Geometric Standards

8/2/2019 Geometric Standards

1/28

SECTION 3

GEOMETRIC STANDARDS

Section

3


2/28

Rural Roads - Design ManualSection 3 Geometric StandardsRepublic of YemenMinistry of Public Works and Highways

SECTION 3

GEOMETRIC STANDARDS

TABLE OF CONTENTS

Page

3.1 Introduction .........................................................................................1

3.2 Sight Distance.......................................................................................13.2.1 Stopping Sight Distance .........................................................1

3.2.2 Intermediate Site Distance.......................................................3

3.3 Superelevation......................................................................................3

3.4 Horizontal Alignment.............................................................................73.4.1 Circular Curves ..................................................................7

3.4.2 Transition Curves ................................................................8

3.4.3 Improving Horizontal Alignment ............................................. 10

3.4.4 Geometric Controls ............................................................ 10

3.4.5 Widening on Curves ........................................................... 11

3.5 Vertical Alignment...............................................................................

123.5.1 Elements of Vertical Alignment...............................................

13

3.5.2 Crest Curves ................................................................... 14

3.5.3 Sag Curves ..................................................................... 17

3.5.4 Gradient ........................................................................ 17

3.5.5 Climbing Lanes ................................................................ 18

3.6 Cross Section ......................................................................................193.6.1 Rationale for Determining Road Widths ...................................... 19

3.6.2 Carriageways and Shoulders..................................................

203.6.3 Cross Slope .................................................................... 23

3.6.4 Passing Places ................................................................. 24


3/28


SECTION 3

GEOMETRIC STANDARDS

3.1 INTRODUCTION

As mentioned in Section 2, the geometric features for most of the rural

roads in Yemen are governed by the natural terrain characteristics.

However, this does not exclude the fact that the geometric design should be

consistent with the traffic volume, composition of traffic and design speed.

This Section provides a summary of the geometric design data and containssufficient information for the majority of roadway design problems. A

Policy on Geometric Design for Highways and Streets by the American

Association of State Highways and Transportation Officials (AASHTO) is

a reference in which the basic theory behind geometric design data is fully

explained. Also, Guidelines for Geometric Design of Very Low-Volume

Roads (ADT400) by AASHTO, and TRRL Road Note No. 6 A guide

to Geometric Design can be consulted.

Several design standards from projects previously undertaken in Yemen

have been reviewed. The Consultants have taken into consideration the

technical aspect combined with the specific requirements of this project in

developing a new set of geometric standards. The review covers the sight

distance, horizontal and vertical alignment and cross sectional elements as

related to traffic volumes and design speeds.

3.2 SIGHT DISTANCE

Sight distance is the length of roadway ahead visible to the driver.

Ability to see ahead is of utmost importance in the safe and efficient

operation of a roadway. If safety is to be built into the roadways, the

designer must provide sight distance of sufficient length in which drivers

can control their vehicles so as to avoid striking an unexpected obstacle on

the traveled way.

Two sight distances are considered in design of bi-directional carriageway

for rural intermediate and village access roads: Stopping Sight Distance

and Intermediate Sight Distance.


4/28


Stopping sight distance is generally determined as the sum of two

distances:

(1) Reaction Distance, the distance traveled by the vehicle from the

instant the driver sights an object necessitating a stop to the instant

the driver actually applies the brakes. This distance depends on the

reaction time of the driver which varies according to the alertness of

the driver. AASHTO Policy on Geometric Design for Highways

and Streets uses a brake reaction time of 2.5s, while AASHTO

Guidelines for Geometric Design of Low-Volume Local Roads

(ADT 400) recommends a reaction time of 2s for rural roads.

(2) Braking Distance, the distance required to stop the vehicle from

the instant the brakes are applied. This distance is a function of the

longitudinal friction factor, and thus deceleration of the vehicle.

The stopping sight distance in the AASHTO Policy is given by the

following formula which has two components corresponding to the two

distances mentioned above:

aV039.0Vt278.0S

2

+=

where,

S = stopping sight distance, m

t = brake reaction time, s

V = design speed, kph

a = driver deceleration, m/s2

Table 3.1 shows a comparison between minimum sight distance standards

for AASHTO and TRL, for very low volume roads.

