One of the primary functions of a pavement is load distribution. Therefore, in order to adequately design a pavement, representative loading characteristics must be presumed about the expected traffic it will encounter. Loads, the vehicle forces exerted on the pavement (e.g., by trucks, heavy machinery, airplanes), can be characterized by the following parameters:

Traffic loads, along with environment, damage pavement over time. The simplest pavement structural model asserts that each individual load inflicts a certain amount of unrecoverable damage. This damage is cumulative over the life of the pavement, and when it reaches some maximum value, the pavement is considered to have reached the end of its useful service life.

Tire loads are the fundamental loads at the actual tire-pavement interface and are generally assumed to be equal for all tires on any given axle. For most pavement analyses, it is assumed that the tire load is uniformly applied over a circular area. Also, it is generally assumed that tire inflation and contact pressures are the same.

While the tire contact pressure and area is of vital concern in pavement performance, the number of contact points per vehicle and their spacing is also critical. As tire loads get closer together, their influence areas on the pavement begin to overlap, especially at depth. At this point, the design characteristic of concern is no longer the single isolated tire load, but the combined effect of all the interacting tire loads. Therefore, axle and tire arrangements are quite important.

Tire-axle combinations (see Figure 2-11) are typically described as:

Other axle configurations exist (tridem [or three axle], quad [or four axle]), but generally represent a small fraction of the entire population.

Federal and state laws establish maximum axle and gross vehicle weights to limit pavement damage. The range of weight limits in the U.S. varies, based on federal and state laws.

Wheel and axle loads for an individual vehicle are not difficult to determine. However, the number and types of wheel/axle loads a particular pavement will be subject to over its entire design life become complicated to determine and are subject to uncertainties in traffic growth and changes to the traffic stream composition over time. Ultimately, it is not the wheel load, but the damage to the pavement caused by each wheel load that is of primary concern.

There are two basic methods for characterizing axle load repetitions:

A typical load spectrum input would be in a form of a table that shows the relative axle load frequencies for each common axle combination (e.g., single axle, tandem axle, tridem axle, quad axle) over a given time period. A separate table with the breakout of trucks by class (classes 4-13) tied to an initial average annual daily truck traffic (AADTT) count and growth rate are also needed. Load spectra data are commonly obtained from weigh-in-motion stations.

Along with load type and repetitions, the load distributions across a particular pavement geometric section must be estimated. For instance, on a six-lane interstate highway (3 lanes in each direction) the total number of loads is probably not distributed exactly equally in both directions. Often, one direction carries more loads than the other. Within that one direction, not all lanes carry the same loading. Typically, the outermost carries the most trucks and is subjected to the heaviest loading.

As a result, pavement structural design should account for these types of unequal load distribution. This is usually accounted for by selecting a “design lane” for a particular pavement. The loads expected in the design lane are either a) directly counted or b) calculated from the cumulative two-direction loads by applying factors for directional distribution and lane distribution.

The 1993 AASHTO Guide offers the following basic equation:

Where:

For instance, on most interstate routes, the outside lane carries a majority of the heavy truck traffic and would be the design lane.

The Transportation Planning and Programming Division (TPP) posts a directional distribution statistic in the

Traffic Analysis for Highway Design

report, but this distribution is related to peak ADT distributions (the 30th highest hourly volume) that affect level of service for geometric design as opposed to loading for structural design.

The assumption made in the

Traffic Analysis for Highway Design

report is traffic loading is equivalent in both directions. If the designer anticipates the truck directional distribution to be different from 50/50 or loads to be significantly greater in one direction, then this concern should be indicated in the request (Form 2124) submitted to TPP for project level traffic data.

Recommended lane distribution factors for both flexible and rigid pavements designs are:

TPP provides traffic projections (“Single Source Traffic Data Operating Procedures” from the

Transportation Planning Policy Manual

, Chapter 3, Section 4). The designer must request a 20-yr. traffic projection for flexible pavements and a 30-yr. traffic projection for rigid pavements from the Traffic Section of TPP.

