaesthetic characteristics. Roads designed to fit theterrain and take advantage of adjacent naturalfeatures create less driving tension and wearinessfor the driver.

Two perspectives have to be considered in theaesthetics of a road:

• the view of the road; and

• the view from the road.

From the community perspective, the impact ofthe road on their outlook, or the appearance of theroad from their residences, is the major factor.This perspective is helped by application of gooddesign principles as discussed elsewhere in thisChapter but it may be necessary to provide otherfeatures such as shielding with vegetation andearth mounds to overcome the intrusion of theroad into the community’s landscape. This isbeyond the scope of this manual.

The view from the road is within the control of thedesigner and application of the design principlesenunciated in this manual should achieve apleasing result.

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Figure 10.1 Well designed roads in harmony with their surroundings produce a pleasing appearance

It is desirable that any point of interest in thevicinity of the road (e.g. areas of high scenicvalue) be able to be viewed by users of the road.The sub-section titled “Development andMaintenance of Views”, Section 3.4.2 of thisManual, discusses this issue.

Figure 10.2 Landscape Mounding Alternatives

10.2.5 Designing for SunPosition

The impact of the sun shining directly into adriver's eyes should be considered in the locationand design of a road. It is not always possible todesign a road to avoid this problem in allcircumstances but careful consideration should begiven to it particularly when conflict situationsoccur. An intersection on the crest of a hill is onesuch case.

Judicious application of curvature, subtle changesin direction and provision of appropriatelandscaping can help to alleviate some of thesituations where sun glare is a problem. Thedifferent sun positions throughout the year makethe analysis more complex but they have to beconsidered. Figures 10.3 and 10.4 provide solarcharts for Queensland and provide a guide fordesigners in assessing the impact of sun positionon a road design.

10.3 Alignment Coordination

10.3.1 General

The importance of alignment coordination cannotbe over emphasised. The appearance, economyand safety of the road design depend on thesuccessful achievement of proper coordination.Details of the individual elements of horizontaland vertical alignment design are provided inChapters 11 and 12 - this Chapter will addresstheir combination.

Coordination of alignment and profile should notbe left to chance but should begin at the conceptstage, during which adjustments can readily bemade. The designer should study long, continuousstretches of highway in both plan and profile andvisualize the whole in three dimensions. (SeeAASHTO, 1994).

If long lengths are not considered, it is possible toproduce discontinuities in appearance eventhough the shorter lengths, taken in isolation, aresatisfactory. (See upper photograph in Figure10.11).

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10Undulating mound

Narrow mounds

Terraced mound

Retained mounds

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The centre of the diagram represents the observer’s position. The heavy curved lines represent the sun’spath for selected dates and latitude 27.5° South and are crossed by lines indicating hours. To find thesun’s position for the required conditions, select the point where the appropriate lines intersect. The sun’saltitude (in degrees above the horizontal plane) is shown by the relation of this point to the concentriccircular lines within the diagram. The direction of the sun’s rays is shown by a line drawn through this pointfrom the outer graduated circle towards the centre.

Figure 10.3 Solar Chart for South Queensland

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The centre of the diagram represents the observer’s position. The heavy curved lines represent the sun’spath for selected dates and latitude 20° South and are crossed by lines indicating hours. To find the sun’sposition for the required conditions, select the point where the appropriate lines intersect. The sun’saltitude (in degrees above the horizontal plane) is shown by the relation of this point to the concentriccircular lines within the diagram. The direction of the sun’s rays is shown by a line drawn through this pointfrom the outer graduated circle towards the centre.

Figure 10.4 Solar Chart for North Queensland

Further emphasis of the importance of theseelements of the design process is given inUS DOT, 1997 as follows:

“The interrelationship of horizontal and verticalalignment is best addressed in the route locationand preliminary design phases of the project. Atthis stage, appropriate trade-offs and balancesbetween design speed and the character of theroad - traffic volume, topography and existingdevelopment - can be made. …

Because they must be complementary, horizontaland vertical geometry must be designedconcurrently. Uncoordinated horizontal andvertical geometry can ruin the best parts andaccentuate the weak points of each element.Excellence in the combination of their designsincreases efficiency and safety, encouragesuniform speed, and improves appearance - almostalways without additional cost.”