Table 3.1 Comparison of Minimum Stopping Sight Distance Standards

Minimum Stopping Sight Distance (m)

AASHTO(1)

TRL Road Note 6(2)Design

Speed KphADT < 250 ADT 250 400 fL Smin

20 15 15 __ __

30 25 30 0.6 2540 35 40 0.55 35

50 45 55 0.50 50

60 60 70 0.47 65

70 75 90 0.43 85

80 95 110

85__

0.40 120


5/28


As discussed in Section 3.5.2, sight distance plays a key role in determining

the minimum lengths of crest vertical curves. Stopping sight distance is

measured from the drivers eyes (eye height, h1) to an object height (h2).

AASHTO policy uses h1 = 1080 mm and h2 = 150mm or h2 = 600mm if the

object is a vehicle. With the increased use of SUVs, the average eye height

has increased, so that h1 could be assumed to be 1.4m and h2 15cm. This

sight distance criterion should be checked for all classes of roads irrelevant

of the number of lanes, traffic volume or pavement type. Measures to be

taken to correct any deficiency include removal of obstacles, excavation of

side slopes or trimming of sharp crest curves.

3.2.2 Intermediate Site Distance

The Intermediate sight distance is the distance needed for two drivers

traveling with design speed to stop before colliding. This criterion is valid

in the case of one-lane roads. For village access roads having a traffic load

of less than 50 vehicles/day the intermediate sight distance can be neglected

if the lane is widened up to at least 4.5m.

Table 3.2 shows the proposed minimum normal and relaxed standards forsight distances related to design speeds for RAP roads.

Table 3.2: Minimum Standards for Sight Distances Related to Design SpeedsMinimum Sight Distance, m

Stopping Intermediate

Rural Intermediate Village Access

Design

Speed,

KPHNormal Relaxed Normal Relaxed

Rural

Intermediate

Village

Access

20 20 15 15 15 50 30

30 35 30 30 25 80 5040 50 40 40 35 110 70

50 65 55 - 45 150 -

60 85 70 - 200 -

80 130 110 - 300 -

100 160 155 - 380 -

120 230 540

Sight distance could also be related to type of terrain if the design speed for

each terrain is specified. It must however be recognized that in each terrain,owing to local topographical changes, higher or lower speeds than

recommended may apply.

Sight distances are also affected by vertical grade and obstacles along the

side of the roadway on horizontal alignment.


6/28


Superelevation may be defined as the rotation of the roadway cross section

in such a manner as to overcome the centrifugal force that acts on a motor

vehicle traversing a curve. On a superelevated carriageway, the centrifugal

force is resisted by:

1. The weight component of the vehicle parallel to the superelevated

surface and

2. The side friction between the tires and the pavement.

It is impossible to balance centrifugal force by superelevation alone,

because for any given curve radius, a certain superelevation rate is exactly

correct for only one operating speed around the curve. At all other speeds,there will be a side thrust outward or inward relative to the center of the

curve, which must be offset by side friction.

The general formula to calculate superelevation for various curve radii is

the following:

e+f = V2

/ 127R

where,

e = Superelevation rate, in meter per meter width of road.

f = side friction factor or coefficient of side friction between vehicle tires

and road pavement.

R = radius of curve, in meters.

V = design speed in kph.

The value of f shall be obtained from an expression which recognizes that

the value of the side friction varies with the speed of travel, the loss in therubber tread and the natural condition of road surface. The acceptable value

borne out by practice on similar roads is given by:

f = 0.19 0.0006V

where V is the same value above.

Superelevation should not be so excessive as to cause a stationary vehicle

to slide down the cross slope, regardless of the nature and condition of theroad surface. Superelevation rate shall not be less than the rate of crown

slope (Table 3.3), i.e. camber or crossfall.

Table 3.3: Horizontal Curve Design Data Maximum Superelevation

Superelevation %


7/28


Superelevation slopes on curves shall extend the full width of the

shoulders, except that the shoulder slope on the low side shall not be less

than the minimum shoulder slope used on tangents.

For 2-lane roadways, different superelevation slopes for each half of the

road shall not be used; superelevation shall remain a plane for the full width

of roadbed, except on transitions.

The axis of rotation for superelevation is usually the centerline of the road.

However, in special cases such as desert roads where curves are preceded

by relatively long tangents, the plane of the superelevation may be rotated

about the inside edge of pavement to improve perception of the curve. Inlevel country, drainage pockets caused by superelevation may be avoided

by changing the axis of rotation from the centerline to the inside edge of the

pavement.

Superelevation transition is the general term denoting the length of

highway needed to accomplish the change in cross slope from a normal

crown section to the fully superelevated section, or vice versa. To meet the

requirements of comfort and safety the superelevation run-off should beeffected uniformly over a length adequate for the likely travel speed. The

superelevation transition can be divided into two sections defined as

follows:

- Tangent Run-off or Run-out: This is the distance in which the level

of the edge of pavement is raised to a horizontal plane with the

centerline grade through the axis of rotation.

