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DESIGN TRAFFIC LOADTRAFFIC COUNT DATA, AXLE WEIGHT DATA, GROWTH OF TRAFFIC

passage of people or vehicles moving on a road

or public highway.

TRAFFIC

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AXLE

A supporting shaft

or member on or

with which a

wheel or a set of

wheels revolves.

AXLE LOAD

The axle load of a wheeled vehicle is the

total weight felt by the roadway for all

wheels connected to a given axle. Viewed

another way, it is the fraction of total vehicle

weight resting on a given axle.

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GENERAL

For the purposes of structural pavement design, it is

necessary to determine the traffic load to be carried

by the pavement during the design life.

The traffic which will pass over the pavement

will be composed of a number of different vehicle

types, each of which will apply a different load.

In order to combine these loads, the concept

of an equivalent standard axle (ESA) based on an

8 tonne axle load has been developed.

GENERAL

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The total number of equivalent standard axles

which will pass over the pavement during the selected

design life is the design traffic load.

For pavement design purposes, axle loads less than

3 tonnes can be ignored. This eliminates private saloon

cars and most jeepneys from the calculations of design

traffic load in equivalent standard axles. As a guide a

vehicle having at least one axle with dual wheels and/or

having more than two axles should be considered.

GENERAL

3 axles

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MAIN PARAMETERS

There are four fundamental factors to be

determined in order to be able to compute the

cumulative equivalent standard axles for

pavement design purposes.These are as follows:

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MAIN PARAMETERS

(a) Existing traffic flows on the length of road under

consideration, subdivided by vehicle class.

(b) A sample of axle weights for each class of heavy

goods vehicle.

(c) Growth rates for each class of vehicles, or an over

all growth rate applicable to all classes of vehicle.

(d) Design life for the pavement.

Conduct classified

t raffic count

Process data to

determine average

annual daily t raffic

(AADT) by vehicles

type

Obtain grow th

factors and apply to

AADT to determine

t raffic by vehicle

class at t ime of

opening road.

Apply grow th factors

to opening traffic by

vehicle class to

determine cumulat ive

t raffic over design life

by vehicles class

Apply equivalent factor by

vehicles class to determine

cumulat ive ESA by vehicle

class over design life

Sum cumulat ive ESA by

vehicle class to determine

total ESA for design life.

Conduct ax le

w eight survey by

vehicles class for

vehicles > 3 tonne

gross w eight

Apply equivalence

factors to ax le

w eights to

determine average

equivalence factor

per ax le

Determine average

equivalence factor

by vehicle type.

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TRAFFIC COUNT DATA

Whether collected by a traffic survey conducted specifically for the project

under consideration, or based on earlier surveys on the same length of road,

the minimum requirements from the traffic survey data are as follows:

(i) Average annual daily traffic (AADT) i.e. average total flow of traffic on the

road in a 24-hour period. This is normally a 2 way flow i.e. the total of the

vehicular flow in each direction.

(ii) The make-up or composition of the AADT in terms of the number of

vehicles in each vehicles classification. This may be presented as a

percentage of flow for each vehicles classification.

AXLE WEIGHT DATA

At the same time as the traffic count, it is desirable to

conduct an axle weight survey.An axle load survey should

be carried out by weighing a sample of vehicles at the

roadside. The sample should be chosen such that a

maximum of about 60 vehicles per hour are weighed.

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Equivalence Factor

Equivalence Factor = Axle Load in kg

8160

4.5

Wheel load (single and dual) Axle load Equivalence Factor

(1000 kg) (1000 kg)

1.50 3 0.01

2.00 4 0.04

2.50 5 0.11

3.00 6 0.25

3.50 7 0.50

4.00 8 0.91

4.50 9 1.55

5.00 10 2.50

5.50 11 3.83

6.00 12 5.67

6.50 13 8.13

7.00 14 11.35

7.50 15 15.48

8.00 16 20.70

8.50 17 27.19

9.00 18 35.17

9.50 19 44.85

10.00 20 56.50

Equivalence Factors For

Different Axle Loads

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TYPE OF VEHICLE

TRUCK W/ 2 AXLES

(6 WHEELS)TRUCK W/ TANDEM REAR WHEELS

(3 AXLES/10 WHEELS)

TYPE OF VEHICLE

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GROWTH OF TRAFFICThe determination of the cumulative total of equivalent

standard axles (ESA) that will pass over the pavement during the

design life must take into consideration the growth or increase in

traffic over the selected designlife.