It is therefore essential that the designer establishthese elements of the design in the concept stage,confirm them in the planning stage and properlydetail them in the design stage. They must beaddressed in the earliest stages of any new roaddevelopment proposal, and maintainedthroughout the planning and design process.

10.3.2 Principles of Co-ordination

Users see the road as a constantly changing three-dimensional continuum, and unless designers takeaccount of this fact, they may not appreciate howthe finished design will appear to road users.Thus, horizontal and vertical alignments must notbe considered in isolation.

The driver sees a foreshortened and, thus,distorted view of the road, and unfavourablecombinations of horizontal and vertical curvescan result in apparent discontinuities in thealignment, even though the horizontal and verticaldesigns each comply separately with theprovisions of Chapters 11 and 12.

The main principle of obtaining good co-ordination between horizontal and verticalalignments is given below:

Ideally vertical elements should be superimposedon horizontal ones in such a way that theintersection points practically coincide with thehorizontal slightly in advance of the vertical, andthe horizontal and vertical curves are of similarlengths.

When horizontal and vertical curves aresuperimposed, the combination of superelevationand vertical profile may cause distortion in theouter pavement edges which could confusedrivers at night ie. give poor driver perception.Computer packages are available which enable adesigner to view the roadway from the driversperspective. The use of these packages can enablea designer to identify such distortions in the outerpavement edges.

Figure 10.5 shows examples of ideal andacceptable co-ordination of horizontal andvertical curves.

10.3.3 Specific Co-ordinationIssues

Some situations in which poor coordination ofvertical and horizontal alignment occurs arediscussed in the following paragraphs.

The vertical curve overlaps one endof the horizontal curve

If a vertical crest curve overlaps either thebeginning or the end of a horizontal curve, drivershave little time to react to the horizontal curveonce it comes into view. This is a particularlyunsafe practice if there is a decrease in 85thpercentile speed at the start of the horizontalcurve. An example of this is shown in the upperdiagram of Figure 10.6.

The defect may be corrected in both cases bycompletely separating the curves. If this isuneconomic, the curves must be adjusted so thatthey are coincident at both ends, if the horizontalcurve is of short radius. If the horizontal curve isof longer radius, they need be coincident at onlyone end.

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The vertical curve overlaps both endsof the horizontal curve

If a vertical crest curve overlaps both ends of asharp horizontal curve, a hazard may be createdbecause a vehicle has to undergo a sudden changeof direction during passage of the vertical curvewhile sight distance is reduced. This creates thesame problems as discussed above.

The corrective action is to make the curvescoincident at one end so as to bring the crest on tothe horizontal curve.

Insufficient separation between thecurves

If there is insufficient separation between the endsof the horizontal and vertical curves, a falsereverse curve may appear on the outside edge-lineat the beginning of the horizontal curve, or on theinside edge-line at the end of the horizontal curve.This is a visual defect and is illustrated in themiddle diagram of Figure 10.6.

Corrective action consists of increasing theseparation between the curves.

Dissimilar length horizontal andvertical geometric elements

A short movement in one plane should not beplaced within a large movement in the other. Aparticular instance where this can lead to safetyproblems is when a small depression in thevertical alignment results in a ‘hidden dip’. Anexample of a hidden dip is shown in the upperdiagram of Figure 10.9. (See also Figure 10.7.)Large depressions such as those shown in thelower diagram of Figure 10.5 are not deemed tobe hidden dips.

Corrective action consists of making both ends ofhorizontal and vertical curves coincident, thusproducing similar length curves. An alternativetreatment is to completely separate the curves.

Figures 10.9 and 10.10 show examples of grosslydissimilar lengths of both horizontal and verticalgeometric elements where short movements inone plane are combined with large movements inthe other.

Long Flat Grades

Long straights with flat grades make it difficultfor drivers to judge the distance and speed ofapproaching vehicles leading to overtakingaccidents. An approaching vehicle more than2500m away on a straight seems to be standingstill but the same situation on a large curveprovides the driver with a changing perspectiveallowing some judgement of speed and distance.This situation is exacerbated at night.