- Superelevation Run-off: This is directly proportional to the total

Superelevation, which is the product of the lane width and the

superelevation rate.

Length of run-off on this basis is directly proportional to the total

superelevation, which is the product of the lane width and the

superelevation rate. Section 3.4.2 below shows how to calculate the

superelevation run-off distance using the rate of pavement rotation method.

Figure 3.1 shows typical details for superelevation runoff.


8/28

Rural Roads - Design Manual

Section 3 Geometric StandardsRepublic of YemenMinistry of Public Works and Highways

DOCS-0974-04 3-6

Figure 3.1 Typical Details for Superelevation Runoff

Inside Edge of

Roadway (P.G.L)

Outside Edge of

Roadway

1 / 4 LA

or 10m Max

Tangent Run out Length of Superelevation Runoff

Length of Application (as shown on the profile)

Total Length of Application

1 / 4 LA

or 10m Max

or 10m Maxor 10m Max

Inside Edge of

Roadway (P.G.L)

Outside Edge of

Roadway1 / 4 LA

or 10m Max 1 / 4 LA

or 10m Max


Outside Edge ofRoadway

Superelevation Application Details LA

+e

+eo

-eo

-e

+e

+eo

-eo

-e

B

B/2e

B/2e

B/2e2

B/2e2

B/2e

B/2e

+e

-eo

-e

+e

+eo

-e

P G L C/L

B/2e2

B/2e2

Sign Convention for CrossfallMethod of Attaining Superlevation of

Pavement Revolved about

Centerlines of Roadways

e

e

Inside Edge of

Roadway (P.G.L)

Outside Edge of

Roadway

1 / 4 LA

or 10m Max

Tangent Run out Length of Superelevation Runoff

Length of Application (as shown on the profile)


1 / 4 LA

or 10m Max

or 10m Maxor 10m Max

Inside Edge of

Roadway (P.G.L)

Outside Edge of

Roadway1 / 4 LA

or 10m Max 1 / 4 LA

or 10m Max


Outside Edge ofRoadway

Superelevation Application Details LA

+e

+eo

-eo

-e

+e

+eo

-eo

-e

B

B/2e

B/2e

B/2e2

B/2e2

B/2e

B/2e

+e

-eo

-e

+e

+eo

-e

P G L C/L

B/2e2

B/2e2

Sign Convention for CrossfallMethod of Attaining Superlevation of

Pavement Revolved about

Centerlines of Roadways

e

e


9/28


3.4 HORIZONTAL ALIGNMENT

3.4.1 Circular Curves

The natural terrain, as mentioned earlier, governs the horizontal alignments.

The roads that are located on escarpments are therefore characterized by the

multitude of hairpin curves that necessitate a back and forth maneuver in

order to make the turn. For these roads, no minimum curvature can be

specified, as speeds will drop to zero during the maneuver.

For the remaining cases, the minimum radii will have to correspond to the

design speeds as per the recommendations listed in Table 3.4, determinedusing the superelevation equation defined above:

R =f)(e127

V2

+

Table 3.4: Horizontal Curve Design Data Minimum Radii (m)

Rural Intermediate Roads

Escarpment

Design

Speed

kph

fmaxFlat/Rolling

emax = 8%

Mountainous

emax = 6% emax =4% emax =6%

VillageAccess

Roads

emax = 6%

20 0.18 - 15(1)

15(1)

15(1)

15

30 0.17 - 30 35 30 30

40 0.17 50 55 60 55 55

50 0.16 80 90 100 90 90

60 0.15 125 135 150 135 135

80 0.14 230 250 -

100 0.12 395 435 -

120__

600 - -

(1) Not applicable for hairpin curves. Minimum radii to be 12-15m in mountainous and

escarpment areas. For hairpin curves a compound curve 15-10-15 may be used instead of

15m or 20m simple curve radius.

The minimum radius is a limiting value for a given design speed

determined from the maximum rate of superelevation and the maximum

side friction factor. Use of sharper curvature for that design speed would

call for superelevation beyond the limit considered practical or for

operation with tire friction beyond safe limits, or both


10/28


Design Speed, Kph 30 40 50 60 70 85 100 120

Side Friction Factor 0.33 0.30 0.25 0.23 0.20 0.18 0.15 0.15

The values for horizontal curve design shown in Table 3.4 should be used

for rural roads when practical. In constrained situations relaxed values

based on reduced design speed shown in Table (3.5) may be used.

