geometric design

Mekelle UniversityEthiopian Institute of Technology-Mekelle

School of civil engineeringRTEC

INTRODUCTION

• Geometric design is the stage of road designprocess where the dimension and layout ofroad elements are proportioned to meet theneeds of road users.

• According to ERA geometric design is the processwhereby the layout of the road through the terrain isdesigned to meet the needs of the road users. i.e.Safety, Comfort, Efficiency, Economy, Environmentetc.

8/16/2016 Highway Geometric Design 2

INTRODUCTION

Highway geometric elements:

– Cross-section

– Sight distance

– Horizontal alignment

– Vertical alignment

– Vertical and lateral clearance

– Intersections etc.


INTRODUCTION

• In geometric design of highways, the following points should be considered:– Volume and composition of traffic – Consistent and compatibility The standards proposed for the different elements should be compatible

with one another.

– include all aspects of geometry of the road Signs, markings, proper lighting, intersections etc.

– Environmental friendly – Economic consideration– Safety– The highway should enable all the road users to use the facility.– Aesthetics


ROAD CROSS SECTION ELEMENTS

• Cross sections define the configuration of a proposed roadway at right angles to the centerline.

• Typical sections show the width, thickness and descriptions of the surfacing courses as well as the geometrics of the graded roadbed, side ditches and side slopes.



Right-of-Way

• It is the width of land secured and preserved to the public for road purposes.

• should be adequate to accommodate all the elements that make up the cross-section of the highway and may reasonably provide for future development.



Carriageway

• The part of the road constructed for use by moving traffic, including traffic lanes, auxiliary lanes such as acceleration and deceleration lanes, climbing lanes, width of median strip and passing lanes.



Shoulders

• The portion of the roadway adjacent to the carriageway.

Purpose• Serves for an emergency stop of vehicles

• Used to laterally support the pavement structure

• Parking facility

• Pedestrian walkway

• Recommended shoulder width for paved road is in the range of 0.5 to 3 m.


ROAD CROSS SECTION ELEMETS

Roadway• consists of the carriageway and the shoulders, parking lanes

and viewing areas.Median• section of divided road that separates lanes in the opposite

directions. • Functions:

• Provide recovery area during emergency• Provide stopping area for left and U-turning vehicles• Provide refuge for pedestrians• Reduce headlight glare

• Median can be either raised, flush or depressed



Crown slope/camber

• Is the cross slope provided to raise middle of the road surface in the transverse direction to drain rain water from the road surface but not being so great as to make steering difficult.

• Normal cross fall should be 3% on paved roads and 4-6% on unpaved roads.



Side Slopes & Back Slopes

• The graded area immediately adjacent to the graded roadway shoulder and side slope toe or ditch.

• Side slopes should be designed to insure the stability of the roadway and to provide a reasonable opportunity for recovery of an out-of-

control vehicle.



• Three regions of the roadside are important when evaluatingthe safety aspects of side slope

the top of the slope (hinge point),

the side slope, and

the toe of the slope

• Embankment or fill slopes parallel to the flow of traffic maybe defined as recoverable, non-recoverable, or critical.



Curbs• Raised structures used mainly on urban roads to

delineate pavement edge and pedestrian walkways.• Curbs are also used:

• To control drainage (part of gutter)• Improve aesthetic• reduction of maintenance operations• Reduce right-of-way

– Are classified as • Barrier curbs – relatively high designed for preventing vehicles

from leaving the road• Mountable curbs – are designed so that vehicles can cross them



Side walks– provided on urban or sub urban roads

• When pedestrian traffic is high along main or high speed roads

• In urban areas, sidewalks are provided along both sides of streets to serve pedestrians access to schools, parks, shopping centers, and transit stops.



Drainage Ditches

• Function of collecting and conveying surface water from the highway right-of-way.

• The depth of channel should be sufficient to remove surface water without saturation of the subgrade.

• should be located and shaped to avoid creating a hazard to traffic safety.


Design controls and criteria

• Refers to the constraints and requirements that a certain highway should fulfill.

• Factors affecting the geometric design of a highway are: (ERA Manual)

Functional classification of the roadThe nature of the terrainDesign vehicleTraffic volume (expected)Design speedDensity and character of adjoining land useEconomic and environmental considerations

• These considerations are not of course, completely independent of one another.


Design controls and criteria

So these considerations can be reduced to:(principal design criteria)

• Functional classification of the road

• Topography

• Traffic

• Design speed

• Design vehicle


Functional classification of the road

• Functional classification is the process by which streets and highways are grouped in to classes or system based of the character of service they are intended to provide.

