Geometric Design 2018.pdf

GEOMETRIC DESIGN OF ROADS AND HIGHWAYS

Concepts of Transportation Engineering 

Transportation Engineering – a branch of civil engineering that deals with the application of technology and scientific principles to the planning, functional design, operation and management of facilities for any mode of transportation in order to provide for the safe, rapid, comfortable, convenient, economical and environmentally compatible movement of people and goods

Elements of Transportation Engineering     



Highway & Traffic Engineering Pavement Engineering Railway Engineering Airport Engineering Water (Harbor Engineering) Transportation Pipeline Transportation

Highway Engineering 



Branch of transportation engineering dealing with the planning, location, design construction and maintenance of highways and with the regulations and control devices employed in highway traffic operations Elements include: traffic loading, shape of road, travelway, shoulders, sidewalks, pavement surfacing, subgrade, pavement foundation, drainage and ditch

Highway Types/Classification of Roads (Political Classification) 





National Roads – form part of the main road trunkline system administered by the DPWH thru its regional and engg district offices, right of way of not less than 20m allocation for improvements Provincial Roads – connect one municipality to another, roads within provincial boundaries, ROW <15m. City Roads- it connects one city/town/ municipality-roads within the urban area, ROW of at least 15m

Modern Highway Location Practice 







Reconnaissance – eliminate costly locations and limit the choice to one or two general routes between controls Route Selection –is to flag the best location within the general route, which will more or less permit the predetermined grade controls Preliminary Survey –to obtain topography of the strip or strip flagged which data will be utilized as the basic framework for the projection of the line Location survey – to transfer the paper projection determined in the off, from the topographic strip map to the actual site in the field





Municipal roads - roads within town/ municipalities, ROW of at least 10m Barangay roads – rural roads, ROW of at least 10m; roads outside the Poblacion or municipality, feeder or farm to market roads

Specify design standards •Local/national standards •Functional classification Design speed, cross sectional features, superelevation

• Topography • Ground soil conditions • Hydrologic condition • Environmental conditions/impacts • Habitat • Existing zoned land use • Functional classification

Specify Major Design Controls •Design vehicle • design speed • volume/capacity/LOS/Access •Horizontal controls (radius, superelevation) • Vertical controls (grades, intersections, utilities) • cross section controls and elements •Earthwork control (excavation, embankments, etc)

Selection of routes •Identify alternate routes • sketch horizontal and vertical alignments •Screen routes and select those for preliminary design

Modifications?

•

Survey Major Site Characteristics

•Horizontal alignment • vertical alignment • cross sections • drainage features • earthworks • environmental impacts

Evaluate Design •Cost (project cost, user cost)

Detailed Design

Modifications?

Conduct (preliminary) design

MINIMUM DESIGN STANDARD PHILIPPINE HIGHWAYS

ROAD DESIGN CONSIDERATIONS Environmental Concerns 1. Potential impacts of road construction on the quality of physical and human condition in the area.

2. Ensure less potential negative impacts on air quality, sound quality, water quality and land characteristics.

3. Reduce undesirable changes in the physical and human condition in area were the construction will take place

ROAD DESIGN CONSIDERATIONS Cultural Concerns 1.Degradation of natural history (palaeotological significance) 2.Cultural Sites

* archaeological sites * burial (cemetery) * religious significance * ethnical domain

GEOMETRIC DESIGN STANDARDS Traffic Flow * influential to vehicle to vehicle interactions * congestion may result in increase in journey time and accident risk

MINIMUM DESIGN STANDARD PHILIPPINE HIGHWAYS

GEOMETRIC DESIGN STANDARDS Traffic Information * use for structural design purposes * basis for traffic demand forecasting * traffic volume estimation

Traffic loading information may be obtained from the following sources:

* Tabulated average 80 Kn equivalent Single Axle Load per Lane * Traffic survey (visual or traffic counting) * Transportation planning models (trip generation models)

GEOMETRIC DESIGN STANDARDS Traffic loading information may be obtained from the following sources:

* Tabulated average 80 Kn equivalent Single Axle Load per Lane Traffic Class

Cumulative Equivalent Traffic (E80/lane)

Description

<0.20 x 106

Very light trafficked, very few heavy vehicles

0.2 – 0.8 x 106

Lightly trafficked roads, light delivery and agricultural vehicles, very few heavy vehicles

3 (Major Collector Roads)

0.8 - 3 x 106

Medium volume of traffic; few heavy vehicles

4 (Highway)

3 - 12 x 106

High volume of traffic and/or many heavy vehicles

5 (Expressway)

12 - 50 x 106

Very high volume of traffic and/ or high proportion of fully laden heavy vehicles

1 (Feeder Roads) 2 (Collector Roads)

GEOMETRIC DESIGN STANDARDS Traffic forecasting

* forecasting of average daily traffic can be projected growth factor (urban area is 20% and rural area 5%) Formula for traffic forecasting :

Pn = Pi (1 + r ) n where: Pn =projected number of traffic Pi = initial average daily traffic r = growth rate n = number of projected years

using a

GEOMETRIC DESIGN STANDARDS Traffic Forecasting/Traffic Design Data

Geometric Design

• number of lanes •width of lanes •design speed

Pavement Design

• type of pavement •thickness of subgrade, subbase, base courses •thickness of pavement

Specify design standards •Local/national standards •Functional classification Design speed, cross sectional features, superelevation

• Topography • Ground soil conditions • Hydrologic condition • Environmental conditions/impacts • Habitat • Existing zoned land use • Functional classification

Specify Major Design Controls •Design vehicle • design speed • volume/capacity/LOS/Access •Horizontal controls (radius, superelevation) • Vertical controls (grades, intersections, utilities) • cross section controls and elements •Earthwork control (excavation, embankments, etc)

Selection of routes •Identify alternate routes • sketch horizontal and vertical alignments •Screen routes and select those for preliminary design

Modifications?

•

Survey Major Site Characteristics

•Horizontal alignment • vertical alignment • cross sections • drainage features • earthworks • environmental impacts

Evaluate Design •Cost (project cost, user cost)

Detailed Design

Modifications?

Conduct (preliminary) design

Highway Curves 





Highway and railroad routes are chosen only after a complete and detailed study of all possible location Route selection usually involves the use of air photos and ground surveys and the analysis of existing plans and maps Route selected is chosen because it satisfies all design requirements with minimal social, environmental and financial impact

  

     

Design Criteria Purpose: The standards provided in this chapter are applicable to new construction, reconstruction, and bridge projects on highways with traffic volumes of over 400 vehicles per day. For each project, the values established for the applicable critical design elements represent the Design Criteria for that project. The chapter defines the following critical design elements and provides values for different classifications of highways and roads: Design Speed Lane Width Shoulder Width Bridge Roadway Width Grade Horizontal Curvature

           

Design Criteria Purpose: Superelevation Stopping Sight Distance Vertical Clearance Travel Lane Cross Slope Structural Capacity Level of Service Control of Access Pedestrian Accommodation Median Width Horizontal Clearance

GEOMETRIC DESIGN STANDARDS  Sample problem: Traffic Forecasting Determine the possible number of average vehicle per day of a rural road if the initial average daily traffic is 200, locality growth rate is 6% and design years is 5 years.