The determination of appropriate growth factors can be a

complex procedure. Unless accurate information is available,

from which predictions can be made with a high degree of

confidence, it is recommended that average growth figures should

be obtained from Planning Services Department of DPWH.

Formula for Traffic Growth

Factor at time of opening year

Where:

i = growth rate, %

n = no.of years from initial study to the opening year

GF = (1 + i)n

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Formula for Traffic Growth

Factor over design lifeUsing summationnotation:

(1 + i)x

n−1

x=0

Where:

i = growth rate, %

n = analysis period, years

GF = (1 + i)n

Analysis Period Years (n) No Growth 2.00 4.00 5.00 6.00 7.00 8.00 10.00

1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

2 2.00 2.02 2.04 2.05 2.06 2.07 2.08 2.10

3 3.00 3.06 3.12 3.15 3.18 3.21 3.25 3.31

4 4.00 4.12 4.25 4.31 4.37 4.44 4.51 4.64

5 5.00 5.20 5.42 5.53 5.64 5.75 5.87 6.11

6 6.00 6.31 6.63 6.80 6.98 7.15 7.34 7.72

7 7.00 7.43 7.90 8.14 8.39 8.65 8.92 9.49

8 8.00 8.58 9.21 9.55 9.90 10.26 10.64 11.44

9 9.00 9.75 10.58 11.03 11.49 11.98 12.49 13.58

10 10.00 10.95 12.01 12.58 13.18 13.82 14.49 15.94

11 11.00 12.17 13.49 14.21 14.97 15.78 16.65 18.53

12 12.00 13.41 15.03 15.92 16.87 17.89 18.98 21.38

13 13.00 14.68 16.63 17.71 18.88 20.14 21.50 24.52

14 14.00 15.97 18.29 19.16 21.01 22.55 24.21 27.97

15 15.00 17.29 20.02 21.58 23.28 25.13 27.15 31.77

16 16.00 18.64 21.82 23.66 25.67 27.89 30.32 35.95

17 17.00 20.01 23.70 25.84 28.21 30.84 33.75 40.55

18 18.00 21.41 25.65 28.13 30.91 34.00 37.45 45.60

19 19.00 22.84 27.67 30.54 33.76 37.38 41.45 51.16

20 20.00 24.30 29.78 33.06 36.79 41.00 45.76 57.28

25 25.00 32.03 41.65 47.73 54.86 63.25 73.11 98.35

30 30.00 40.57 56.08 66.44 79.06 94.46 113.28 164.49

35 35.00 49.99 73.65 90.32 111.43 138.24 172.32 271.02

Note: The above growth factors multiplied by the first year traffic estimate will give the total volume

of traffic expected during the analysis period

Annual Growth Rate, Percent

Traffic Growth Factors

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DESIGN LIFE

Since pavements, new or rehabilitated, are usually designed for

period ranging from 10 years to 20 years or more, it is necessary to

predict the ESA for this period of time i.e. the performance period.

The performance period often referred to as the design period or

design life of the pavement is defined as the period of time that an

initial (or overlaid) pavement will last before reaching its terminal

serviceability.

For roads in the Philippines, it is normal to use a design life of

10 years for asphalt surfaced roads and 20 years for concrete

surfaced roads.

UNEQUAL TRAFFIC LOADS

In some circumstances, it may be determined that the

traffic loading in one direction along a road is much

greater than that in the opposite direction. In this case,

design traffic loads computations should be carried out

for each direction and the highest loading used for

pavement design purposes.

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EXAMPLE CALCULATION of DESIGN

TRAFFIC LOAD DETERMINATIONThe following data are results of an earlier traffic counts and axle weight

surveys:

(a) Initial AADT (Year of Study: 1995) = 3,000 vehicles per day

(b) Buses make up 5% of the AADT, 2 axle trucks 8% and others 4%

(c) Equivalence factors have been established as follows:

Buses = 1.2 per bus

2 axle trucks = 1.85 per truck

Other trucks = 1.37 per truck

(d) Growth rates to determine AADT by vehicle class at opening year:

Buses = 3%

2 axle trucks = 5%

Other trucks = 5%

The opening year is 1998 and the selected design life is 20 years (rigid

pavement). Determine the design traffic load.

Conduct classified

t raffic count

Process data to

determine average

annual daily t raffic

(AADT) by vehicles

type

Obtain grow th

factors and apply to

AADT to determine

t raffic by vehicle

class at t ime of

opening road.