Roller Coaster Grading

Long straight sections are prone to “rollercoaster” grading (see Figure 10.7) with the addedpotential for hidden dips. Designers should takecare that these features are not incorporated intothe design by using appropriate curvature in bothplanes and checking lines of sight for hidden dips.

Optical Summit

Figure 10.8 illustrates a phenomenon known as“Optical Summit” brought about by thecombination of vertical and horizontalalignments. The horizontal curve does not overlapthe two vertical curves and the vertical curves aretoo small for the large movements in the otherplane. The appearance has been improvedconsiderably by the adjustments to bothalignments as shown in the figure. The horizontalcurve has been lengthened to encompass both ofthe sag vertical curves and the sag curves havebeen lengthened to be more in keeping with thescale of the other movements.

This example also illustrates the benefit of usingperspective views during the design process tohighlight potential problems while they can stillbe corrected at minimal cost.

10.4 Fitting the Road to theTerrain

10.4.1 General

A visually attractive and physically unobtrusiveroad will be one that integrates well with theenvironment through which it passes.

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“A general rule for designers is to achieve aflowing line, with a smooth and naturalappearance in the land, and a sensuous, rhythmiccontinuity for the driver. This effect results fromfollowing the natural contours of the land, usinggraceful and gradual horizontal and verticaltransitions, and relating the alignment topermanent features such as rivers and mountains.”(Aesthetics in Transportation, US DOT, 1980).

The opposite of this occurs with designs using acombination of long tangents with short curveswhether in the horizontal or vertical plane.Figures 10.10 and 10.11 illustrate examples of theresults achieved.

In urban areas, the type of surroundings willdepend on the land use of the areas through whichthe alignment passes. The hard lines of anindustrial area will be complemented more by adesign which incorporates features such asretaining walls and other structures more inkeeping with that urban form. The urban form ofthe area being traversed will dictate the detail ofthe design whatever that form. A length of urbanroad may call for several changes in the designconcept as the road passes from one form toanother.

Split carriageways in urban setting

In other areas it will be rare that the terrainprecisely matches the speed that must be used forthe design of the alignment elements. Earthworkswill be needed to mould the road into the terrainbut careful attention to the principles in thisChapter will minimise the intrusion caused by this.

It was pointed out in Section 6.1 that drivers willadopt a speed related to the horizontal alignmentand the terrain. If the horizontal alignment can be

designed to match the terrain, the road will beconsistent with driver expectations, aestheticallypleasing and economical. Figure 10.12 shows twoalignments through the same hill, one generallyfollowing the contours and producing a morepleasing result than the other at a much lower costin earthworks.

The use of continuous liberal curves is preferredto long tangent, short curve design. An example ofthis is shown in Figure 10.5.

Varying the width and slope of batters

In flat open terrain, long straight road sections arecommon, but generally there is advantage inavoiding excessive lengths of straight road. Agentle curvilinear design (consisting of large radiihorizontal curves) always helps to keep theoperating conditions ‘under control’ and at thesame time, affords scope for far more sympatheticfitting of the road to the terrain.

In addition, safety is enhanced by making thedrivers more aware of their speed, by allowingthem to make better assessment of the distancesand speeds of other vehicles, by reducingheadlight or sun glare in appropriatecircumstances, and by reducing boredom andfatigue.

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Figure 10.5 Ideal and Acceptable Co-ordination of Vertical and Horizontal Alignments

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Figure 10.6 Poor Co-ordination of Vertical and Horizontal Alignments

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Hidden Dips

880 CLP

Plan

Profile

Perspective

Figure 10.7 Roller Coaster Grading resulting in hidden dips

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Figure 10.8 The “Optical Summit” Effect

For additional information on curvilinearalignment design in flat terrain, refer Section10.11.

10.4.2 Approaches to Design

Designers have to exercise their creativity andingenuity as well as their knowledge and skill inroad design to produce the most effective results.Some of the ways in which they can improve thelikelihood of more pleasing and effective designsare discussed below.