Sol’n: Working Formula: Pn = Pi (1 + r )n Given: Pi = 200 ADT, r = 6% and n = 5 Pn = 200 (1 + 0.06) 1 =212 Pn = 200 (1 + 0.06) 2 =224.72~225 Pn = 200 (1 + 0.06) 3 =238.20 Pn = 200 (1 + 0.06) 4 =252.50 Pn = 200 (1 + 0.06) 5 =267.65~268

GEOMETRIC DESIGN STANDARDS Factors that affect the Design Speed * Traffic volume and composition * Topography (Flat, Rolling, Mountainous) * Classification of Road * Roadside environment (land uses next to the road, pedestrian, accesses to the road) * Function of the road (local or through traffic)

GEOMETRIC DESIGN STANDARDS Safety * optimizing by linking geometric elements to design speed * design standards must take into account the environmental road conditions, traffic characteristics and drivers behavior * potential collision risk

Design Speed * design varies with different terrain * provide appropriate consistency between geometric elements * mountainous and rolling areas speed limits as low as 40 kph * review of design speed to ensure that they relate to current circumstances

GEOMETRIC ELEMENT DESIGN Horizontal Alignment

* Design Features : Horizontal Curve and Horizontal Tangent * Obtaining the values of design control for horizontal curves and tangents * Determination of the location type of characteristics of horizontal curves. A. Sight Distance – most important element among many elements in horizontal alignment design. The driver’s ability to see ahead

contributes to safe and efficient operation of the road

Truck Sight Obstruction

Car Line of sight

GEOMETRIC ELEMENT DESIGN Horizontal Alignment B. Superelevation (banking)– help to counteract the centripetal acceleration produced as a vehicle rounds a curve.

Vehicle tire

1. Superelevaton requirement

f

V2 R = -------------------127 (e + f)

e

R = radius of the curve V = vehicle speed (kph) e = rate of superelevation f = coefficient of friction between the tire and the road surface

road surface

GEOMETRIC ELEMENT DESIGN Horizontal Alignment 2. Superelevation skidding resistance V2 ---------------127 R

> (e + f),

Skidding resistance condition V2 --------------- < 0.22 127 R 3. Overturning due to superelevation

Truck

GEOMETRIC ELEMENT DESIGN Horizontal Alignment Things to consider in the design of road superelevation 1. comfortableness for running 2. separation of very slow moving vehicles such as bicycle 3. climatic condition of the region 4. type of area 5. terrain activity

GEOMETRIC ELEMENT DESIGN Sample Problem: The design speed of a rural road is 40 kph with a horizontal curve radius of 200 meters. The coefficient of friction of the surface of the road is 0.15 and the superelevation is 8%. Can this speed be safely maintained on the road? Solution: v = 40kph R = -------------------f = 0.15 127 (e + f) e= 0.08 R = 200 m. V2

Check the design speed V = √ 127 x (0.08 + 0.15) x 200 = 76.43 kph therefore the design speedof 40 kph is safe to maintain because the horizontal curve radius is quite long for a driver to comfortably drive.

GEOMETRIC ELEMENT DESIGN Vertical Alignment * V.A. has a strong influence upon the construction cost, the operating cost of vehicles using the road and in combination with the horizontal alignment also on the number of accidents. * it should be designed to the highest standard of consistency and economy * Designed to be aesthetically pleasing Two Major Aspects of Vertical Alignment * vertical curvature – governed by vertical sight distance and comfort criteria * gradient –related to vehicle performance and level of service

GEOMETRIC ELEMENT DESIGN Plan : Horizontal Alignment

Profile: Vertical Alignment

GEOMETRIC ELEMENT DESIGN Vertical Alignment 1. Vertical Curves are required to provide smooth transition

between consecutive gradients and the simple parabola is recommended for these. The most important geometric consideration governing vertical curvature is the sight distance. Line of sight

Road Surface

GEOMETRIC ELEMENT DESIGN Vertical Alignment 2. Gradient

* needs to be considered from the standpoint of both length and steepness, and the speed at which heavy vehicles enter the gradient.

* effect of a steep grade is to slow down the heavier vehicles and increase operating cost Recommended Standards for maximum grades (%) to Design Speed Design Speed Topograph y

30

40

50

60

65

70

75

80

Flat

6

5

4

3

3

3

3

3

Rolling

7

6

5

4

4

4

4

4

Mountaino us

9

8

7

6

6

5

-

-

GEOMETRIC ELEMENT DESIGN Vertical Alignment Technique to compute the road slope or gradient level difference slope = ----------------------------------- * 100 length Level Difference Road gradient or slope

L = Length

GEOMETRIC ELEMENT DESIGN •Case Study: Determine the expected annual average daily traffic over the required design period in one direction of the road. Given: a. The predicted growth rate is assumed as follows: 1990-1994 = 6% 1994-1996 = 7% 1996-2001 = 4% 2001-2008 = 8% b. Initial traffic data = 650 ADT c. Total traffic distribution by the number type of vehicle per day in one direction as follows: car/jeep = 44% buses = 25% trucks = 31% d. Assume the 80 kn equivalent single axle load. Cars and jeeps are not considered to make significant damage to the pavement: Bus factor = 0.50 Trucks factor = 1.0

GEOMETRIC DESIGN FEATURES, PARAMETERS AND STANDARDS Geometric element

Design features

a) Cross section

Traffic way,carriage way, median, shoulders, parking lane, roadside

b) Horizontal alignment

Horizontal curve and horizontal tangent

Design parameters No of carriageways  no of lanes per carriageway Width and cross slope Characteristics of median Characteristics of shoulders/parking lanes 

Number of curves Characteristics of curves Length of curve Available sight distance 

Design standards Minimum width or lane  maximum and minimum cross slope 

Minimum radius and length of circular curve  minimum clearance Minimum length of transition curve 

GEOMETRIC DESIGN FEATURES, PARAMETERS AND STANDARDS Geometric element c) Vertical alignment

Design features vertical curve  vertical tangent 

d) Intersection zone diagram

Intersection approach zone Intersection area

e) Superelevation diagram

Superlevation rate, Superelevation runoff Tangent runoff

Design parameters types, location and length of curve Length and grade of tangent Available sight distance 

Design standards min and max length of vertical curve  max grade of vertical alignment 

number, length ad width of lanes on approach zone  type of intersection area 

Superelevation rate  superelevation runoff length  tangent runoff length 

Maximum rate of superelevation 

Geometric Design 

Geometric Design for transportation facilities includes the design of geometric cross section, horizontal alignment, vertical alignment, intersections, and various design details.

goals of geometric design    

maximize the comfort safety, economy of facilities while maximizing their environmental impacts

FUNDAMENTALS OF GEOMETRIC DESIGN      

geometric cross section vertical alignment horizontal alignment super elevation intersections various design details.

