Traffic Monitoring Guide

Appendix H. TRAFFIC DATA FOR PAVEMENT DESIGN

H.1 BACKGROUND

The goal of pavement design is to obtain a set of specific pavement construction parameters, such as types of construction material and construction methods; thickness of various courses, including base, structural layer; and a surface course to achieve pre-determined performance criteria. Traffic, climate, soil, and other geological data are typically used as inputs for pavement design. Traffic data shall cover the entire life expectancy, which is between the opening year and the last year of life expectancy (Figure H-1). The last year of life expectancy of pavement is often referred to as the design year.

FIGURE H-1 TEMPORAL DATA SEQUENCE

Source: Federal Highway Administration.

Traditionally, traffic impact on pavement design is through the concept of equivalent single-axle loads (ESAL). ESAL is the equivalent number of 18,000 lbf (pound force) single-axle loads that would produce the same amount of damage over the pavement design life. Traffic modeling and design traffic professionals project cumulative ESAL data from all vehicles for the entire pavement life expectancy.

NCHRP Project 1-37A’s Mechanistic-Empirical Pavement Design Guide enables a dynamic approach where pavement performance can be modeled with very specific traffic related parameters. The new process requires significant traffic data from demand forecasting professionals, which in turn necessitates significant field monitoring data.

H.2 VEHICLE RELATED TERMINOLOGIES

The FHWA’s 13 vehicle category classification system relies on vehicle axle spacing to differentiate various vehicles. Vehicle data classified under this system are referred to as vehicle class information or data.

Vehicle axle configuration refers to the different axle configurations that a vehicle may have. Under each vehicle axle configuration, the weight on the axle/axles is called axle loading. Axle load reflects both the weight of a vehicle itself and the cargo it carries.

H.3 TRAFFIC DATA ASSOCIATED WITH ESAL

To design pavement for a travel lane, AASHTO’s Guide for Design of Pavement of Structures offers the formula for traffic data as listed below

TABLE H-1 TRAFFIC DATA SUMMARY

Source: Federal Highway Administration.

w₁₈= D_D × D_L × W₁₈

Where:

w₁₈ is the ESAL for the design lane. W₁₈8 represents the ultimate traffic data needed for pavement design.

D_D is the traffic directional factor for a two way roadway.

D_L is a lane traffic splitting factor for a roadway having more than one lane in each direction.

W₁₈ is cumulative two way ESAL projected for a roadway segment.

H.3.1 W₁₈ – CUMULATIVE TWO WAY EASL

The W₁₈ is the cumulative dual directional ESAL covering the entire life expectancy. To compute W₁₈, data on number of axles for various axle configurations under various axle loads is needed. While the number of axles for various axle configurations under various axle loads can be obtained from traffic projection professionals, axle load equivalence factors are to be obtained from AASHTO’s Guide Appendix D.

H.3.2 ONE OF MANY POTENTIAL W₁₈ COMPUTATION PROCEDURES

1. Obtain Column A, B, and C information from AASHTO’s Guide for Design of Pavement Structures Appendix D.

2. Fill out Column D with forecasted cumulative traffic measured in number of axles by axle load from traffic forecasting professionals.

3. Column E = Column C × Column D.

4. Summarize Column E to obtain the W₁₈.

TABLE H-2 ESAL COMPUTATION ILLUSTRATION

Source: Federal Highway Administration.

H.4 TRAFFIC DATA ASSOCIATED WITH MECHANISTIC AND EMPIRICAL PAVEMENT DESIGN

NCHRP Project 1-37A’s Mechanistic-Empirical Pavement Design Guide requires the extensive input of traffic data. The following are traffic data items required for the process

TABLE H-3 ILLUSTRATION OF MAF COMPUTATION FROM MADTT

TMAF_ij=MADTT_ij/[∑(MADTT)_j] × 12 MAF_ij; MAF_ij is the monthly adjustment factor for month i and vehicle class j truck

Source: Federal Highway Administration.

TABLE H-4 VEHICLE CLASS DISTRIBUTION COMPUTATION ILLUSTRATION

TABLE H-6 ILLUSTRATION OF SINGLE AXLE LOAD FACTORS FOR JANUARY, FEBRUARY AND DECEMBER

Source: Federal Highway Administration.

H.4.2 7B. TANDEM, 7C. TRIDEM, AND 7D. QUAD AXLE VEHICLES

For tandem axle vehicles, the axle weight group starts at 6,000 lbs. and ends at 82,000, lbs. with increments of 2,000 lbs. For both tridem and quad axle vehicles, the axle weight group start at 12,000 lbs. and ends at 102,000 lbs. with increments of 3,000 lbs.

TABLE H-7 AVERAGE NUMBER OF AXLES

TABLE H-8 TRAFFIC DATA SUMMARY

Source: Federal Highway Administration.

H.5 SUMMARY

Traffic data is one of the most critical elements in pavement design. Even though all traffic data used in pavement design reflects future traffic conditions, traffic monitoring programs and traffic monitoring data provide the needed ground truth in the projection processes including, but not limited to, establishing historical trend and model calibration and validation.