In cases where the existing curve has a radius less than those listed, and

widening entails land acquisition, high excavations or high fills, signs shall

be posted to reduce the speed to correspond to the adopted radius.

Table (3.5) Horizontal Curve Design Data Minimum Radii (m)

for Reduced Design Speed


Flat / Rolling Mountainous Escarpment

Village

Access

Roads

Design

Speed

kph

Reduced

Design

Speed

kph

fmax

emax = 8%

emax = 6% emax = 4% emax = 6% emax = 6%

20 20 0.180__

15(1)

15(1)

15(1)

15

30 25 0.170__

20 25 20 20

40 30 0.170 30 30 35 30 30

50 40 0.170 50 55 60 55 55

60 50 0.160 80 90 100 90 90

80 65 0.145 150 160 __ __ __

100 85 0.135 265 290 __ __ __

120 100 0.125 385 __ __ __ __

(1) Not applicable to hairpin curves. Minimum radii shall be 12-15m. in mountainous and escarpment areas. Forhairpin curves, a compound curve of 15-10-15m may be used instead of a 15m or 20m simple curve.

Note: The above data are for constrained situations based on reductions in design speed up to 15 kph. These areapplicable to roads with ADT 250-400 vpd with limited heavy vehicle traffic (see AASHTO Guidelines for

Geometric Design of Very Low-Volume Local Roads (ADT400)).

3.4.2 Transition Curves

Transition curves are not normally used in Yemen. This section, explains

how to design them if they are required.

T iti b i t d b t t t d i l t


11/28


Several methods exist for the calculation of transition curves and may be

used in most situations. The rate of pavement rotation method has been

adopted here. The rate of pavement rotation is defined as the change in

crossfall divided by the time taken to travel along the length of transition atthe design speed. The length of transition curve is derived from the

formula:

3.6n

V.eLs =

where Ls = Length of transition curve (meters)

e = Superelevation of the curve (meters per meter)

V = Design speed (km/h)N = Rate of pavement rotation (meters per meter per second)

The same values of rate of change of pavement rotation should be used to

calculate the minimum length (Lc) over which adverse camber should be

removed on a tangent section prior to the transition:

3.6n

V.eL nc =

where Lc = Length of section over which adverse camber is removed

(meters)

en = Normal crossfall of the pavement (meters per meter).

The length of transition curve (Ls) is used to apply the superelevation, with

the adverse camber removed on the preceding section of tangent (Lc). The

change from normal cross-section to full superelevation at the start of thecircular curve is achieved over the superelevation run-off distance which is

the sum of Ls and Lc.

The resulting combination of horizontal alignment design and

superelevation for different design speeds is presented in Table 3.5 below:

Table 3.6: Horizontal Curve Design Data

Minimum Superelevation Transition LengthSuperelevation Transition Length, m

Rural Intermediate RoadsDesign Speed,

KphFlat/Rolling Mountains Escarpment

Village Access

20 - - 18 27

30 29 19 29


12/28


3.4.3 Improving Horizontal Alignment

The major criteria for considering improvements to the horizontal

alignment are the following:

1. Safety

2. Grade profile

3. Type of Roadway

4. Design speed

5. Topography

6. Cost (Construction, Maintenance, Operation)

Of these considerations, safety comes first. Therefore, the stopping sight

distance shall be adequate at all points of the roadway.

The grade profile shall be considered next in mountainous and escarpment

section. Critical grades are commonly encountered on existing roads

located in these sections. The possible improvement of these grades by

adjusting the horizontal alignment should be investigated in the cases

where such an adjustment does not entail major earthworks orencroachment into private property.

The road types that are considered in this Manual are the rural intermediate

roads and the village access roads. The standards for the horizontal

alignment will vary for each of these two road types.

The design speed in turn controls sight distance and hence safety.

Topography controls both curve radius and design speed to a large extent.

The economics of construction, maintenance and operation must be

balanced carefully against other factors in order to produce the safety

alignment consistent with the level of design.

3.4.4 Geometric Controls

The general geometric controls of horizontal alignments that could be

looked at in considering improvements for the rural intermediate roads arethe following:

Location of curves: Alignment shall be as direct as possible but consistent

with topography. A flowing alignment consistent with the contours is

aesthetically more pleasing than one with long tangents. Natural slopes and


13/28


Curve Length and Central Angle: Winding alignment composed of short

curves shall be avoided since it results in erratic operation. In general, the

length of curve should be at least 100 m long for a central angle of 5

degrees. The minimum length shall be increased 30m for each 1 degreedecrease in the central angle. Sight distance or other safety considerations

shall not be sacrificed thereby. In general, the central angle of each curve

shall be as small as physical conditions permit, in order to achieve the

shortest possible route.