GEOMETRIC CROSS SECTION The primary consideration in the design of cross sections is drainage. Highway cross sections consist of traveled way, shoulders (or parking lanes), and drainage channels. Shoulders are intended primarily as a safety feature. Shoulders provide: accommodation of stopped vehicles emergency use, and lateral support of the pavement. Shoulders may be either paved or unpaved. Drainage channels may consist of ditches (usually grassed swales) or of paved shoulders with berms of curbs and gutters.

Austin, TX N.W. Garrick April 2008

5000 ft

B

A

I-95 East Lyme

5000 ft

B A

Autobahn 3 Aschaffenburg

5000 ft

A

B

New York Thruway

5000 ft B 2

1

A

Merritt Parkway

1000 ft

B 2

1

A

Blue Ridge Parkway

1000 ft

B

A

Atlantic Ocean Road, Norway

PLS DON’T TRY THIS ON YOUR PROJECT!!!!!

Importance of Geometric Design

Importance of Geometric Design  The geometric design of a highway deals with the dimensions and layout of visible features of the highway such as alignment, sight distance and intersection.

Optimum efficiency- Maximum safety- Reasonable cost  The geometrics of highway should be designed to provide optimum efficiency in traffic operations with maximum safety at reasonable cost.  The designer may be exposed to either planning of new highway network or improvement of existing highways to meet the requirements of the existing and anticipated traffic. 12500 PCU/ day 2500

2015

Year

2035

Improving Geometric Elements in Stages  Improving in stages is very difficult and more expensive  Design geometric features at the initial stage itself considering future traffic scenario

For example, in 2015 the road may be designed for 80 kmph. But the designer has to consider for 120 kmph as the road may be upgraded in future. Accordingly Sight distance, super-elevation etc are to be designed.

Elements of Geometric Design of Highways  Cross section elements

Width of Formation and pavement Surface characteristics land Cross slope of pavement SD available in horizontal curve

 Sight distance consideration SD available in Vertical curve SD available at intersections

 Horizontal alignment details Super-elevation Transition curves

 Vertical alignment details  Intersection elements

DESIGN CONTROL CRITERIA

Design Factors  The geometric design of highways depends on several design factors which control the geometric elements are: Design Speed Topography Traffic Factors Design hourly volume and capacity Environmental and other factors

Design Speed Most important factor Decided based on overall requirement of highway In India depends on category of road design speed assigned (NH/SH/MDR/ODR/VR) Design speed varies with topography

Design Speed Every geometric elements depends on speed For example, Pavement surface characteristics Width & side clearance Sight distance requirements Radius of curve Super-elevation Summit or valley curve

Topography

Plain Terrain

Design Speed of NH/ SH 100 Kmph

80 Kmph

Mountainous Terrain

50 Kmph

Rolling Terrain

Traffic Factors Different vehicles have different characteristics like height, weight, braking nature etc.

Design Hourly Volume Flow fluctuate with time Off-peak versus Peak Uneconomical to design for PH traffic Reasonable traffic value called design hourly volume DHV for urban 8-10% ; for rural 12-18%

Environmental & Others Factors such as aesthetics, landscaping, noise pollution and other local conditions should be given due consideration in the design of road geometrics Grade-separated intersection for expressways to achieve higher speed standards

Technique for Horizontal and Vertical Alignment 

Design of horizontal and vertical alignment of a road consists of two major tasks 



Obtaining the values of design controls for horizontal curves and tangents (requires the use of the parameters of the quality of design) Involves the determination of the location type and characteristics of horizontal curves

Horizontal Alignment





Components of the horizontal alignment. Properties of a simple circular curve.

Estimation of Control Values of Horizontal Alignment Parameters 

Estimation of control values of horizontal alignment parameters involves the estimation of the threshold values of circular and transition curves and horizontal tangents 

a) Danger of Skidding exist when: V2 > e + fs, the value should not exceed 0.22 127 R

where: e = superelevation V = speed (Kph) fs = coefficient of side friction R = radius of the curve (m)

Horizontal Alignment

Tangents

Curves

Tangents & Curves Tangent Curve Tangent to Circular Curve

Tangent to Spiral Curve to Circular Curve

Simple curve elements

Simple curve in full superelevation

Compound curve

Geometric Design Standards

Road Classification

Carriageway Width (m)

Shoulder Width (m)

i) Single lane

3.75

2 x 1.25

ii) Double Lane

7.00

2 x 0.9

Major District Roads and Other District Roads

3.75

2 x 0.5

Village Roads

3.00

2 x 0.5

National Highways

90

Geometric Design Standards

91

Geometric Design Standards

92

Geometric Design Standards

93

Geometric Design Standards  Reverse curves are needed in difficult terrain.  It should be ensured that there is sufficient length between the two curves for introduction of requisite transition curves.

94

Geometric Design Standards   

Curves in same direction separated by short tangents, known as broken – back curves. Should be avoided, as far as possible, in the interest of aesthetics and safety and replaced by a single curve. If this is not feasible, a tangent length corresponding to 10 seconds travel time must at least be ensured between the two curves.

95

Geometric Design Standards 

Compound curves may be used in difficult topography but only when it is impossible to fit in a single circular curve.



To ensure safe and smooth transition from one curve to the other, the radius of the flatter curve should not be disproportional to the radius of the sharper curve.



A ratio of 1.5:1 should be considered the limiting value.

96

Geometric Design Standards  Set Back Distance  Requisite sight distance should be available to sight the inside of horizontal curves.  Lack of visibility in the lateral direction may arise due to obstruction like walls cut, slopes, wooded areas, high crops etc.

97

Geometric Design Standards  Vision Berm  Where there is a cut slope on the inside of the horizontal curve, the average height of sight line can be used as an approximation for deciding the extent of clearance.  Cut slope shall be kept lower than this height at the line demarcating the set back distance envelop, either by cutting back the slope or benching suitably, which is also generally known as vision berm.

98

Geometric Design Standards  Vertical Alignment  The vertical alignment of a hill road need to be adaptive by: •

Adopting mild vertical grades for reduced potential for erosion of road bed.

•

Designing vertical profile compatible with natural topography for optimum and balanced cut-fill quantities hence generate less spoil.

•

Keeping finished road level and fill slopes higher than the high flood level (HFL).

•

Avoiding interception with water table line which cause wet pavement layers.

•

Optimizing the cut height at landslide and rock fall prone areas.

•

Ensure Easy Access to Properties.

•

Ensure Safer Junction Design.

