Traffic Terms and Concepts

1
Traffic Terms and Concepts

• Why do we need to concern ourselves with traffic
  when we design pavements?
• Traffic is what LOADS the pavement

The following presentation contains references to
Figure 6.01 and Table 6.01 which are posted
under subsection 2.6 of the course notes on the
instructors website. Viewer discretion is
advised as some scenes contain material of a
graphic nature.
2

• Traffic loads are cyclic (repetitious)

3

• Repeated, cyclic loads on an structure eventually
  result in structural fatigue

4

• We see the result of this fatigue as pavement
  damage or distress

5
6.01 Explain the concept of load equivalency and
define the standard unit load/configuration used
in pavement design technology.

• heavy vehicles cause damage to pavements
• the heavier the load per axle, the more damage
• in order to assess the damage caused by the many
  different types/configurations of vehicles, one
  specific load/configuration was adopted as the
  standard

6

• The standard adopted is the 18,000 lb single axle
  load, a truck with a single rear axle
• the rear wheels each transmit 9000 lb loads to
  the pavement

9000 lb
9000 lb
18,000 lb 80 kN
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• a load equivalency factor gives the number of
  repetitions of the standard load/configuration
  that would cause an equivalent amount of damage
  as one pass of the specific vehicle
• eg., a load equivalency factor of 2.5 means that

one pass of a specific vehicle
causes an equivalent amount of damage as two and
a half passes of the standard vehicle
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6.02 Define the following terms a) ESAL
b) ITN c) DTN

• a) the standard load and axle configuration
  to which all other load and axle configurations
  are converted when evaluating traffic loads for
  pavement structural design
• ESAL Equivalent Single Axle Load ( 80 kN)

9000 lb
9000 lb
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• ITN (Initial Traffic Number)
• the average number of ESAL's/day in the first
  year of a pavement design analysis period

• DTN (Design Traffic Number)
• the average number of ESAL's/day over the entire
  pavement design analysis period
• The total ESAL applications over the design
  analysis period divided by the number of traffic
  days
• eg., 6,000,000 ESALs over 20 years 300,000
  ESALs per year or 1,000 ESALs per day for 300
  truck days per year (i.e., DTN 1000)

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6.03 Apply the Asphalt Institutes model to
determine ITN.

• Find Figure 2.01 in the course notes.

• Example shows
• Locate WT on scale D.
• Locate HT on scale C.
• Extend line to pivot line, B.
• Locate L on scale E.
• Extend line from E through B to ITN on scale A.

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Nomenclature

• WT Average Gross Vehicle Weight of heavy trucks
  in kips
• HT Average daily number of Heavy Trucks in the
  design lane
• L the legal axle limit in kips
• ITN the Initial Traffic Number

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EXAMPLE

• A new 4-lane road pavement is to have an AADT of
  12,500 vpd with 10 heavy trucks whose average
  gross vehicle weight is 40,000 lb. If the legal
  single axle load limit is 20,000 lb, find the
  ITN.
• With no indication otherwise, assume two-way
  traffic operation and a directional split of 50.
  Remember, AADT is bidirectional!
• 12500 x 0.5 6250 vpd in one direction
• 6250 x 0.1 625 heavy trucks per day in one
  direction

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• Use Table 6.01 from the course notes to find the
  Lane Distribution Factor, LDF
• For 4 lanes,
• 5000lt12500lt15000
• LDF 0.80
• HT 625 x 0.80
• HT 500

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So, WT and HT then L and ITN
40 kips
500 tpd
20 kips
520 ESALs/day
Or use the equation
15
ESALs/day