Traffic Terms and Concepts

1. Traffic Terms and Concepts Whydo we need to concern ourselves with traffic when we design pavements? Traffic is what LOADS the pavement The following presentation contains references toFigure 6.01andTable 6.01which 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.

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 tothe pavement 9000 lb 9000 lb 18,000 lb = 80 kN

7. 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

8. 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 (= 80kN) 9000 lb 9000 lb

9. 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 theentire 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)

10. 6.03 Apply the Asphalt Institute’s 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.

11. 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

12. EXAMPLE A new 4-lane road pavement is to have an AADT of 12,500vpd 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

13. Use Table 6.01 from the course notes to find the Lane Distribution Factor, LDF: For 4 lanes, 5000<12500<15000 LDF = 0.80 HT = 625 x 0.80 HT = 500

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

15. ESALs/day