99

Geometric Design Standards 

Vertical Alignment     

Vertical curves are introduced for smooth transition at grade change. Both Summit curves and Valley curves should be designed as Square parabola. The Length of vertical curves is controlled by sight distance requirements. Curves with greater length are aesthetically better. Recommended gradients for different terrain conditions, except at hair pin bends, are given below:

Classification of Gradient

Mountainous Terrain and Steep Terrain more than 3000 m above MSL

Ruling Gradient

5%

Limiting Gradient Exceptional

Steep Terrain up to 3000 m above MSL

Mountainous

Steep

6%

5%

6%

6%

7%

6%

7%

7%

8%

-

-

100

Design of Hair-pin Bends 

At unavoidable circumstances Hair-pin Bends may be designed as Circular Curve with Transitions or as Compound Circular curves. Design Criteria for Hair-pin Bends As per IRC:SP:48-1998 and IRC:52- 2001

Description

Criteria

Min Design Speed

20 Km/h

Min Roadway width at apex

NH/SH

11.5m (Double lane) 9.0m (Single lane)

MDR/ODR

7.5m

Village Roads

6.5m

Min radius for the inner curve

14 m

Min Length of transition Curve

15 m

Gradient

Maximum

1 in 40 (2.5%)

Minimum

1 in 200 (0.5%)

Max Super elevation

1 in 10 (10%)

Minimum Intervening distance between the successive hair pin bends

60m 101

Illustrations of Hair-pin Bends

102

Climbing Lane 

 

Climbing Lane shall be provided in order to address the necessity of making available separate lane for safe overtaking for vehicle travelling uphill. IRC:52-2001, IRC:SP:73-2015 and IRC:SP:84-2014 mandates for provision of Climbing lanes but no warrants are provided. AASHTO provides the guidelines for the provision of Climbing lanes: 

Up Grade traffic flow rate in excess of 200 vehicles per hour.



Up Grade truck flow rate in excess of 20 vehicles per hour.



One of the following conditions exists:



•

A 15 km/h [10 mph] or greater speed reduction is expected for a typical heavy truck.

•

Level of Service ‘E’ or ‘F’ exists on the grade.

•

A reduction of two or more levels of service is experienced when moving from the approach segment to grade.

In addition, safety considerations may justify the addition of a climbing lane regardless of grade or traffic volumes.

103

Other Geometric Design Aspects  Escape Lane  Grade Compensation at Curves  Passing Places  Vertical and lateral Clearances  Widening at Curves  Co-ordination of Horizontal and Vertical Alignments  Tunnels

Passing Places

Escape lane

Widening at Curves 104

Typical Section for Tunnels 

Typical Cross section for 3-lane Tunnel as per IRC SP 91-2010

105

DESIGN OF HORIZONTAL ALIGNMENT

106

Estimation of Control Values of Horizontal Alignment Parameters 

b) Danger of Overturning 1) At low speeds, there is a likelihood of offtracking while at high speeds drivers generally experience difficulty in steering their vehicles and thus take the outer side of the beginning of the curve 2) To reduce the effects of these, some extra width of pavement often provided on curves

Estimation of Control Values of Horizontal Alignment Parameters 

Danger of Overturning 

The major requirements are to determine the following:   

Minimum radius and length of curve Maximum rate of superelevation Minimum amount of widening

Estimation of Control Values of Horizontal Alignment Parameters 

Danger of Overturning X V2 > x + ye y

e

127 R

y – xe

Where: y & x = coordinates of the center of gravity of the design vehicle (Y=1.5 and x = 1.2 m_

Estimation of Control Values of Horizontal Alignment Parameters 

Minimum Radius and Length of Curve 

The appropriate radius of a circular curves can be considered to be one that prevent overturning of heavy vehicles, ensures safety of each vehicle and its occupants and satisfies the sight distance requirement

Rdmin = Max (Rsr; Rso; Rsd )



Rdmin = Max (Rsr; Rso; Rsd ) 

 



Rdmin = minimum desirable radius of a circular curve Rsr=radius which satisfies the skidding prevention Rso=radius which satisfies overturning of the trucks on the curve Rsd=radius which satisfies sight distance requirements

Rdmin = Max (Rsr; Rso; Rsd ) Rdmin = minimum desirable radius of a circular curve Rsr=radius which satisfies the skidding prevention V2 Rsr = ------------------------------------127 (emax + fsmax)

em = m a x i m u m v a l u e o f s u p r e e l e v a t i o n ( r u r a l a r e a s = 0 . 0 8 a n d u r b a n a r e a s = 0 . 0 4 T o a x 0 . 0 6 AASHTO Values of Side Friction Design Speed (kph)

48

64

80

96

104

112

120

128

Side Friction

0.16

0.15

0.14

0.13

0.13

0.12

0.11

0.11



Rdmin = Max (Rsr; Rso; Rsd ) 

Rdmin = minimum desirable radius of a circular curve 

Rsr=radius which satisfies the skidding prevention



V2 Rsr = -------------------127 (emax +fsmax)

fsmax = 0.37 {0.0000214 *Vd2 - 0.0064Vd + 0.77}



Rdmin = Max (Rsr; Rso; Rsd ) 

Rdmin = minimum desirable radius of a circular curve 

Rso=radius which satisfies overturning of the trucks on the curve

V2 [y –xe] Rso = ---------------------127 [x +ye]



Rdmin = Max (Rsr; Rso; Rsd ) 

Rdmin = minimum desirable radius of a circular curve 

Rsd=radius which satisfies sight distance requirements

(SD)2 Rsd =--------------SD = sight distance 8m m =distance from curve obstruction

Minimum Desirable Rate of Superelevation 

Rsd

Line of sight

obstruction

sight distance

Superelevation 



Banking or superlevation is necessary to counteract the centrifugal force that is acting on the vehicle The value of maximum superelevation, e, may range from 6% to 12% depending on the terrain of the area where the highway will traverse

Superlevation

C L

Kawazu-Nanadaru Loop Viaduct/Bridge

Minimum Desirable Rate of Superelevation 

To ensure that a vehicle is traveling at the 99th percentile speed on a curve if minimum radius will experience an acceptable lateral acceleration 2 ed = M i n [ e ; V / 2 8 2 R d m i n ] m a x m a x

ed = m a x d e s i r a b l e v a l u e o f m a x s u p e r e l e v a t i o n r a t e

Minimum Desirable Amount of Extra Widening on Circular Curve 

Extra widening is often needed on curve because: 1) vehicles occupy a greater width on curves because their rear wheels generally track inside front wheels in rounding a curve, 2) drivers generally experience difficulties in steering their vehicle in the center of a lane

Extra Widening requirement 0.5nl2

0.105V

Wdmin=------------ + ---------Rdmin

Extra widening

(Rdmin)0.5

Wdmin=desirable min amount of widening n= number of lanes l =length of wheel base of the design vehicles (m) Rdmin =min desirable redius of curve

Widening on Road Curves Standard Widening of Curves Radius (m)