Apply grow th factors

to opening traffic by

vehicle class to

determine cumulat ive

t raffic over design life

by vehicles class

Apply equivalent factor by

vehicles class to determine

cumulat ive ESA by vehicle

class over design life

Sum cumulat ive ESA by

vehicle class to determine

total ESA for design life.

Conduct ax le

w eight survey by

vehicles class for

vehicles > 3 tonne

gross w eight

Apply equivalence

factors to ax le

w eights to

determine average

equivalence factor

per ax le

Determine average

equivalence factor

by vehicle type.

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Solution

Initial AADT = 3,000 vehicle per day

Buses = 0.05 x 3000 = 150 per day

2 Axle Trucks = 0.08 x 3000 = 240 per day

Other Trucks = 0.04 x 3000 = 120 per day

2. Apply growth factors to determine AADT by vehicle class at opening year.

1. Determine average annual daily traffic (AADT) by vehicles type.

From 1995 (Year of Study) to 1998 (Opening year) = 3 years

Therefore growth by class is as follows:

Buses = 150 x 1.033 = 164 per day

2 Axle Trucks = 240 x 1.053 = 278 per day

Other Trucks = 120 x 1.053 = 139 per day

3. Apply growth factors to each class to determine total number of axles that will pass over the

pavement in the design life.

Analysis Period Years (n) No Growth 2.00 4.00 5.00 6.00 7.00 8.00 10.00

1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

2 2.00 2.02 2.04 2.05 2.06 2.07 2.08 2.10

3 3.00 3.06 3.12 3.15 3.18 3.21 3.25 3.31

4 4.00 4.12 4.25 4.31 4.37 4.44 4.51 4.64

5 5.00 5.20 5.42 5.53 5.64 5.75 5.87 6.11

6 6.00 6.31 6.63 6.80 6.98 7.15 7.34 7.72

7 7.00 7.43 7.90 8.14 8.39 8.65 8.92 9.49

8 8.00 8.58 9.21 9.55 9.90 10.26 10.64 11.44

9 9.00 9.75 10.58 11.03 11.49 11.98 12.49 13.58

10 10.00 10.95 12.01 12.58 13.18 13.82 14.49 15.94

11 11.00 12.17 13.49 14.21 14.97 15.78 16.65 18.53

12 12.00 13.41 15.03 15.92 16.87 17.89 18.98 21.38

13 13.00 14.68 16.63 17.71 18.88 20.14 21.50 24.52

14 14.00 15.97 18.29 19.16 21.01 22.55 24.21 27.97

15 15.00 17.29 20.02 21.58 23.28 25.13 27.15 31.77

16 16.00 18.64 21.82 23.66 25.67 27.89 30.32 35.95

17 17.00 20.01 23.70 25.84 28.21 30.84 33.75 40.55

18 18.00 21.41 25.65 28.13 30.91 34.00 37.45 45.60

19 19.00 22.84 27.67 30.54 33.76 37.38 41.45 51.16

20 20.00 24.30 29.78 33.06 36.79 41.00 45.76 57.28

25 25.00 32.03 41.65 47.73 54.86 63.25 73.11 98.35

30 30.00 40.57 56.08 66.44 79.06 94.46 113.28 164.49

35 35.00 49.99 73.65 90.32 111.43 138.24 172.32 271.02

Annual Growth Rate, Percent

Traffic Growth Factors

For 5% growth rate, use the table below.

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Buses = 164 x 26.87 x 365 = 1,608,461

2 Axle Trucks = 278 x 33.06 x 365 = 3,354,598

Other Trucks = 139 x 33.06 x 365= 1,677,299

For 3%, use the following formula:

GF = (1 + i)x

n−1

x=0

GF = (1 + 0.03)x

20−1

x=0

GF = 26.87

Therefore,

Buses = 1,608,461 x 1.2 = 1,930,153

2 Axle Trucks = 3,354,598 x 1.85 = 6,206,006

Other Trucks = 1,677,299 x 1.37 = 2,297,900

5.Total ESAL.

4. Multiply each class by respective equivalence factors to determine ESAL.

Total ESAL = 1,930,153 + 6,206,006 + 2,297,900 = 10,434,059

Thus, the total ESA is 10,434,059. Therefore, the design traffic

load is per lane of carriageway which is 50% of ESA equal to 5.217 x 106

ESA.