• Road can be classified in many ways for instanceTraffic volumeSurface conditionLevel of service etc.

However, classification based on the speed and accessibility is the most generic one.

Mobility: The ability to move goods and passengers to their destination. (in a reasonable time)

Accessibility: the ability to reach desired destination


Classification(Cont.)


ERA functional classifications

Trunk roads

Access roads

Collector roads

Link roadsRoads linking centers of national or international importanceand have over 300 first year AADT, although values canrange between 75-10,000 AADT.

Roads linking centers of provincial importance and their AADTs between 25-1,000 .

Roads linking locally important centers to each other, to a more important center, or to higher class roads and their fist year AADTs between 25-300 .

Any road link to a minor center such as market and local locations with AADT less than 150..

Roads linking centers of international importance and roads terminating at international boundaries and have a present AADT 1000 and as low as 150.

Feeder roads


Nature of terrain

• The geometric design elements of a road depend on the traverse terrain through which the road passes. Traverse terrain properties are categorized into four classes as follows:

Flat or gently rolling terrain

Rolling terrain

Mountainous terrain

Escarpment


Topography(Cont.)


Traffic Volume and Composition

• Traffic data indicates the service for which the road is being planned and directly affects the geometric elements such as width, alignment, etc.– Traffic volume – AADT, ADT, PHV, DHV– Directional distribution – the percentage of traffic volume

flowing in each direction– Traffic composition – the percentage of different types of

vehicles in the traffic stream – different types of vehiclesare converted into passenger car unit to design a road width

– Traffic projection – using the design period of a road (5-20years)a reliable traffic projection should be madeconsidering the following elements


Traffic(Cont.)

o Traffic projection (cont’d.):–• Current traffic – currently using the existing road• Normal traffic growth – anticipated growth due to population

growth or change in land use• Diverted traffic – traffic that switches to a new facility from near by

roads• Converted traffic – traffic resulting from changes of mode• Change of destination traffic – traffic that has changed to different

destination due to new or improved transport and not changes inland use

• Development traffic – traffic due to improvement on adjacent landdevelopment that would have taken place had the new or improvedroad not been constructed

• Induced traffic – traffic that did not previously exist in a any formbut results when new or improved transport facilities are provided


Design vehicle

• In practice a mix of different vehicle types uses a road at any time

• However, a road designed for truck-trailers while it is mainly used by passengers car is uneconomical, and designing a road for passenger cars while there are significant number of trucks using it is again not appropriate.

• Hence, a largest vehicle that uses the road in frequent bases is considered as a Design Vehicle.


Design vehicle(Cont.)

• Both the physical characteristics and turning capability of vehicles are controls in geometric design.

• Vehicle characteristics and dimensions affecting design include power to weight ratio, minimum turning radius and travel path during a turn, and vehicle height and width.

• The road elements affected include the selection of maximum gradient, lane width, horizontal curve widening, and junction design.


Design vehicle(Cont.) As per ERA Design Manual there are four general classes

of design vehicles


Design vehicle(Cont.)


Design speed

• it is a single most important factor that affect most of the geometric design elements

• it directly governs Sight distance, horizontal curve radius, length of vertical curve, etc.

• different drivers travel at different speed depending on their behavior. However, for GD we need a single representative speed, known as design speed.

• Design speed : is the highest continuous speed at which vehicle can travel with safety when weather condition is conducive.

• Usually design speed is taken as the 85th percentile speed.


Design speed(Cont.)

• It is selected from standards for designing specific section of road considering the terrain, land use, classification of the road, etc.

• Using road functional classification selection, terrain type and design traffic flow, a design class, or standard, is selected from Table 2-1, with reference to the design parameters associated with that class as given in Tables 2-2 through 2-17 the allowable geometric constraints are fixed.(see ERA 2013 geometric design manual)


ELEMENTS OF GEOMETRIC DESIGN

»Sight Distance

»Horizontal Alignment• The tangent, or straight section

• Horizontal circular curve

• The transition curve (spiral) and

• super elevation

• Widening

• Stopping sight distance on horizontal curves

»Vertical Alignment


Horizontal Alignment

• Is a plan view which shows the horizontal curves and tangents or straights.

• Horizontal alignment deals with the design of the directional transition of the highway in a horizontal plane.

• Horizontal alignment consists of: » Horizontal tangents

» Circular curves and

» Possibly transition curves



Horizontal alignment should meet these general considerations: Alignment should be straight if possible within physical and

economic constraints Alignment should be consistent. Try to avoid sharp curves at the

ends of long tangents and sudden changes from gently to sharply curving alignment.