Design Speed (kph) 40

50

60

70

80

100

120

50

1.75

80

1.50

1.50

100

1.25

1.25

125

1.0

1.25

1.25

150

1.0

1.25

1.25

180

1.0

1.25

1.25

1.25

200

0.75

1.0

1.0

1.0

250

0.75

0.75

1.0

1.0

1.25

300

0.75

0.75

1.0

1.0

1.0

400

0.50

0.75

0.75

0.75

1.0

1.0

500

0.50

0.50

0.75

0.75

1.0

1.0

0.50

0.50

0.75

0.75

1.0

1.0

0.50

0.50

0.75

0.75

1.0

0.50

0.75

0.75

0.75

0.50

0.75

0.75

0.50

0.75

600 800 1000 1200 1500

Transition Curve 





A transition curve is sometimes needed to improve the driver’s operation and comfort and make steering easier and more accurate for a vehicle This is necessary because the centrifugal force which acts on a vehicle as it enters a circular curve may result in a lateral jerk (rate of change of centrifugal acceleration) which can cause a discomfort to the driver and passengers of the vehicle It is used to produce a smoother appearing transition that is more accurate to the character of the alignment

Transition Curve 



The most commonly used type of transition curve is a spiral curve (clothoid) which is used to enable a gradual introduction of the centrifugal force (or slower rate of change of lateral acceleration) It shall be designed between circular curves or between straight alignment and circle when radius of curvature is 750m or degree of curve > 1 to satisfy different condition of optic condition and dynamic condition



Optic Condition- the change in direction (deflection) shall be > 3degrees expressed as A = R/3 or Ls = R/9 The offset of the circular curve due to the transition curve shall be > 0.50 m to present a satisfactory aspect in perspective, expressed as: Ls = 12R

Limit between these two conditions is R = 972m ; Ls = 108m



Dynamic Condition Ls > V [e +e’] where e’ =NC (normal crown), % Superelevation Runoff Condition: Ls > SR (superelevation runoff) Adopt Ls > the length of corresponding to the most constraining condition

Minimum Desirable Length of Spiral Curve (Ls) 

A) Rate of increase of centrifugal acceleration must be constant and smaller than an established limit V3 Ls =-----------RC

Ls= length of spiral curve V = speed, kph

c = rate of increase of centrifugal value varies between 1&3

acceleration. Its

Minimum Desirable Length of Spiral Curve (Ls) 

Criterion 1: Rate of increase of centrifugal acceleration and superelevation is incorporated (Lsc) 2 Lsc = 0.022 Vd { [ V / R ] – 1 2 7 e } d d m i n d m a x



Criterion 2: Length must be consider to enable the introduction of the designed superelevation Lss = ed * 0 . 5 [ W + W e ] / R S ( i f p a v e m e n t i s r o t a t e d a b o u t t h e c e n t e r ) m a x Lss = ed * [ W + W e ] / R S ( i f p a v e m e n t i s r o t a t e d a b o u t t h e m a x



  

i n n e r e d g e )

Where: Lsc, Lss = length of transition curve to fulfill rate of change of lateral acceleration and to fulfill the rate of introduction of the designed superelevation requirements respectively RS allowable relative slope (0.2% for 1 lane, 0.5% for 2 lanes, 1% for 3 lanes and 2% for 4 lanes) ed = m a x i m u m d e s i r a b l e r a t e o f s u p e r e l e v a t i o n m a x W and We = normal pavement width and the extra widening at the circular curve, respectively Lsd = M A X ( L s c , L s s ) m i n

L s = m i n i m u m d e s i r a b l e l e n g t h o f t r a n s i t i o n s p i r a l d m i n

CEE 320 Winter 2006

Superelevation Transition

FYI – NOT TESTABLE

CEE 320 Winter 2006

Superelevation Transition

from AASHTO’sA Policy on Geometric Design of Highways and Streets 2001

Superelevation Runoff/Runout from AASHTO’sA Policy on Geometric Design of Highways and Streets 2001

CEE 320 Winter 2006

FYI – NOT TESTABLE

FYI – NOT TESTABLE New Graph

CEE 320 Winter 2006

Superelevation Runoff - WSDOT

from the 2005 WSDOTDesign Manual , M 22-01

FYI – NOT TESTABLE

Spiral Curves

No Spiral

CEE 320 Winter 2006

Spiral

from AASHTO’sA Policy on Geometric Design of Highways and Streets 2001

FYI – NOT TESTABLE

CEE 320 Winter 2006

No Spiral

Spiral Curves

CEE 320 Winter 2006

• • • •

Involve complex geometry Require more surveying Are somewhat empirical If used, superelevation transition should occur entirely within spiral

CEE 320 Winter 2006

Desirable Spiral Lengths

from AASHTO’sA Policy on Geometric Design of Highways and Streets 2001

Superelevation Road Section View CL 2%

2%

Road Plan View

Superelevation Road Section View CL 1.5%

2%

Road Plan View

Superelevation Road Section View CL

1%

2%

Road Plan View

Superelevation Road Section View

0.5%

CL

2%

Road Plan View

Superelevation Road Section View CL -0.0%

2%

Road Plan View

Superelevation Road Section View

-0.5%

CL 2%

Road Plan View

Superelevation Road Section View -1%

CL

2%

Road Plan View

Superelevation Road Section View -.5%

CL

2%

Road Plan View

Superelevation Road Section View

-2%

CL

Road Plan View

2%

Superelevation Road Section View -3%

CL

3%

Road Plan View

Super elevation Road Section View -4%

CL

4%

Road Plan View

Superelevation Road Section View -3%

CL

3%

Road Plan View

Superelevation Road Section View -2%

CL

2%

Road Plan View

Superelevation Road Section View -1.5%

CL

2%

Road Plan View

Superelevation Road Section View -1%

CL

2%

Road Plan View

Superelevation Road Section View -0.5%

CL

2%

Road Plan View

Superelevation Road Section View -0.0%

CL

2%

Road Plan View

Superelevation Road Section View 0.5%

CL

2%

Road Plan View

Superelevation Road Section View 1%

CL

2%

Road Plan View

Superelevation Road Section View 1.5%

CL

2%

Road Plan View

Superelevation Road Section View 2%

CL

2%

Road Plan View

In actual design of a horizontal curve, the engineer must select appropriate values of e and fs .





Super-elevation value ‘e’ is critical since 





high rates of super-elevation can cause vehicle steering problems at exits on horizontal curves and in cold climates, ice on road ways can reducefs a n d v e h i c l e s a r e f o r c e d i n w a r d l y o f f t h e c u r v e b y g r a v i t a t i o n a l f o r c e s .

Values of e ‘ ’ and f‘ s ’ c a n b e o b t a i n e d f r o m A A S H T O s t a n d a r d s .

Horizontal Curve Fundamentals 





For connecting straight tangent sections of roadway with a curve, several options are available. The most obvious is the simple curve, which is just a standard curve with a single, constant radius. Other options include; 



compound curve, which consists of two or more simple curves in succession , and spiral curves which are continuously changing radius curves.