Tangents or Straights Affording Passing Opportunities: An 800m tangent

is considered adequate for the purpose of providing passing opportunities

on 2-lane roadways. Passing tangents shall be provided as frequently aspossible in keeping with the terrain. Shorter radii ensuring greater length of

intervening tangent shall be preferred to sweeping curves of large radii

which reduce the length of intervening tangents. However, sharp curves at

the end of passing tangents and especially long tangents shall not be used.

Compound Curves: These shall be avoided in general. On a compound

curve the shorter radius shall be least 2/3 of the longer radius. The total arc

length of a compound curve shall not be less than 100m.

Curvature on Fills: Other than flat curvature should be avoided on high,

long fills. In the absence of cut slopes, shrubs, trees, etc., above the

roadway, it is difficult for drivers to perceive the extent of curvature and

adjust their operation to the conditions.

These design controls should be checked for the existing roads under

consideration. Design solutions should be developed within the specific

budget constraint associated with every road.

3.4.5 Widening on Curves

Pavement widening is needed on certain open curves because:

a. A large vehicle or truck occupies a greater width, requiring allowance

for the swept path of the vehicle as it follows a curved path, and

b. The drivers have some difficulty in steering their vehicles to hold tothe center of the lane and to allow them to maneuver when

approaching other vehicles.

The required amount of widening is dependent on the characteristics of the

vehicles using the road, the radius and length of the curve, and lateral


14/28


applied on the inside edge of pavement only, and preferably attained over

superelevation runoff length. Widening values are given in Table 3.5.

Figure 3.2 shows how carriageway widening on curves is graduallyattained from the inside of the curve.

Table 3.7: Horizontal Curve Design Data

Widening on Curves for all Road Types

Pavement Widths, mRadius

4.0-4.9 5.0-5.9 6.0-6.9 7.0


15/28


curvature design standards for such alignments are to be reduced to the

minimum in order to avoid land acquisition.

The major criteria for considering improvements to the vertical alignmentare the following:

1. The grade line is a reference line by which the elevation of the

pavement and other features of the highway are established. Though

controlled mainly by the topography, other factors such as horizontal

alignment, safety, sight distance, speed, construction costs and the

performance of heavy vehicles on a grade should be considered.

2. All portions of the grade line shall meet sight distance requirements for

the design speed classification of the road.

3. In level terrain, the elevation of the grade line is often controlled by

drainage considerations. In rolling terrain a reasonably undulating

grade line is desirable from the standpoint of operation and

construction economy.

4. Two vertical curves in the same direction separated by a short section

of tangent grade shall in general be avoided, particularly in valley

curves.

5. It is desirable to reduce the grades at intersections. Turns are

negotiated with reduced mechanical wear and fuel consumption, and

increased safety.

6. The standards listed in Tables 3.6 and 3.7 should be met in terms of

maximum gradient and minimum K-values.

7. In order to avoid drainage problems in flat and level grades on

uncurbed pavements, the pavement has to be adequately crowned to

drain the surface laterally.

3.5.1 Elements of Vertical Alignment

The two main components of vertical alignment are:

i. Vertical curvature, which is governed by sight distance and comfort

criteria, and


16/28


2

L

x

200

L.Gy

=

where y = vertical distance from the tangent to the curve (meters)

x = horizontal distance from the start of the vertical curve

(meters)

G = algebraic difference in gradients (%)

L = length of vertical curve (meters)

3.5.2 Crest Curves

The provision of ample sight distance for the road design speed represent

the main control for safe operation on crest curves.

The minimum lengths of crest curves are designed to provide sufficient

sight distance during daylight conditions. Conditions normally do not allow

full overtaking sight distance and the design should aim to reduce the

length of crest curves to provide minimum stopping sight distance in order

to allow for increasing overtaking opportunities on the gradients on eitherside of the curve.

Two conditions exist when considering minimum sight distance criteria on

vertical curves. The first is where sight distance is less than the length of

the vertical curve, and the second is where sight distance extends beyond

the vertical curve. Consideration of the properties of the parabola results in

the following relationships for minimum curve length to achieve the

required sight distances:

For S < L:

( )221

2

200

.

hh

SGLm

+=

For S > L:( )

G

hhSLm

2

212002+

=

Where Lm = minimum length of vertical crest curve (meters)

S = required sight distance (meters)

G = algebraic difference in gradients (%)

h1 = driver eye height (meters)

h bj t h i ht ( t )


17/28


Tables 3.8 and 3.11 show the two vertical alignment design parameters for

various terrain types: minimum vertical curvature in terms of K-values, and

maximum gradient.

