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The method considers traffic in terms of the cumulative number of standard
axles (8160 kg) to be carried by the pavement during the design life.
This requires the following information:
- Initial traffic in terms of CVPD
- Traffic growth rate during the design life
- Design life in number of years
- Vehicle damage factor (VDF)
- Distribution of commercial traffic over the carriage way.
Initial traffic
Initial traffic is determined in terms of commercial vehicles per day (CVPD).
For the structural design of the pavement only commercial vehicles are
considered assuming laden weight of three tonnes or more and their axle loading
will be considered.
Estimate of the initial daily average traffic flow for any road should normally
be based on 7-day 24-hour classified traffic counts (ADT).
In case of new roads, traffic estimates can be made on the basis of potential
land use and traffic on existing routes in the area.
Traffic growth rate
Traffic growth rates can be estimated (i) by studying the past trends of
traffic growth, and (ii) by establishing econometric models.
If adequate data is not available, it is recommended that an average annual
growth rate of 7.5 percent may be adopted.
Design life
For the purpose of the pavement design, the design life is defined in terms of
the cumulative number of standard axles that can be carried before
strengthening of the pavement is necessary.
It is recommended that pavements for arterial roads like NH, SH should be
designed for a life of 15 years, EH and urban roads for 20 years and other
categories of roads for 10 to 15 years.
Vehicle Damage Factor
The vehicle damage factor (VDF) is a multiplier for converting the number of
commercial vehicles of different axle loads and axle configurations to the
number of standard axle-load repetitions.
It is defined as equivalent number of standard axles per commercial vehicle.
The VDF varies with the axle configuration, axle loading, terrain, type of
road, and from region to region.
The axle load equivalency factors are used to convert different axle load
repetitions into equivalent standard axle load repetitions. For these
equivalency factors refer IRC:37 2001.
The exact VDF values are arrived after extensive field surveys.
Vehicle distribution
A realistic assessment of distribution of commercial traffic by direction and
by lane is necessary as it directly affects the total equivalent standard axle
load application used in the design.
Until reliable data is available, the following distribution may be assumed.
- Single lane roads: Traffic tends to be more channelized on single
roads than two lane roads and to allow for this concentration of wheel load
repetitions, the design should be based on total number of commercial vehicles
in both directions.
- Two-lane single carriageway roads: The design should be based on 75
% of the commercial vehicles in both directions.
- Four-lane single carriageway roads: The design should be based on
40 % of the total number of commercial vehicles in both directions.
- Dual carriageway roads: For the design of dual two-lane carriageway
roads should be based on 75 % of the number of commercial vehicles in each
direction.
For dual three-lane carriageway and dual four-lane carriageway the distribution
factor will be 60 % and 45 % respectively.
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