Basic Geometry

Tangent

Horizontal Curve

Tangent

Tangent Vs. Horizontal Curve 



Predicting speeds for tangent and horizontal segments is different May actually be easier to predict speeds on curves than tangents 



Speeds on curves are restricted to a few well defined variables (e.g. radius, superelevation) Speeds on tangents are not as restricted by design variables (e.g. driver attitude)

Elements of Horizontal Curves PI E

T

M

L

 



PC

PT

R

R 

   

Stopping Sight Distance and Horizontal Curve Design SSD

Ms

Highway Centerline

Sight Obstructio n

Rv

s

Critical inside lane

170

171

172

Source:CalTrans Design Manual online, http://www.dot.ca.gov/hq/oppd/hdm/ pdf/chp0200.pdf

173

Same as point E of GB

174 Source: Iowa DOT Standard Road Plans

Same as point E of GB

With Spirals

175 Source: Iowa DOT Standard Road Plans RP-2

With Spirals

Tangent runout (A to B)

176

With Spirals

Removal of crown

177

With Spirals

Transition of superelevation

Full superelevation

178

179

Selection of a type of horizontal curve   

Is a transition curve needed? Does a simple curve fit the site condition? If not, what is the most appropriate compound curve that fits the site conditions?

Selection of a type of horizontal curve To answer the questions, a designer needs to know the threshold values of circular and transition curves Step 1: Compare Rdmin with Vd3/432: if Rdmin is less, then transition spirals are required, otherwise, only a circular curve is required. Step 2: Compare the value of angle of transition curve with the total deviation angle (angle between the two intersecting straights). If the angle of transition is equal to half of deviation angle, then transition curves can be used throughout. Otherwise, a combination of circular and transition curve is required 

Selection of a type of horizontal curve To answer the questions, a designer needs to know the threshold values of circular and transition curves Step 3: Check whether the curve obtained through steps 1 and 2 is compatible with the site conditions. If yes, use the curve. If not, select a suitable compound curve. Two criteria must be satisfied. 1) minimum radius of the compound curve must not be less than the minimum desirable radius 2) adequacy of the curve selected must be checked using this figure:



Selection of a type of horizontal curve 

After selecting the appropriate type of horizontal curve, TL12, the length of hor tangent between two curves must be check to ensure that it is greater than the critical length of tangent which is given as CTL = MAX [{10R10.5}; 2Vd] CTL = critical tangent length and R1 is the radius of the larger curves (consider the max radius) If TL12 is less than CTL, a compound curve must be used to replace the curves 1 and 2



Sample Problem 

The design speed of a highway is 80 kph. However, due to economic and other reasons, a horizontal curve of radius of 200m is on the highway. Can this speed be safely maintained on the road? If not, what should be done?

Solution: Step 1) Estimate the actual value of superelevation on the horizontal curve. Assuming that emax =0.07. Using the eqn of (80)2/282*200]

edmax = MIN [0.07;

e = 0.113 edmax = 0.07 (not safe)

Solution: Step 2) Compare the friction demand and maximum available friction Based from the friction demand (fd)= [(80)2 / 127 * 200] – 0.07 = 0.18 fsmax =0.37{0.0000214 * Vd2 – 0.0064 Vd + 0.77} = 0.37{0.000214 * (80)2 – 0.0064 (80) + 0.77} = 0.146 Since fd > fsmax; 80 kph is not safe speed on the curve The maximum desirable speed is given as {127*200 (0.07 + 0.146)}2 = 74 kph Thus, the speed on the curve should be restricted to 74 kph through use of signs and markings

Sample 2 

A truck with a center of gravity at x=1.2m and y=1.5m is expected to travel on a circular curve. The design speed on the curve is 80 kph and the minimum desirable sight distance is 150m. In addition, the expected distance of the obstruction from the road center line is 10m. Determine the minimum desirable values of the curve parameters.



Solution 

Step 1) Checking of skidding condition (80)2/127Rsr = e + fsmax since it is generally recommended that emax + fsmax should not be greater than 0.22. (6400)/127Rsr = 0.22 Rsr = 230 m

Step 2) Check condition of stability against overturning V2/127Rso = [x+ye]/[y-xe] X

V=80 e=emax: 6400/127Rso = [x+ye]/[y-xe] y

= [1.2+(1.5*0.07)]/[1.5-(1.2*0.07)] = 0.92 Thus, Rso =6400/(127*0.92)

e V2 > x + ye 127 R

y – xe

Rso =55m

Step 3: Check Sight Distance condition Rsd = (SD)2/8m Rsd = (150)2 / (8*10)= 280m Step 4: Estimate the minimum desirable value of radius Considering all the conditions: Rdm = M A X [ 2 8 0 , 2 3 0 , 5 5 ] i n Therefore, Rdmin = 280m

Step 5: Estimate the minimum desirable maximum rate of superelevation 

Assume em = 0 . 0 7 a x

2 edm = M I N [ 0 . 0 7 ; V / 2 8 2 * R d m i n ] a x

2 edm = M I N [ 0 . 0 7 ; ( 8 0 ) / 2 8 2 ( 2 8 0 ) ] a x = MIN [0.07; 0.081] edm = 0 . 0 7 V2 > x + ye a x 127 R y – xe Checking adequacy of results: There will be no problem of overturning if That is [6400/127*280] =0.18 This is less than [1.2+(1.5*0.07)/[1.5-(1.2*0.07)] = 0.92 Therefore the combination of R=280 and e=0.07 is OK!

Technique for Design of Vertical Alignment The vertical alignment of a road consist of tangent grades connected with parabolic vertical curves (Crest or sag curves).Vertical alignment is the profile view of the centerline of the road consisting of tangent grades connected by vertical curves

Vertical Curves 

Vertical Curves can be circular or parabolic curves. Parabolic curves are preferred by many agencies because they provide a constant rate of curvature 



Maximum and Minimum Length of a Symmetrical Crest Vertical Curve Maximum and Minimum Length of Symmetrical Sag Vertical Curve

Gradient or Grade 





Gradient is the rate of rise and fall on any length of road with respect to the horizontal. The gradient and length of tangent depend generally on the terrain and the design speed. In general, maximum grade is considered to be 10-12% for a length of tangent of 150m.

