ESAL Calculations

The Mn/DOT flexible pavement design methodology and nomographs are given in Chapter 5 of the Geotechnical and Pavement Manual.

Prior to completing a pavement design, the Mn/DOT method requires that all traffic be converted into Equivalent Single Axle Loads (ESALs), which is a means of equating various axle loads and configurations to the damage done by a number of 18,000 pound single axles with dual tires on pavements of specified strength over the design life of the pavement.

There are four steps to forecasting cumulative ESALs. District traffic forecasters, the Traffic Forecast and Analysis Section, and consultants can provide assistance. The four steps are:

1. Determination of Two-Way Average Annual Daily Traffic (AADT)
2. Determination of Two-Way Vehicle Classification Volumes
3. Determination of Flexible ESAL Factors
4. Determination of Design Lane ESAL Distribution
   

An electronic spreadsheet called MNESALS, is available from the Traffic Forecast and Analysis Section, or at the Traffic Forecast and Analysis Section. Another spreadsheet is provided below.

1. Determination of Two-Way AADT

Traffic Assessment is outlined here.

The firsts step of the ESAL determination is to find the AADT. The second part involves assigning the total AADT into different vehicle classifications.

AADT and/or Heavy commercial Average Daily Traffic (HCADT) forecasts may have already been approved during the Preliminary Design phase of a project's development. If no AADT forecasts exist, any of the following methods can be used to forecast traffic. Detailed descriptions of these methods and data collection techniques associated with each method can be obtained from Traffic Forecast and Analysis Section personnel.

• Inferred growth rate;
• Project AADT as a function of population, employment, or other independent variables;
• Least squares projection using historic AADT.
• Trunk highway AADTs prior to 1986 generally require axle correction factor adjustment.;
• Analysis of trends and patterns within a travel corridor; and
• Use of computer generated travel demand estimates.

Mn/DOT traffic flow maps are available from Mn/DOT and provide current estimates of AADT for the trunk highways, county road systems, and the municipal state aid system. If necessary, additional AADT estimates can be obtained from the Traffic Forecast and Analysis Section staff.

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2. Determination of Two-way Vehicle Classification Volumes

Next, assign a percentage of the total two-way AADT to each vehicle classification. The number of vehicle classifications depends on the number of uniquely defined, average ESAL factors. In Minnesota, there are eight sets of average ESAL factors that are generally used in ESAL forecasts. Number of axles and body configuration usually provide the basis for defining vehicle classifications when quantifying ESAL factors. Prior to assigning the vehicle distribution, evaluate the existing vehicle classification for a highway project segment, or segments close to the project. Vehicle classification site listing and maps are available from the Traffic Forecast and Analysis Section. If no data exists on or near the project, additional vehicle classification data should be collected. If data cannot be collected, refer to the table below (table 4-4.2 in the Geotechnical and Pavement Manual) for values that may be used to estimate vehicle classification volumes.

SU = Single Unit Trucks
TST = Tractor Semi Trailer
ax = axle

After determining current vehicle classification volumes for a site on a proposed highway project, determine how the volumes for each classification may change in the future. If historical data from the vehicle classification data record suggests that each vehicle classification volume as a percent of AADT has remained relatively stable through time, you may assume that future vehicle classification volumes can be estimated by multiplying each vehicle classification percent of current AADT by the AADT for the future year. If historical data or anticipated land development in the area suggests that one classification will increase or decrease at a different level, each classification can be projected independently. Documentation is required for all traffic assumptions and calculations.

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3. Determination of Flexible ESAL Factors

Average vehicle ESAL factors, formerly known as SN18 factors, represent daily, axle based, ESAL factors, summed by vehicle within a vehicle classification and averaged within a year, at one or more weigh-in-motion (WIM) data collection sites. Most WIM sites are located on roadways with ten-ton, single-axle weight limits with no springtime restrictions. The table below gives average ESAL factors for each vehicle classification, and for various types of roadway. Unless detailed axle weight, commodity and truck body type, and haul direction data is available, use the default ESAL factors given below.

Average ESAL factors by vehicle classification from WIM
Revised February 1, 1997

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4. Determination of the Design Lane ESAL Distribution

ESAL Calculator:This on-line spreadsheet should only be used if you do not have access to the MNESALS electronic spreadsheet.

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