Table 3.8 shows the minimum K-values for the following conditions using

the equations above:

1. Stopping sight distance measured from eye height h1 of 1.080m to a

stopped vehicle, i.e. object height h2 = 0.6m. K-values are for

ADT


18/28


Table 3.8 Minimum Vertical Curvature Values

for Very-Low Volume Roads

(1) K-values are for higher risk locations for ADT 100-250 vpd and all locations for

250-400 vpd. K-values are for H1 = 1080 mm and h2 = 600 mm representing a stoppedvehicle.

(2) K-values are based on stopping sight distance measured from eye height of 1.05m and

an object height of 0.2m.

Table 3.9: Vertical Alignment Design Data Minimum K-Value for Curves

K- Value


Flat/Rolling Mountainous Escarpment

Village Access

Roads

Design

Speed

KphCrest Sag Crest Sag Crest Sag Crest Sag

20 - - - 1 2 1 2

30 - - 3 4 3 4 3 4

40 18 20 5 8 5 8 5 8

50 28 35 9 11 9 11 - -

60 55 55 14 15 14 15 - -

80 85 75 18 20 - - - -

100 160 105 22 22 - - - -

120 250 - - - - - - -

AASHTO ADT


19/28


access roads. No-passing signs should be erected where the available sight

distance does not allow overtaking.

3.5.3 Sag Curves

It has been assumed that adequate sight distance will be available on sag

curves in daylight. However, at night, visibility is limited by the distance

illuminated by the headlamp beams, and minimum sag curve length for this

condition is given as:

For S < L:

( )

tan.200

.

1

2

Sh

SGLm

+=

For S > L:( )

G

ShLm

tan.200 1 +=

Where h1 = headlight height (meters)

= angle of upward divergence of headlight beam (degrees)

Appropriate values for h1 and are 0.6 meters and 1.0 degrees respectively.The use of these equations can lead to requirements for unrealistically long

vertical curves as, especially at higher speeds, sight distances may be in

excess of the effective range of the headlamp beam, particularly when low

meeting beams are used. Thus, the only likely situation when the above

equations should be considered for use is on the approaches to fords and

drifts and other similar locations where flowing or standing water may be

present on the road surface. Most of these structures occur on low speed

road where headlamp illumination is more likely to reach the full sightdistances.

It is recommended that, for most situations, sag curves are designed using

the driver comfort criterion of vertical acceleration (Table 3.10). The

K-values used are given in Table 3.8.

Table 3.10: Minimum Levels of Acceptable Vertical Acceleration

Design Speed km/h 120 100 85 70 60 50 40 30

Vertical acceleration

(Proportion of g in m/sec2)

0.05 0.06 0.07 0.08 0.08 0.09 0.10 0.10

3.5.4 Gradient


20/28


For very low levels of traffic flow with only a few four-wheel drive

vehicles, the maximum traversable gradient is in excess of 20 per cent.

Small commercial vehicles can usually negotiate a 19 per cent gradient,

whilst two-wheel drive trucks can successfully tackle gradients of 15-16per cent except when heavily laden.

Gradients of 10 per cent or over will usually need to be paved to enable

sufficient traction to be achieved, as well as for pavement maintenance

reasons.

As traffic flows increase, the economic disbenefits of more severe

gradients, measured as increased vehicle operating and travel time costs,are more likely to result in economic justification for reducing the severity

and/or length of a gradient. On the higher design classes of road, the lower

maximum recommended gradients reflect the economics, as well as the

need to avoid the build up of local congestion. However, separate economic

assessment of alternatives to long or severe gradients should be undertaken

where possible or necessary.

Table 3.11: Vertical Alignment Data Maximum Gradient

Gradient, %

Rural Intermediate RoadsDesign Speed

KphFlat/Rolling Mountainous Escarpment

Village Access

20 - 14(1) 15(2) 15(3)

30 - 11 11 11

40 8 10 10 10

50 7 9 9 -

60 6 8 8 -

80 5 7 - -

100 5 5 - -

120 5 - - -

(1) Maximum gradient for new roads. For existing alignments may be relaxed to18%.