Control Grades for Design Level

Rolling

Mountainous

Freeway/ Xpressway

3-4%

4-5%

5-6%

Rural Arterials

3-5%

4-6%

5-8%

Urban Arterials

5-8%

6-9%

8-11%

Collector/ Secondary Local or Minor Street

4-7% (rural) 5-10% (rural) 5-9% (urban) 6-12% (urban) 5-8%

6-11%

6-12% (rural) 7-14% (urban) 10-16%

Min and Max Length of a Symmetrical Crest Curve Vcldmin = Max {Lsd;Lc;La} Vcldmin=min desirable length of crest vertical curve Lsd=length of curve to satisfy the sight distance requirement Lc=length of curve to satisfy the comfort requirement La=length of curve to satisfy appearance requirement

Min and Max Length of a Symmetrical Crest Curve SD2A Lsd =--------------------------

where: A=algebraic difference in grades

A=G1 G 2

(a+b+2c0.5+b0.5)200 a=eye height above the road surface b=object height above the road surface Lc =[V2A]/389 La = 2V

Based fro AASHTO Vcldmin=51A,

A is in %

Min and Max Length of a Symmetrical Sag Curve 

For sag curves, the main sight distance criterion to be satisfied is night visibility Vsldmin=Max[Lnv;Lc;La]

Vsldmin=min desirable length of sag vertical curve SD2A Lnv=min length of sag curve to satisfy night Lnv=----------------------- visibility [2h+2SDtanθ]100 h=height of headlight above the surface (assume to be 0.6m) θ=beam angle, 1 degree

Min and Max Length of a Symmetrical Sag Curve s

θ Lc = [V2A]/389 La = 2V

Design Controls for Crest Vertical Curves

from AASHTO’sA Policy on Geometric Design of Highways and Streets 2001

from AASHTO’sA Policy on Geometric Design of Highways and Streets 2001

Design Controls for Crest Vertical Curves

Sag Vertical Curves

Light Beam Distance (SSD)

G1

headlight beam (diverging from LOS by β degrees) PVT

PVC

h1

PVI

L

For SSD < L L



A  SSD  200  h1

2

 S tan  

G2

L

 2  SSD  

h2 = 0

For SSD > L 200  h1

  SSD A

 tan



GEOMETRIC ELEMENT DESIGN

Combination of Horizontal Alignment and Vertical Alignment • The combination of horizontal and vertical alignment is the final check of design and belongs to one of the most difficult procedure in geometric design • Harmony of geometric alignment • Design should satisfy safety running, visual and psychological comfortableness. • Economical and less negative impact in environment

Combination of Horizontal Alignment and Vertical Alignment Route Location

Determination of Horizontal Alignment

Combination of Horizontal and Vertical Alignment

Determination of Vertical Alignment

GEOMETRIC ELEMENT DESIGN

Combination of Horizontal and Vertical Alignment

Good design

Bad design

Plan

Plan

Profile

Profile

Comment: A very satisfactory appearance results when vertical and horizontal curves coincide. Keep vertical curve within horizontal curve

Comment: This combination is dangerous as the reverse curvature of the alignment is obscured from the driver’s view by the crest.

Plan

Plan

Profile

Profile

Comment: Ideal coordination between HA & VA, vertices of curves coinciding, creating a rich combination

Comment: The summit vertical curve restricts the driver’s view of the level crossing (or road intersection, start of a horizontal curve or other hazard) and produces a dangerous situation

Road Intersection

GEOMETRIC ELEMENT DESIGN

Combination of Horizontal and Vertical Alignment

Good design Plan

Bad design Plan

Bridge

Profile

Obstructio n

Profile Comment: This is the most effective way to display a bridge. Bridge is visible to the driver.

Comment: This situation always look bad. It is much better to begin the detour before the driver is aware of the reason for it.

General Idea of Design: 



RH(m)

HA & VA should be superimposed; keep vertical curve within horizontal curve. Keep the balance of VA & HA RVm=100[LVC/g2-g1),%]

RHm

RVm=100[LVC/g2-g1)%]

500

10000

1000

10000

700

12000

1100

30000

800

16000

1200

40000

900

20000

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 



CROSS SECTION IS THE VERTICAL PLANE (SECTION) PERPENDICULAR TO THE LINEAR DIRECTION OF THE ROAD CROSS SECTION DESIGN INVOLVES TWO MAIN TASKS 



USE THE VALUES OF THE QUALITY OF DESIGN TO ESTIMATE THE CONTROL VALUES OF ROADWAY AND ROADSIDE DESIGN PARAMETERS JUDGEMENT ON THE BASIS OF TERRAIN CONDITION, COST, ETC

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Built up areas: cross section consists of basically different types of traffic ways, parking spaces and medians. The set of traffic ways for vehicles is known as carriageway; the combination of carriageways, parking spaces and medians is a roadway MEDIAN CARRIAGEWAY

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 



FOR ROADS OUTSIDE THE BUILT UP AREAS: CROSS SECTION CONSISTS OF ROADWAY (WITH ONE OR MORE CARRIAGEWAYS) AND A ROADSIDE FOR INTERSECTIONS, CROSS SECTION GENERALLY CONSISTS OF OPEN SPACES AND TRAFFIC

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Cross Section Design involves the determination of the following:  



Appropriate type of cross section Appropriate configuration of the type of cross section Appropriate dimensions of each of the elements of the configiration

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Estimation of Control Values of Design Parameters a) Width of lane,W = Wv + S + S m f

W = w i d t h o f d e s i g n v e h i c l e ( m ) v Sm = s p a c e r e q ’ d f o r l a t e r a l m o v e m e n t s ( m ) = 0.15-0.30 @ V=30 kph = 0.25-0.40@ V=50 kph = 0.40-0.50@V=70 kph or above Sf= space req’d due to fear of sidewalk or object (trees, parked veh, etc) = 0.25-0.40 for sidewalk; 0.50-0.60 for objects @ V = 30 kph = 0.35-0.50 for sidewalk; 0.70-0.80 for objects @ V = 50 kph = 0.50-0.60 for sidewalk; 0.90-1.0 m of robjects @ V = 70 kph

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Estimation of Control Values of Design Parameters 

For Built Up Areas 

W = (Vd/80) + 2



Width of a bicycle lane 



Wbc = [2=3a] / 4 by side = 2
a=design number of bicycles riding side

For other type of conveyor (road) Wsc = Wv + 0.50 Wsc =minimum desirable width of special conveyor Wv =width of special conveyor

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Minimum Width of Safety Enhancement Places (Shoulders and Parking Lanes) 



The minimum desirable width of a safety enhancement place depends on whether the place is designated to be used as a shoulder or a parking lane. In the case of a shoulder the usable width is usually between 0.6 and 3.6m-depending on the class of road and traffic volume. The usable width of a shoulder can be determined as:   

Ws = minimum desirable useable width o shoulder (m) Wv = width of design vehicle (m) Cw = clearance (usually between 0 and 1.5)

In parallel parking, the minimum and desirable width are 3.0 and 3.6m respectively

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Minimum Desirable Number of Traffic Lanes for each Category of Conveyors 

The desirable number of traffic lanes for each designated category of conveyors is dependent on the design entry flow rate and design service flow rate and can be estimated as:

Ndmin = DDFR/DSFR Ndmin = desirable minimum number of lanes DDFR = design demand flow rate in pcph (per car per hour) DSFR = design service flow rate (pcph per traffic lane)

