U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000


Skip to content
Facebook iconYouTube iconTwitter iconFlickr iconLinkedInInstagram

Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

 
REPORT
This report is an archived publication and may contain dated technical, contact, and link information
Back to Publication List        
Publication Number:  FHWA-HRT-13-090    Date:  April 2016
Publication Number: FHWA-HRT-13-090
Date: April 2016

 

MEPDG Traffic Loading Defaults Derived From Traffic Pooled Fund Study

CHAPTER 2 - Review of MEPDG traffic loading defaults

This chapter covers the following topics:

ASSESSMENT OF METHODOLOGY USED FOR ORIGINAL TRAFFIC LOADING DEFAULTS

The research team reviewed the original methodology for generating MEPDG traffic loading defaults, and the results of that review are summarized in this section.

Data Selection Criteria for Development of MEPDG Defaults

The researchers used the following data selection criteria to identify data for the development of the original MEPDG traffic defaults:

Development of the default axle load spectra was based on data from 134 sites. Defaults for axle spacing in tandem and tridem axle configurations were based on data from 26 sites, and axles per truck type defaults were based on data from 16 sites. All the defaults were based on LTPP data collected up to 1999 for General Pavement Studies (GPS) sites that passed rudimentary LTPP quality checks.

Summary of Traffic Loading Defaults and Methods Used

WIM data were used to generate the following global traffic loading defaults when sufficient traffic weight data were unavailable:

Determination of Expected Errors in Traffic Estimates Using Original Methodology

The NCHRP Project 1-37A report includes a procedure to estimate the expected error of the site-specific traffic estimates based on the amount of data collected at a site, given the variation in the data and the selected confidence interval.(3) The following equation was used to calculate the expected error in estimating the daily number of trucks for each vehicle class.(6)

Figure 1. Equation. Expected error in estimating the daily number of trucks for each vehicle class. e open parenthesis VC subscript k closed parenthesis subscript j equals Z divided by n to the power of 0.5 times open parenthesis sigma divided by mu closed parenthesis subscript k, j.

Figure 1. Equation. Expected error in estimating the daily number of trucks for each vehicle class.

Where:

e(VCk)j = Expected error for vehicle class k in season j.

Z = Confidence interval coefficient.

n = Number of sampling days.

σ = Standard deviation of the number of class k vehicles in the population during season j.

μ = Mean number of class k vehicles in the truck traffic population during season j.

This approach accounts for errors in truck volume estimates associated with data availability and variability of data due to natural fluctuations in truck volumes during the season. However, it does not account for the errors associated with WIM equipment performance or expected level of accuracy of WIM systems, nor does it provide an estimate of expected errors in axle loads.

APPLICABILITY OF THE ORIGINAL MEPDG TRAFFIC LOADING DEFAULTS

The research team identified, reviewed, and compiled studies published prior to 2009 that evaluated the reasonableness of the original MEPDG traffic defaults or MEPDG sensitivity to traffic inputs. (See references 7-10.) Most of the identified studies focus on sensitivity of pavement design to MEPDG defaults (level 3 inputs) versus site-specific or regional traffic data (level 1 inputs) and present State-sponsored research projects with the focus on State implementation of the MEPDG. Depending on pavement type, distress type, climatic region, State, and functional type of road, the effect of load spectra on pavement design or performance prediction ranged from low to high.

Understanding the Physical Meaning of the Original Default NALS

Some of the criticism regarding the original MEPDG traffic loading defaults comes from unusual shapes reported as a default NALS. The following example demonstrates that the use of the average NALS for each axle group type for a specific truck class is the reason why some NALS do not exhibit some of the typical loading features or patterns that have been reported for specific roadway segments (i.e., appearing like a "stretched" distribution with longer tail of the distribution (higher percentages of overloads and higher percentage of light loads)).

Figure 2 compares the default NALS for tandem axle for truck class 9 to the loading patterns from three loading spectra for SPS TPF sites within the same functional classification (rural other principal arterial (ROPA)). As shown, site-specific NALS are significantly different, and one may question the applicability or adequacy of the default tandem spectra.

