Traffic Terms and ConceptsTraffic Terms and ConceptsWhyWhy do we need to concern ourselves with do we need to concern ourselves with

traffictraffic when we design pavements? when we design pavements?Traffic is what Traffic is what LOADSLOADS the pavement the pavement

The following presentation The following presentation contains references tocontains references to Figure Figure

6.016.01 andand Table 6.01Table 6.01 which are which are posted under subsection 2.6 of posted under subsection 2.6 of

the course notes on the the course notes on the instructor’s website. Viewer instructor’s website. Viewer discretion is advised as some discretion is advised as some scenes contain material of a scenes contain material of a

graphic naturegraphic nature..

Traffic loads are cyclic (repetitious)Traffic loads are cyclic (repetitious)

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

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

6.01 Explain the concept of load 6.01 Explain the concept of load equivalency and define the equivalency and define the

standard unit load/configuration standard unit load/configuration used in pavement design used in pavement design

technology.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 The standard adopted is the 18,000 lb single axle load, a truck with a single rear axleaxle load, a truck with a single rear axle

• the rear wheels each the rear wheels each transmit 9000 lb transmit 9000 lb loads loads toto the pavementthe pavement

9000 lb

9000 lb

18,000 lb = 80 18,000 lb = 80 kNkN

a a load equivalency factorload equivalency factor gives the number of gives the number of repetitions of the standard load/configuration that repetitions of the standard load/configuration that would cause an equivalent amount of damage as would cause an equivalent amount of damage as one pass of the specific vehicle;one pass of the specific vehicle;

eg., a load equivalency factor of eg., a load equivalency factor of 2.52.5 means that… means that…

one pass of a one pass of a specific vehiclespecific vehicle

causes an equivalent causes an equivalent amount of damage as amount of damage as two and a half passes two and a half passes of the standard of the standard vehiclevehicle

==

6.02 Define the following terms:6.02 Define the following terms: a) ESAL a) ESAL b) ITN b) ITN c) DTN 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 designESAL = Equivalent Single Axle Load (= 80 kN)

9000 lb9000 lb

ITN (Initial Traffic Number)ITN (Initial Traffic Number) the average number of ESAL's/day in the the average number of ESAL's/day in the

first year of a pavement design analysis first year of a pavement design analysis periodperiod

DTN (Design Traffic Number)DTN (Design Traffic Number) the average number of ESAL's/day over the average number of ESAL's/day over

thethe entire pavement design analysis entire pavement design analysis periodperiod

The total ESAL applications over the The total ESAL applications over the design analysis period divided by the design analysis period divided by the number of traffic daysnumber of traffic days

eg., 6,000,000 ESAL’s over 20 years = eg., 6,000,000 ESAL’s over 20 years = 300,000 ESAL’s per year or 1,000 ESAL’s 300,000 ESAL’s per year or 1,000 ESAL’s per day for 300 truck days per year (i.e., per day for 300 truck days per year (i.e., DTN = 1000DTN = 1000))

6.03 Apply the Asphalt Institute’s 6.03 Apply the Asphalt Institute’s model to determine ITN.model to determine ITN.

Find Figure 2.01 in Find Figure 2.01 in the course notes.the course notes.Example shows:Example shows:

1.1. Locate Locate WTWT on scale on scale DD..

2.2. Locate Locate HTHT on scale on scale CC..

3.3. Extend line to pivot Extend line to pivot line, line, BB..

4.4. Locate Locate LL on scale on scale EE..5.5. Extend line from Extend line from EE

through through BB to to ITNITN on on scale scale AA..

NomenclatureNomenclature:: WT = Average Gross Vehicle Weight WT = Average Gross Vehicle Weight

of heavy trucks in kipsof heavy trucks in kips

HT = Average daily number of Heavy HT = Average daily number of Heavy Trucks in the design laneTrucks in the design lane

L = the legal axle limit in kipsL = the legal axle limit in kips

ITN = the Initial Traffic NumberITN = the Initial Traffic Number

EXAMPLEEXAMPLEA new A new 44-lane road pavement is to have an -lane road pavement is to have an

AADT of AADT of 12,50012,500 vpdvpd with with 10%10% heavy heavy trucks whose average gross vehicle trucks whose average gross vehicle weight is weight is 40,000 lb40,000 lb. If the legal single . If the legal single axle load limit is axle load limit is 20,000 lb20,000 lb, find the ITN., find the ITN.

With no indication otherwise, assume two-With no indication otherwise, assume two-way traffic operation and a directional way traffic operation and a directional split of 50%. split of 50%. Remember, AADT is Remember, AADT is bidirectionalbidirectional!!

A.A. 12500 x 0.5 = 6250 vpd in one direction12500 x 0.5 = 6250 vpd in one directionB.B. 6250 x 0.1 = 625 heavy trucks per day in 6250 x 0.1 = 625 heavy trucks per day in

one directionone direction

Use Use Table 6.01Table 6.01 from the course from the course notes to find the notes to find the Lane Distribution Lane Distribution Factor, Factor, LDFLDF::

For 4 lanes,For 4 lanes,

5000<5000<1250012500<150<1500000

LDF = 0.80LDF = 0.80

HT = 625 x 0.80HT = 625 x 0.80

HT = 500HT = 500

TABLE 6.01

MTO LANE DISTRIBUTION FACTORS

(After: "Pavement Design and Rehabilitation Manual", 1990)

Lane Distribution

2 lanes All 1.00

Highway Type AADT (vpd) Factor, LDF

< 5000

5000 - 15000

0.85

0.804 lanes

< 15000 0.60

15000 - 25000 0.55

15000 - 25000 0.75

> 25000 0.70

6 lanes25000 - 40000 0.50

> 40000 0.45

So, WT =So, WT =

and HT =and HT =

then L =then L =

and ITN =and ITN =

40 40 kipskips

500 500 tpdtpd

20 20 kipskips

520 520 ESALs/dayESALs/day

Or use the Or use the equation:equation:

9474.6

4648.34220.10658.1

10LWTHTITN

9474.6

4648.34220.10658.1

10LWTHTITN

9474.6

4648.34220.10658.1

102040500

ITN

1.8859312

5.321967.1896.752ITN

5198.518 ITN ESALs/day