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Minimum Desirable Number of Traffic Lanes for each Category of Conveyors

DDFR = [AADT*EK*PF*DD]/ PHF AADT = forecast average annual daily traffic EK = percent of AADT during the peak hour = 0.12-0.20 for rural routes = 0.07 – 0.12 for urban routes PF = proportion of conveyor category in a peak period traffic stream DD = directional distribution factor = proportion of traffic of road which moves in the major direction PHF = peak hour factor

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN 

Minimum Width of Median 



The minimum width of a median or center reserve, when necessary, can be between 2m up and 30 m depending on the available right of way. The minimum desirable width depends on the purpose of the median. A median may be designed for safety purposes in terms of (1) preventing accidents caused by crossover traffic, headlight glare distraction and traffic turning left from through lanes, (2) provide refuge for pedestrians crossing the highway

TOOLS AND TECHNIQUES FOR CROSS SECTION DESIGN • Cross Slope or Camber of lanes or Crossfall – slope provided to the surface of a lane in the transverse direction to drain off water from the surface.

Types of Surface

Range of camber in Areas of Rainfall Range Heavy

To

Light

Cement Concrete & High Type Bituminous surface

2.0%

To

1.70%

Thin Bituminous Surface

2.5%

To

2.0%

Water Bound Macadam, and gravel Pavement

3.0%

To

2.5%

Earth Surface

4.0%

To

3.0%

Roadside Slopes Shoulder

Cut Slope

Carriageway

Shoulder

1 S

Fill Slope

Shoulder

Carriageway

Shoulder

1 S 

Main design parameters for the roadside slope  Roadside slopes (embankment or cut) – the alternatives are either to select a steep slope near the roadway and shield it with a traffic barrier or select a flat slope  Drainage ditches – this will base from the slope of the drainage

Highway Geometric Design Process Specify Design Standards • Local agency, • Functional Design

Survey Major Site Characteristics • Topography • Ground/soil conditions • Hydrologic conditions • Environmental conditions/impacts

(Design Speed, cross sectional features, superelevation)

• Habitat • Existing/zoned land use

Specify Design Standards Local Agency  Functional classification (Ex. Design speed, cross sectional, superelevation) 

     

Highway Geometric Design Process



Survey Major Site Characteristics Topography  Ground/soil condition  Hydrologic condition  Environmental condition  Existing/zoned land use

Specify Major Design Controls Design vehicl Design speed Volume/capacity/LOS Horizontal controls(radius, superelevation) &Vertical controls (grades, intersection) Cross section controls/elements Earthwork controls (excavation, embankments)

Select Route  Identify alternate routes  Sketch horizontal and vertical alignments  Screen routes and select those for preliminary design





Conduct (Preliminary) Design Horizontal Alignment  Vertical Alignment  Cross sections  Drainage features  Earthwork  Environmental impacts

Evaluate Design Costs (project costs, user costs)  Environmental impacts

Detailed Design

Modificati ons?

Existing ground surface

Propose road surface

Types of Road Intersection 

The type of intersection may be divided intro 3 categories. One is from its shape, and one is from its structural type and another is from its operational type. 

Shape 

Three-leg intersection

T-type

Y-type



Four Leg intersection

Right Angle

Oblique



Multi leg intersection



Rotary intersection (Roundabout)

Structural Type of Intersection 



Intersection at Grade: two or more roads intersection or join at the same level Grade separated intersection: when one highway crosses another at grade, the capacity is reduced  

Structures with interchange Structures without intechange

GEOMETRIC DESIGN STANDARDS Traffic Forecasting/Traffic Design Data

Geometric Design

• number of lanes •width of lanes •design speed

Pavement Design

• type of pavement •thickness of subgrade, subbase, base courses •thickness of pavement

GEOMETRIC DESIGN STANDARDS Traffic Information * use for structural design purposes * basis for traffic demand forecasting * traffic volume estimation

Traffic loading information may be obtained from the following sources:

* Tabulated average 80 Kn equivalent Single Axle Load per Lane * Traffic survey (visual or traffic counting) * Transportation planning models (trip generation models)

GEOMETRIC DESIGN STANDARDS Traffic loading information may be obtained from the following sources:

* Tabulated average 80 Kn equivalent Single Axle Load per Lane Traffic Class

Cumulative Equivalent Traffic (E80/lane)

Description

<0.20 x 106

Very light trafficked, very few heavy vehicles

0.2 – 0.8 x 106

Lightly trafficked roads, light delivery and agricultural vehicles, very few heavy vehicles

3 (Major Collector Roads)

0.8 - 3 x 106

Medium volume of traffic; few heavy vehicles

4 (Highway)

3 - 12 x 106

High volume of traffic and/or many heavy vehicles

5 (Expressway)

12 - 50 x 106

Very high volume of traffic and/ or high proportion of fully laden heavy vehicles

1 (Feeder Roads) 2 (Collector Roads)

GEOMETRIC DESIGN STANDARDS

Traffic forecasting

* forecasting of average daily traffic can be projected growth factor (urban area is 20% and rural area 5%) Formula for traffic forecasting :

Pn = Pi (1 + r ) n where: Pn =projected number of traffic Pi = initial average daily traffic r = growth rate n = number of projected years

using a



GEOMETRIC DESIGN STANDARDS Sample problem: Traffic Forecasting

Determine the possible number of average vehicle per day of a rural road if the initial average daily traffic is 200, locality growth rate is 6% and design years is 5 years.

Sol’n: Working Formula: Pn = Pi (1 + r )n Given: Pi = 200 ADT, r = 6% and n = 5 Pn = 200 (1 + 0.06) 1 =212 Pn = 200 (1 + 0.06) 2 =224.72~225 Pn = 200 (1 + 0.06) 3 =238.20 Pn = 200 (1 + 0.06) 4 =252.50 Pn = 200 (1 + 0.06) 5 =267.65~268

GEOMETRIC DESIGN STANDARDS Safety * optimizing by linking geometric elements to design speed * design standards must take into account the environmental road conditions, traffic characteristics and drivers behavior * potential collision risk

Design Speed * design varies with different terrain * provide appropriate consistency between geometric elements * mountainous and rolling areas speed limits as low as 40 kph * review of design speed to ensure that they relate to current circumstances

Transportation System Description: Networks and Data 

Study Area   

Delineation of the study area Subdivision of the study area into zones Definition of zone centroids

Propose road

Transportation System Description: Networks and Data 

Transportation models are being used to make prediction and forecasts of future changes in usage of traffic facilities for sake of facility design, control and operation.

Travel Choice Model System    

Trip Generation Model Trip Distribution Model Modal Choice Model Route Choice

Zonal Data

Trip Production

Trip frequency choice

Trip Attraction

Transport Networks

Trip Distribution

destination choice

Modal Split Travel Resistances

modal choice

Period of day

Assignment

Time choice

route choice

Network loads, travel times, etc