How to determine Design Traffic Loading (Based on Nigerian Manual Guide)

Step 1: Determination of Annual Average Daily Traffic (AADT)

The first step to determine the design traffic loading over the design life of a pavement is to determine the baseline traffic flows. The estimate of the baseline traffic flows should be the Annual Average Daily Traffic (AADT) or Average Daily Traffic (ADT) currently using the ruote (if an existing ruote), or expected to use the route on opening to traffic (if a new ruote), classified into vehicle categories (See FHWA vehicle categories):

Federal Highway Administration (FHWA) Vehicle Classes

1. Motorcycles
2. Passenger Cars
3. 4 – Tyre Trucks
4. Buses
5. 2 – Axle 6-Tyre Trucks
6. 3 – Axle Trucks
7. 4 + Axle Trucks
8. 3 – 4 Axle Single – Trailer Combinations
9. 5 – Axle Single-Trailer Combinations
10. 6 + Axle Single-Trailer Combinations
11. 5 – Axle Multi-Trailer Combinations
12. 6 – Axle Multi-Trailer Combinations
13. 7 + Axle Multi-Trailer Combinations

The AADT is defined as the total annual traffic summed for both directions (of traffic) and divided by 365. It is usually obtained by recording actual traffic flows over a shorter period from which the AADT is then estimated, taking seasonal variation into account. For long projects, large differences in traffic along the road may make it necessary to estimate the flow at several locations. It should be noted that for structural design purposes, traffic loading in one direction is required.

In order to reduce error, it is recommended that traffic counts to establish ADT at a specific site conform to the following practices:

1. The counts are for seven consecutive days.
2. The counts on some of the days are for a full 24 hours, with preferably one 24-hour count on a weekday and one during a weekend. On the other days, 12-hour counts should be sufficient, with the 24-hour count used to determine an appropriate ratio to estimate the 24-hour counts from the 12-hour counts.
3. Counts are avoided at times when travel activity is abnormal for short periods, for example, month-end, public holidays, etc.
4. If possible, seven-day counts should be repeated several times throughout the year.

Step 2: Determination of Equivalent Standard Axles (ESAs)

The average ESAs per vehicle class are determined from axle mass surveys which are usually of two types: static weighting which requires vehicles to be stationary, and dynamic weighting which does not allow for static weighting of vehicles. Dynamic weighting is commonly used at sites such as multi-lane highways or where the terrain and traffic flow do not allow static weighting of all the vehicles or to obtain a representative sample.

Based on a survey carried out in Nigeria in 2008, the typical ESAs per heavy vehicle used for design purposes are presented below:

Table 1: Typical ESAs per heavy vehicle

Conversely, the AADT for each vehicle class (k) may be converted to Equivalent Standard Axles (ESAs) using the suitable equivalency factor which is expressed as:

F = (P/80)ⁿ

Where,

n = relative damage exponent (a value that depends on the type of pavement, the pavement failure mechanism, and its state)

F = load equivalency factor

P = axle load (kN) per vehicle class, k

It is common practice to convert a one-directional flow of traffic flow to ESAs. However, if there is information that shows the difference between the flows in each direction, the higher of the two directional values should be used for the design.

Table 2: Typical values of AASHTO load equivalency factors for 80 kN (18,000 lb)

Step 3: Projection of the Average Daily ESAs (ADE)

The AADT or ADT is projected to ADE which is calculated as the sum of the product of the ADT per vehicle class (k), and the average ESAs per vehicle class (k).

Mathematically, ADE_initial = ∑ (ADT_k  x ESAs_k)

Where,

ADE_initial = Initial Average daily ESAs

ADT_k = average daily traffic per vehicle class ‘k’

ESAs_k = Equivalent standard axle of 80 kN per vehicle class ‘k’

Step 4: Determination of Cumulative Equivalent Standard Axle (ESAs) Loading

This is the final stage of calculation of the design traffic loading which is simply the cumulating of the average daily number of ESAs on one lane at the opening of the new road to traffic which is projected and cumulated over the design life of the pavement.

Mathematically, the cumulative ESAs per lane is given by the expression:

ESA_total = ADE_initial x f_y

Where,

f_y = cumulative factor or traffic growth factor

f_y = 365 x (1 + 0.01i) x [[(1 + 0.01i)^y – 1]/ (0.01i)]

Where,

y = pavement structural design period

i = traffic growth rate (%)

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