(2) For Hairpin bends, the maximum gradient should not exceed 6% at centerline of

curve + 15m from each approach. For existing roads, it can be relaxed to 10%.(3) May be relaxed for existing roads for sections where changes in alignment are

not cost-effective.

Note: The length of the maximum gradient in this case should not exceed 200m for new

roads otherwise the speed should be dropped below 20kph. Sections with agradient greater than 10% should be considered for paving. For existing roads

the length of the maximum gradient may be relaxed to 300m.


21/28


journey times and reduced vehicle operating costs. Benefits will increase

with increases in gradient, length of gradient, traffic flow, the proportion of

trucks, and in overtaking opportunities. The effect of a climbing lane in

breaking up queues of vehicles held up by a slow moving truck willcontinue for some distance along the road.

Experience has shown that climbing lanes are unlikely to be justified other

than on a small proportion of roads with heavy flows.

As climbing lanes will be used largely by trucks and buses, they must be a

minimum of 3.0 meters in width. They must be clearly marked and, where

possible, should end on level or downhill sections where speed differencesbetween different classes of vehicles are lowest to allow safe and efficient

merging manoeuvres.

3.6 CROSS SECTION

3.6.1 Rationale for Determining Road Widths

The cross section of a roadway is made up of:

Number and width of lanes Shoulder width Cross slopes Pavement type Side slopes Drainage Right-of-way width

Lane and shoulder widths are be determined according to the traffic

volume, traffic composition and vehicle speed, and characteristics of the

terrain. The cross section may need to vary over a particular route because

the controlling factors are changing. The basic requirements are, however,

that changes in the cross section shall not be made unnecessarily, that the

cross section standards shall be uniform within each subsection of the route

and any changes of the cross section shall be effected gradually and

logically over a transition length.

In certain cases, however, it may be necessary to accept isolated reductions

in cross section standards, for example when an existing narrow bridge or

other structure has to be retained In such cases a proper application of


22/28


Road class D: Village Access Roads with low volume of traffic (1000vpd): a running surface width of 6 7m allows vehicles in opposing

directions to pass safely without the need to slow down or move laterally in

their lanes.

Economic considerations call for minimization of road width in order to

reduce construction and maintenance costs, whilst being sufficient to carry

the traffic flows efficiently and safely.

Table 3.12 shows the recommended values for carriageway, shoulder and

formation widths for various classes of roads.

Figure 3.1 shows typical road cross section with dimension ranges.

3.6.2 Carriageways and Shoulders

As the maximum width of a vehicle is 2.5m, the lane width should be 3.0

3.5 m. For the higher classes of roads a lane width of 3.5m is prescribed.For low volume traffic of mostly light vehicles on rural local access roads,

a lane width of 2.75m and even 2.50m may be acceptable. For roads with

substantial commercial traffic, the paved width should exceed the lane

width in order to reduce the cost of shoulder maintenance and lessen the

wheel load concentration at the pavement edges.

One of the constraints in designing the cross sections, is to ensure that the

works are limited to the existing right-of-way. This limits the need forexpropriation. The cross section along the rural intermediate roads and the

village access roads will follow the existing platform width, varying

generally between 3m and 7m with variable shoulders on both sides. Figure

3.1 shows the typical cross sections along with the number of lanes varying

between one and two lanes per section.


23/28



DOCS-0974-04 3-21

Table 3.12 Summary of Standards for Various Cross Section Elements

Cross Slopes (% )

Road Function

Approx

Range of

Traffic Flow

(ADT)

No. of LanesLane width

(m)

C/W width

(m)

Shoulder width

(m) Pavement Shoulders Formation

Rural Intermediate(Governorate)

400-3000 2 3.0-3.5 6-7 0.0-1.5 2.0% 2-3% 2-3%

Tertiary(District)

100-1000 2 2.5-3.0 5-6 0.0-1.0 2.0% 2-3% 2-3%

Feeder(Village Access)


24/28



DOCS-0974-04 3-22

ROAD MARKING

2-3%

2-3% 2-3%

2-3%

2%2%

FORMATION WIDTH

6 - 8

1 LANE1 LANE

2.5 3.5 2.5 3.5 0.5 1.50.5 1.5

CARRIAGEWAY

GRAVELSHOULDER

GRAVELSHOULDER

C

L

Min0.5

1.0

ALL DIMENSIONS ARE IN METERS

Crossfall

Figure 3.3 Typical Cross Section Terminology and Dimensions

Slope normally

1V:2H for depthof 2m, or inaccordance with

type of soil and

depth

Slopeaccording to

type of soil and

depth of Cut.

For existing

alignments a

slope of 1:10may be used

Drainage ditch

usually V-shaped.