• Careful attention to detail will alert the designerto potential improvements in the design -

- Minor alignment shifts

- Rounding of batters, shoulders and tabledrains

- Flattening fill and cut batters, particularlywhere they are small (may also be able toavoid the use of guard rail)

- Adopting constant minimum batter catchpoints to transition from cut to fill

- Using independent alignment and gradingfor each direction of traffic

- Cut and cover tunnels to overcome bothurban and rural problems of separation

Blending batters with the roadside

• Use of a multidisciplinary team will assist thedesigner to reach the right conclusions. Theteam must be in place at the start of the processto achieve the best result (see 10.1 above). Forthe most complex designs, in addition to theroad designer, the team may include -

- Landscape Architect

- Urban Designer/ Architect

- Environmental Scientist

- Town Planner

- Structural Engineer

- Road Engineers (drainage, pavement,geotechnical)

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Location principles - choice of alignment

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Figure 10.9 Poor Co-ordination of Alignments

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Figure 10.10 Poor Co-ordination of Alignments

An example of visually poor alignment with unrelated horizontal and vertical curves, and broken backedhorizontal curves

A short movement in one plane should not be placed within a large movement in the other

Long straight grades combined with short vertical curves on a straight alignment produce a visually poorresult

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Figure 10.11 Fitting the Road to the Terrain

For smaller projects, the size and expertise ofthe team would have to be tailored to the needsof that project. However, most projects willneed to have input from many of thesespecialists in any case and the designer shouldensure that the appropriate questions are put tothem and suitable solutions generated.

• Preservation of / Integration with natural andman made features can enhance the fit of theroad to the landscape and capture notablepoints of interest for the driver and passengersof the vehicles using the road. And it willreduce the intrusion of the road into thelandscape. These features can often create an“aiming point” to justify a change in directionof the road thereby introducing interest intowhat may otherwise be a boring vista. Suchfeatures include -

- Unique rock formations

- Isolated hills / hills of interesting shapes

- Patches of rain forest

- Feature trees (or clumps of trees)

- Streams and lakes

- Man made features (historic building;isolated property buildings; windmills)

Figure 10.12 illustrates examples where the roadhas been designed sympathetically with thelandscape compared with the alternatives whichignored the impact on the landscape.

10.5 Curvilinear Design inFlat Terrain

The traditional approach to the design of roads onextensive flat plains has been to use long tangentswith relatively short horizontal curves betweenthem. In some cases, this has createdexceptionally long lengths of straight resulting inmonotonous driving conditions.

While the straight sections provide someorientation, the road is uninteresting as it is totallypredictable. The constant sameness of view makesdriving monotonous and fatiguing leading to thedriver losing all sense of pace, time and real speedwith the distinct possibility of the driver falling

asleep.

This can be explained by considering the effect ofspeed on perception and vision. As speedincreases -

• concentration increases;

• the point of concentration recedes;

• peripheral vision diminishes;

• foreground detail begins to fade; and

• space perception becomes impaired.

Thus the higher the speed, the further ahead thedriver focuses and the more concentrated theangle of vision becomes. This restriction of vision(sometimes called “tunnel view”) is the processused to induce hypnosis and sleep. Unless thepoint of concentration is made to move aroundlaterally by means of a curvilinear layout of theroad, driving will become hypnotic causing thedriver to fall asleep.

To achieve space perception, it is necessary tohave a lateral component of movement to allowthe driver to assess relative changes in the size andposition of objects. This enables the driver todiscern movement and its direction. Horizontalcurves provide this lateral component, the rate ofmovement depending on the radius of the curve.

The concept of a continuously curving alignmentwith constant, gradual and smooth changes ofdirection evolved and the curvilinear alignmentconcept of long flat circular curves joined byspiral transitions was born. Inherent in thisconcept was the idea that curves would vary up to30,000m radius. If the curve radii are confinedwithin the limits of 10,000m to 30,000m, the needfor transitions disappears.

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10A shorter route, but more cutting and

a scarred landscape

A skyline cutting, straight

alignment with vertical curve

Re-aligned to fit landscape;

additional excavation on

road side to eliminate

residual mound.

A longer route, but less cutting

and a road blended to the land.