For pavement design purposes, external and internal requests for traffic projections should be coordinated with the district director of Transportation Planning and Development (TPD). The district will use Form 2124, Request for Traffic Data¹. External requests for traffic data for purposes other than pavement design must be immediately referred to the open records coordinator assigned to the district, division, or office that received the request.

CAUTION: Close coordination is needed with the district TPD to ensure timely turnaround of traffic data for pavement structural design. For quick turnaround, check ONLY box “1” on Form 2124. For check box “1” data, time frame to receive data should be no more than 7-10 business days.

If the district has need of other data options on the form (environmental studies, line diagrams, or corridor analyses), the district submitter should make clear on Form 2124 that box “1” requirements should be given priority. The data for check box “1” will be sent to the district in the quick turnaround time frame with the other requirements sent at a later date.

Units of Measurement. Currently, traffic loading is only rendered in terms of ESALs (versus axle load spectra). This is the standard method of evaluating loads for highways designed by the department. ESALs are evaluated for flexible and rigid pavements differently as a result of empirical relationships developed following the AASHO Road Test. Estimates for each type of pavement will be different. ESAL estimates will also vary slightly based on the overall pavement structure (also a result of empirical relationships).

Loading Estimates. The default structure used by the Traffic Section of TPP for traffic loading estimates is an 8 in. rigid slab or a flexible pavement of structural number (SN) 3. The SN is a structural thickness index used in the AASHTO 93 method of flexible pavement design. These structural classes (8-in rigid slab or SN=3 for flexible) are for highways with light to moderate traffic, but ESALs generated for these structures result in more conservative structural thicknesses and are preferred for comparative design when considering alternate pavement types. An option for highways with moderate to heavy traffic is to provide TPP with the closest estimate for the appropriate structural class in terms of SN (flexible pavement) or thickness in inches (rigid slab).

Projections and Special Factors. The designer must ensure that all pertinent information regarding characteristics of the project that influence traffic loading are made known to traffic analysts in TPP. Use Form 2124, Request for Traffic Data1, to request traffic data. For structural pavement design, supply administrative data on Form 2124 including:

Not specifically included on this form is a field for identifying roadbeds. When multiple roadbeds exist along a corridor, identify the required roadbed by mainlanes or frontage roads and by direction, if necessary. Make clear which lanes traffic data are being requested for so the design will reflect traffic loading specifically for the intended project scope.

Districts should review the “Traffic Analysis for Highway Design” report to verify reasonableness. Trucks have the largest impact on ESALs and, therefore, the simplest screening check is to verify the percentage of truck traffic in the average daily traffic (ADT) through observation. A simple ratio of actual truck percentages versus those estimated by TPP can be applied to the TPP estimated ESALs as a quick update for design purposes. A 48-hour continuous classification count within the project limits should be considered for jobs where extreme traffic loading is anticipated. Assigning reasonable truck factors (ESALs/truck) and growth rates to project cumulative ESALs over the design period can be performed using the DARWin® 3.1 ESAL generator or similar tool to compare against TPP supplied traffic data.

There may be occasions where the traffic forecast data supplied by TPP becomes slightly dated due to project letting delays or the original data supplied is for the wrong analysis time interval (e.g., 30-yr. data needed, but only 20-yr. data requested). In these cases, the designer may use TRAFFIC6.xls or similar routine to interpolate or extrapolate ADT and cumulative ESAL data using the same formula embedded within FPS. This formula automatically adjusts 20-yr. traffic data to match the user-selected analysis period. Use with caution; for job locations where traffic patterns have recently changed or will change in the near future, major roadways, or other jobs where the original traffic forecasts are more than 5 years old, the designer is encouraged to request an updated forecast from TPP.

For more information on traffic projections, refer to Chapter 1, Section 4 “

10430: Obtain traffic data

” in the Project Development Process Manual

.