Other shapes may

also be used.

Surfacing

Base

Subgrade

l d lbli f


25/28


The shoulder widths depend upon the availability of Right-of-Way, type of

terrain and the type of the road base (bound or unbound). These are

observed to fall in the range of 0 to 2 meters. No paving is generally needed

for shoulders except in locations where water is likely to penetrate at theedge of the pavement which is an area particularly vulnerable to structural

damage. Shoulders should also be paved if the level of traffic flow

approaches the upper limit for a particular design class. In such cases a

surface dressing or other seal may be applied.

For 2-lane paved roads with carriageway width greater than 5 meters, full

shoulders may be omitted in mountainous and escarpment type terrain

where the costs of achieving desired cross sections are very high. In thiscase the minimum paved width shall be 5.5 meters and side drains and edge

barriers should be given special considerations.

For single lane roads the carriageway width shall be 3.0m. Shoulders

widths may be 0-1.5m depending on traffic volume, mix and terrain.

Two lane roads should be delineated by continuous lines at least 10cm wide

situated on the shoulder immediately adjacent to the running surface.

Centerline markings are also recommended on roads of at least 5m width.

3.6.3 Cross Slope

Cross slope (crossfall) is needed on all roads to assist in the draining of

water into side drains. However it should not be so great as to be hazardous

by making steering difficult.

The normal cross slopes are a function of the type of pavement. Forbituminous pavements, the normal cross slope is generally taken as 2.0%.

The normal crossfall should be designed as shown in Figure 3.1.

In the case of rural roads the shoulders are generally not paved. Their

normal cross fall should be 2 - 3% to ensure faster drainage rate. In case the

carriageway is superelevated the shoulder should follow the same

Superelevation rate. The cross slopes of the formation shall be 2-3%. For

unpaved roads, a cross slope of 3% shall be used.

Applying steeper crossfalls to the formation not only improve drainage

performance of various pavement layers, but also provide a slightly greater

thickness of base material at the edge of the pavement where the bearing

capacity is the smallest due to the least confinement, and thus where the

R l R d D i M lR bli f Y


26/28


the excavation of ditches on tracks through steep sidelong ground. In such

cases drainage details should be provided. Figure 3.4 shows this concept

with is advantages and disadvantages. Drainage channel shape and slope of

cutting are determined according to soil and terrain types.

Side Slopes

V1 V2Sound Rock Weathered Rock Sity Sand H


27/28


places and the potential difficulty of reversing. In general, passing places

should be constructed at the most economic locations as determined by

terrain and ground condition, such as at transitions from cut to fill, rather

than at precise intervals.

The length of individual passing places will vary with local conditions and

the sizes of vehicles in common use but, generally, a length of 20 meters

including tapers will cater for most commercial vehicles on roads of this

type.

A clear distinction should be drawn between, passing places and lay-bys.

Lay-bys may be provided for specific purposes, such as parking or busstops, and allow vehicles to stop safety without impeding through traffic


28/28



DOCS-0974-04 3-26

Cross-Section with Cross-fall to Valley Side Cross-Section with Cross-fall to Mountain Side

Advantages Disadvantages Advantages Disadvantages

- no side drains required, resulting insubstantial reduction in earthworks.

- less cross-drainage structures required

- evenly spread surface water runoff along

road edge reduces erosion problems.

- potentially dangerous for vehicles slidingwhen surface slippery

- careful maintenance of surface required toensure water drains evenly over shoulders

- when gradient exceeds 8 percent, cross-fall must be changed to mountain side.

- safer for vehicles in wet and slipperyconditions

- wider formation improves sight distance

- critical outside edge of road less prone to

damage- controlled surface drainage outlets

- more earthworks because of the increasedwidth to accommodate drainage.

- higher back slopes requiring protection.

- frequent cross-drainage structures required

- more expensive

Source: WB Technical Paper 496.

Figure 3.5 Alternative Cross Sections in Mountainous Terrain

Shoulder

50 - 100

C

L

Carriageway200-250

Catch water drains whererequired; masonry linedchannel and/or polythenesheet to avoid water from

seeping into slope material

Drain60 - 80

3% -5%

In situ soil oroptional gravel

Bio-engineering

slope protection onslopes below and

above road

Side drain: in weakmaterial to be

masonry linedNote: cut and fill to balance, avoidspoil as much as possible

Construction steps: to allow for carefulexcavation with minimal disturbance ofnatural slope and regular, well compactedfill layers on stable ground

Documents

Geometric Standards