Avoid the use of minimum radii whenever possible Avoid horizontal curves on bridges On minor roads, curves should have a minimum length of 150 m for

a deflection angle of 5° and that this length should be increased by 30 m for every 1° decrease in deflection angle.

On major roads and freeways, the minimum curve length in meters should be three times the design speed in km/h.



• Tangent Section Straight portion of roads Provide better visibility & better appearance

Guidelines

• Long tangent should be avoided as they are monotonous for drivers and cause headlight dazzle on straight grade

• More pleasing appearance and higher road safety can be obtained by winding alignment

• Short straight b/n curves should not be used• Straights b/n curves turning in the same direction should have length

greater than 6V• Straights b/n the end and beginning of non transitioned reverse circular

curve should have length greater than 2/3 of the total super elevation runoff



Horizontal Curves

Used to make transition from tangent to tangent

Used to prevent demolishing of important places

It is one of the most important features influencing the efficiency and safety of highways.



Types of horizontal curves:»Simple circular curves

»Compound Curves

» Reverse Curves

»Broken back curves

»Switch back curves

» Transitional Spirals


Horizontal curves

• Simple Circular Curves Has a constant radius

Are designated either by the degree of curvature (D) or radius of the curve (R).


∆=deflection angleL=Length of CurveC=Chord LengthR=Radius of CurvatureM=Middle OrdinateE=External DistanceT=Length of TangentP.I.=Point of IntersectionTC=Tangent to CircleCT=Circle to Tangent

Horizontal curves

Formula for simple curve• Tangent (T):)

T = R*tan (Δ/2)• External distance (E):

E = R*(Sec (Δ/2) – 1 or E = T*tan (Δ/4)• Middle ordinate (M):

M = R*(1‐ Cos (Δ/2)) • Long chord(C): straight‐line distance from PC to PT.

C = 2R*Sin (Δ/2)• Length of Curve (Lc): distance from PC to PT along the curve.

Lc = 20* Δ/D or Lc = R*π* Δ/180• Stations of PC, PI, and PT:

PC = PI – TP T = PC + Lc or PT = PI + T


Horizontal curves

Degree of Curvature

Is the centeral angles subtending a standard arc or chord length of 20m.

Arc Definition

Chord Definition


360

220 R

D

RD

92.1145

R = 10 / Sin (D/2)

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51

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Compound curvesconsisting of a series of two or more simple curves of different radii turning in the same direction.

180

180

sin)180sin(sinsin

2tan

2tan

22

11

22

11

2121

12

222

111

21

RL

RL

tHVT

tGVT

ttttHVGV

Rt

Rt

Horizontal curves

Reverse curve

• Are adjacent curves that curve in opposite directions.

• Should be used for low‐speed roads such as in mountainous terrain

• Reverse curves are seldom recommended. They are absolutely NOT recommended for high‐speed roads. For high‐speed roads, we must provide a tangent section that will allow full development of super elevation at both ends.


Horizontal curves


Super‐elevation

• Is the raising of the outer edge of the road along a curve in‐order to counteract the effect of radial centrifugal force in combination with the friction between the surface and tyres developed in the lateral direction.

• To provide safety and comfort while navigating through curves at higher speeds.


Minimum radius of curve

The design of radius of roadway curves should be based on an appropriate relationship between design speeds and curvature and on their relationship with super elevation (roadway banking) and side friction.

Minimum radius from the stability of vehicle


FcRmV /2

mgf

N

h

Nf

RmVFc

mgN

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/2

m

Stability on Super‐elevated SurfaceForces & Equilibrium


gR

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e1

Super‐elevation


1

1

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2

22

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2

gR

v

gR

v

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vCos

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vSin

WSinCosgR

WvWCosSin

gR

Wv

WSinCosgR

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m

m

mm

m

m

But the term has a very small value and could be ignored for all practical purposes. Check using typical values like V=50km/hr; m=0.16; and R=100m.

gR

v 2m

Super‐elevation


R

V

gR

ve

egR

vTanThus

81.9

6.3

,

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m V=Km/hrR=me=m/mm=dimensionless

m

max127

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Thus, the minimum radius of curve is given by;

Super‐elevation

Side friction coefficients are dependent on

• vehicle speed;

• type, condition and texture of roadway surface;

• weather conditions; and

• type and condition of tires

Table: side friction for paved roads

Table: side friction for unpaved roads


Super‐elevation

• Maximum value of super‐elevation is controlled by:» Climatic conditions: frequency & amount of

snow/icing» Terrain condition: flat vs. mountainous» Area type: rural vs. urban» Frequency of very slow moving vehicles

• As per ERA manual 4 % for urban and 8 % for rural.• A rate of 4 % or 6 % is applicable for urban design in

areas with little or no constraints.


