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 instructor’s website. Viewer
discretion is advised as some scenes
contain material of a graphic nature.
 Traffic loads are cyclic (repetitious)
 Repeated, cyclic loads on an structure eventually
result in structural fatigue
 We see the result of this fatigue as pavement
damage or distress
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
• 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
 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
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
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 ESAL’s over 20 years = 300,000
ESAL’s per year or 1,000 ESAL’s per day for 300
truck days per year (i.e., DTN = 1000)
6.03 Apply the Asphalt Institute’s model
to determine ITN.
Find Figure 2.01 in
the course notes.
1.
2.
3.
4.
5.
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.
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
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!
A. 12500 x 0.5 = 6250 vpd in one direction
B. 6250 x 0.1 = 625 heavy trucks per day in one
direction
Use Table 6.01
from the course
notes to find the
Lane Distribution
Factor, LDF:
TABLE 6.01
MTO LANE DISTRIBUTION FACTORS
(After: "Pavement Design and Rehabilitation Manual", 1990)
Highway Type
AADT (vpd)
Lane Distribution
Factor, LDF
2 lanes
All
1.00
< 5000
0.85
5000 - 15000
0.80
15000 - 25000
0.75
> 25000
0.70
< 15000
0.60
15000 - 25000
0.55
25000 - 40000
0.50
> 40000
0.45
For 4 lanes,
5000<12500<15000
4 lanes
LDF = 0.80
HT = 625 x 0.80
HT = 500
6 lanes
So, WT =
40 kips
and HT =
500 tpd
then L =
20 kips
and ITN = 520 ESALs/day
Or use the equation:
ITN

HT

1 . 0658
WT
10
1 . 4220
6 . 9474
L
3 . 4648

HT
ITN 
1 . 0658
WT
10
ITN 
500
1 . 0658
ITN 
L
3 . 4648

6 . 9474
40
10
1 . 4220
1 . 4220
20
3 . 4648
6 . 9474
752 . 6 189 . 7 32196
.5 
8859312 . 1
ITN  518 . 8  519 ESALs/day