Figure 10.12 Examples of Road-landscape Compromise

Curvilinear alignment from the driver’s viewpoint

Typical long tangent from the driver’s viewpoint

The radius to use depends on several factorsincluding the type of topography and the driver'sdesired speed of travel, the preferred radiusdepending on how far ahead the driver can see theroad. At high speeds, the driver looks from 300mto 600m ahead and a curve should be at least thislong to be visually significant when the driver ison it. A curve ceases to be visually significant ifthe visible part of the curve does not accomplish aturn of more than two degrees (e.g. for a curve ofradius of 15,000m, the visible length of roadwayshould be at least 524m).

It is desirable to adopt a three-degree deflection

angle as a minimum resulting in curve radiibetween 3000m and 30,000m. Where extensiveflat plains occur, it is desirable that overtakingsight distance is provided on as much of the roadas possible. This can be achieved on a 15 000mradius curve, and an alignment with this radius asa minimum will have continuous overtaking sightdistance.

The larger the radius, the closer the alignmentbecomes to a straight line and the less theadvantages of the curve become. There is littlepoint in using radii larger than 30 000m for thisreason.

Table 10.1 Curvilinear Alignment - MinimumRequirements Summary

Factor ValueRadius • Min. 3000m; Max. 30,000mDeflection • Min. 2° for visible part of the

curve (e.g. for 15,000m radius,a length of 524m is required).

• Desirable 3° (requires curvesfrom 3000m to 30,000m radius).

• Over 30,000m, the roadappears straight - there is nopoint in using curves of largerradius than this.

Overtaking • 15,000m radius curves provideSight Distance continuous overtaking sight

distance.

The advantages of a curvilinear alignment are:

• driving is more pleasant as the road unfoldsitself smoothly with no unexpected checks;

• judging distance to an approaching vehicle iseasier for the driver and therefore betterdecisions on overtaking manoeuvres can bemade;

• the view ahead is constantly changing;

• monotony can be reduced by directing the roadtowards interesting features of the countrysidefor short periods; and

• night driving is more comfortable in that theapproaching headlight glare is significantlyreduced. On long straights, headlights becomevisible and annoying from a long distance away

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(up to 15km), but on a curvilinear alignment,the glow of the approaching headlightsbecomes visible before the lamps becomevisible, the rate of approach can be betterjudged and the disability glare is significantlydelayed.

Where the road passes through a forested area, thecurvature of the road creates a continuousbackdrop of trees providing excellent conditionsfor detecting the continuing angular change. Thelarger radii alignments have worked well in thesecases. On treeless plains, some of the effect of thecurvilinear alignment is lost. In thesecircumstances, the smaller radii would be moreeffective.

New large radius curvilinear alignment adjacent toold, long tangent/short curve alignment

In more undulating topography, the combinationof the vertical curvature with the large radiushorizontal curves becomes important and theprinciples discussed in 10.3 above must beapplied. For example, short sag vertical curvesmay be accentuated by the large radius horizontalcurves creating the appearance of a kink. It maybe necessary to adopt a vertical alignment withsignificantly larger radius curves to avoid the

appearance of kinks. Radii in both dimensionswill have to be tailored to meet the circumstances,based on the extent of the driver's vision of theroad ahead.

The principles of curvilinear alignment can beapplied in a wide range of circumstances using awide range of curve radii together with spirals.The vast plains of the inland lend themselves tothis approach and roads designed on theseprinciples provide an improved result for theusers.

References

AASHTO (1994): “A Policy on the GeometricDesign of Highways and Streets”.

MacPherson, R.A. and L.J. Louis (1976):“Curvilinear Alignment Design in the SouthWestern Plains” - Queensland Roads.

Tunnard, C. and B. Pushkarev (1963): “ManMade America: Chaos or Control”.

U.S. Department of Transportation (US DOT)(1980): “Aesthetics in Transportation”.

U.S. Department of Transportation (US DOT)(1997): “Flexibility in Highway Design”.

Relationship to OtherChapters

• Closely related to Chapters 11 and 12 – theyprovide the details, this chapter provides theguidelines for putting them together;

• The cross section and how it interacts with thehorizontal and vertical alignments also affectsthe operation and aesthetics (Chapter 7);

• The principles of this chapter have to beconsidered in planning – Chapter 2;

• Alignment issues arise in Chapters 4, 13, 14,15, 16, 20 and 22.

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