Super‐elevation transition

• super elevation should be introduced and removed uniformly over the length adequate for likely travel speeds.(comfort and safety)

Superelevation transition section consists: Superelevation runoff tangent run out sections

Superelevation runoff: ‐ is the length of highway needed to accomplish the change in cross‐slope in the outside‐lane (flat) to of fully Superelevation.

Is a function of the design speed and rate of super elevation.

Tangent run‐out(crown runoff): ‐ consists of the length of the roadway needed to accomplish a change in outside‐lane cross slope from the normal cross slope rate to zero (flat).

The removal rate is usually the same as the Superelevation removal rate is usually the same as the Superelevation runoff rate.



• In curves without transition

In the design of curves without spirals the super‐elevation runoff is considered to be that length beyond the tangent run out.

67 % of the Superelevation runoff isdeveloped on tangent and 33 % on circular curve.



In curves with transition In alignment design with spirals the Superelevation

runoff is affected over the whole of the transition curve. The length of runoff is the spiral length with the tangent to spiral (TS) at the beginning and the spiral to curve (SC) at the end. The change in cross slope begins by removing the adverse cross slope from the lane or lanes on the outside of the curve on a length of tangent just ahead of TS (the tangent run out). Between the TS and SC (the Superelevation runoff) the traveled way is rotated to reach the full Superelevation at the SC



Methods of Attaining Superelevation• Four methods are used to transition the pavement to

a super elevated cross‐section. These methods include– Revolving a travelled way with normal cross‐slopes about

the centerline profile.– Revolving a traveled way with normal cross‐slopes about

the inside‐edge profile.– Revolving a travel way with normal cross‐slope about the

outside‐edge profile.– Revolving a straight cross‐slope traveled way about the

outside‐edge profile.



• Undivided highways are usually superelevated with the axis of rotation at the roadways centerline.

• Multi‐lane highways with depressed medians are usually superelevated with the axis of rotation at the median edges of the traveled way.


Transition curves

• When driving over simple horizontal curves, there is an abrupt change from a tangent to a circular arc at the PC

• are curves which provide a gradual change in curvature from tangent to a circular path

• have got a radius of curvature gradually changing from infinity to the designed radius.

• have constant rate of change of radius of curvature.• Is convenient for the application of super‐elevation• Transition curves are not normally required for

» Curves with large radius» Roads with lower classification


Transition curves

• The length of transition should be determined from the following two conditions: The rate of change of radial acceleration adopted in the

design should not cause discomfort to the drivers. If C is the rate of change of acceleration,

Ls = 0.0215V3 / (C*Rc)

Where: V = speed (Km/hr)Rc = radius of the circular curve (m)

The rate of change of super elevation (super elevation application ratio) should be such as not to cause higher gradients and unsightly appearances. Ls=new

where: w ‐is width of roadn ‐is The rate of raising the outer edge above the centre line e‐ rate of Superelevation


Stopping Sight Distance On Horizontal Curves


Widening of Highway Curves

• Extra width of pavement necessary on curves.

• Widening of the carriageway where the horizontal curve is tight is usually necessary to ensure that the rear wheels of the largest vehicles remain on the road when negotiating the curve; and, on two lane roads, to ensure that the front overhang of the vehicle does not encroach on the opposite lane. Widening is therefore also important for safety reasons.



• The turning effect of a vehicle (rear wheel relative to the front wheel) requires widening of the lane width.

• The traveled way on horizontal curves is widened to accommodate curve negotiation

• Widening is necessary for smaller radius curves or greater curvatures and/or narrower road lane width and embankments

• Off tracking: is the characteristic of all vehicles that the rear wheels do not follow precisely the same path as the front wheels when they negotiate a horizontal curve or make a turn.

• Total widening is computed by adding the mechanical widening andpsychological widening.



• The widening required can be calculated


R

V

R

LnWc

102

2

B = length of wheel baseR = radius of curveV = design speed (Km/hr)n = number of lanesWc=total widening


Widening – Methods• On a simple curve (i.e. with no spirals) widening

should be applied on the inside edge of a pavement only. For curves with spirals, widening could be applied on the inside (only) or could be equally divided b/n the inside and outside

• Widening should be attained gradually over the super elevation runoff length but shorter lengths are sometimes used (usually this length is 30 –60m).

• Widening is costly and very little is gained from a small amount of widening.



• Widening on high embankments is recommended for design classes DC8 through to DC4.

• Curve widening is generally not applied to curves with a radius greater than 250 meters regardless of the design speed or the lane width


geometric design

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