Figure 2. Graph. Comparison of three tandem axle loading patterns to the MEPDG default NALS for tandem axles for truck class 9. This graph shows a comparison of three tandem axle loading patterns to the Mechanistic-Empirical Pavement Design Guide (MEPDG) default normalized axle load spectra (NALS) for tandem axles for truck class 9. The x-axis shows the tandem axle load ranges in pounds, and the y-axis shows percentage of tandem axles from 0 to 20 percent. There are four series of lines shown in the figure that correspond to the various loading patterns: lightly loaded, bimodal, heavily loaded, and MEPDG default. The lightly loaded curve is represented by a continuous green line and green triangular markers for data points and has a peak of 20 percent at 10,000 to 11,999 lb and a second peak of 9.5 percent at 30,000 to 31,999 lb. The bimodal curve is represented by a continuous blue line and crosses for data points and has a peak of a little over 12 percent at 12,000 to 13,999 lb and a second peak of 10.5 percent at 32,000 to 33,999 lb. The heavily loaded curve is represented by a continuous burgundy line and square markers for data points and has a peak of a little over 5.5 percent at 12,000 to 13,999 lb and a second peak of 19 percent at 32,000 to 33,999 lb. The MEPDG default curve is represented by a continuous black line and diamond markers for data points and has a peak of a little less than 8 percent at 12,000 to 13,999 lb and a second peak of 6 percent at 30,000 to 31,999 lb.

Figure 2. Graph. Comparison of three tandem axle loading patterns to the MEPDG default NALS for tandem axles for truck class 9.

The NCHRP 1-37A research team computed an average NALS for each axle group type and truck class using data from all available sites.(3) The averaging method is used in figure 3, where three different SPS TPF NALS for the same road functional class are averaged and compared to the default NALS for tandem axles. The average of the three site-specific distributions has bi-modal distribution and shows more resemblance to MEPDG defaults.

Figure 3. Graph. Average tandem axle loading patterns for truck class 9. This graph shows the average tandem axle loading patterns for truck class 9. The x-axis shows the tandem axle load ranges in pounds, and the y-axis shows percentage of tandem axles from 0 to 20 percent. There are two series of lines shown in the figure that correspond to the various loading patterns: combined average values and Mechanistic-Empirical Pavement Design Guide (MEPDG) default. The combined average values curve is represented by a continuous purple line and circular markers for data points and has a peak of 12 percent at 10,000 to 11,999 lb and a second peak of 13 percent at 32,000 to 33,999 lb. The MEPDG default curve is represented by a continuous black line and black diamond markers for data points and has a peak of a little under 8 percent at 12,000 to 13,999 lb and a second peak of 6 percent at 30,000 to 31,999 lb.

Figure 3. Graph. Average tandem axle loading patterns for truck class 9.

In summary, the MEPDG default values represent an average NALS from multiple sites across the United States, including light, bi-modal, and heavy loading patterns. These defaults are based on data collected using a variety of data collection equipment with various maintenance and calibration procedures applied. Thus, the bi-modal distribution for tandem axles shown in figure 3 is somewhat smoothed out because of the averaging process. The averaging process was used because the amount of variation within the same functional classification found during NCHRP 1-37A was about the same as between all functional classifications, and only one global set of defaults was developed. However, the question still remains-are these overloads real or a result of measurement errors or improper calibration of the WIM equipment? This question was not addressed during NCHRP Project 1-37A; it was assumed that the overloads were real values.(3)

Applicability of Default NALS

The current state of knowledge does not provide a conclusive answer whether the original defaults could be used successfully by States that move forward with MEPDG implementation. Early findings suggest that while some States find the NALS defaults applicable for their local conditions, others find a need for regional defaults or some other groupings of NALS for improving on the accuracy of pavement design. States that have percentages of heavy and overloaded axles (over 75 percent of legal weight limit) similar to the ones used in default NALS are more likely to benefit from the defaults than States that have significantly different percentages of heavy and overloaded axle loads.

Applicability of Axles per Truck Numbers and Axle Spacing

The values computed under the NCHRP 1-37A study and included in MEPDG software were based on data from 16 LTPP sites that did not have well-documented vehicle classification algorithms.(3) Additionally, some differences existed in State-defined vehicle classification schemes. Therefore, it is expected that some inconsistencies or truck misclassifications are present in the data used to determine the average number of axles per truck type by axle group and the average axle spacing.

CONCERNS AND LIMITATIONS OF THE ORIGINAL MEPDG TRAFFIC DEFAULTS

The following lists provide a summary of concerns andissues found in application of the original MEPDG traffic defaults.

Data processing and data quality concerns include the following:

Issues associated with the methodology include the following:

RECOMMENDATIONS FOR ENHANCEMENT OF THE ORIGINAL MEPDG TRAFFIC LOADING DEFAULTS

Several recommendations for enhancements or alternatives for the default NALS are as follows:

Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000
Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101