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Development of Truck Payload Equivalent Factor (TPEF)

3.0 Data Sources

Analysis of payload distribution is based on two data sources, namely VIUS and VTRIS. A brief detail of the data sources are described below.

3.1 VIUS (Vehicle Inventory and Use Survey)

The VIUS database was created as a result of the vehicle inventory survey. The survey is normally conducted every five years as part of the economic census. The latest VIUS data is for the year 2002. The survey sample is drawn from each state's registration records and the process involves formatting a questionnaire to compile all required information. The questionnaire, with the vehicle registration number, is sent to the appropriate owner to furnish details of the vehicle whose registration is imprinted. The responses are computer-edited thoroughly and corrected if necessary.

VIUS provides physical and operational characteristic data of nationwide private and commercial truck fleet. The physical characteristic data include weight, number of axles, overall length, body type, etc. for medium and heavy trucks. The operational characteristic data include commodities handled, distance traveled, mileage, etc.

3.2 VTRIS (Vehicle Traveler Information System)

WIM is used by many states to process vehicle classification and truck weight data using the VTRIS software. The data collected by WIM are reported to VTRIS via an online data source maintained by FHWA. The WIM data reported for VTRIS are checked for accuracy and validated with the assistance of the VTRIS software before inclusion of the data into the database. The details of an error can be viewed and corrected – it is optional to include the data error in the database. The VTRIS system validates, edits, summarizes, and generates reports on vehicle characteristics.

The VTRIS summary provides an option to summarize the data for the required station or functional class in various data output formats called "W" form. The system also allows the user to generate reports W-2 to W-7 for a selected station or functional class. The ADT can be averaged hourly, monthly, or annually. W-tables are designed to provide a standard format of the outcome of the vehicle weighing and classification efforts. The report consists of the station numbers, functional class, method of averaging, name of the state, and year for all the tables from W-2 to W-7. The following describes the contents of the W-tables:

  • W-2 – Contains a summary of the vehicles counted and the vehicles weighed
  • W-3 – Reports weight data and is used for payload distribution analysis
  • W-4 – Contains equivalency factors by direction report including information on truck axle loading and their effect on flexible and rigid pavement based on ESAL
  • W-5 – Reports weight data and is used for payload distribution analysis
  • W-6 – Contains overweight vehicle reports
  • W-7 – Reports the distribution of overweight vehicles

4.0 Technical Approach

The VIUS database is used for the analysis of truck payload distribution based on various categories; for example, commodity handled, travel distance, and body type, for all 50 states including the District of Columbia. For VTRIS, only "Estimated Average Carried Load" and gross vehicle weight (GVW) distribution by FHWA vehicle class were carried. These data are reported in the VTRIS W-3 and W-5 tables. The payload analysis conducted is based on the VIUS GVW vehicle group. The VTRIS FHWA vehicle groups are described next.

4.1 Gross Vehicle Weight Groups

The gross vehicle weight (GVW) groups are listed under the VIUS variable ADM_GVWR. The eight GVW groups include:

Group 1 – Less than 6,000 lbs.
Group 2 – 6,001 to 10,000 lbs.
Group 3 – 10,001 to 14,000 lbs.
Group 4 – 14,001 to 16,000 lbs.
Group 5 – 16,001 to 19,500 lbs.
Group 6 – 19,501 to 26,000 lbs.
Group 7 – 26,001 to 33,000 lbs.
Group 8 – More than 33,000 lbs.

The average payload for each GVW group is calculated by subtracting the empty weight of the truck as reported under the VIUS variable WEIGHT EMPTY from the truck average weight as reported under the VIUS variable WEIGHT AVG. A new database consisting of only the fields required for the analysis is created from the VIUS data in the Statistical Analysis Software (SAS). Also, the records with missing values for the VIUS variable WEIGHT AVG and WEIGHT EMPTY were considered invalid and excluded to maintain the accuracy of the results.

4.2 FHWA Vehicle Groups

FHWA vehicle groups in the VTRIS database are different from the VIUS vehicle groups. VTRIS data are available only for 36 states and the analysis is carried only for those states. The payload distribution is based on the nine FHWA vehicle classes listed below:

Class 1 – Single Unit: 2-axle
Class 2 – Single Unit: 3-axle
Class 3 – Single Unit: 4-axle or more
Class 4 – Truck/Tractor Trailers: 4-axle or less
Class 5 – Truck/Tractor Trailers: 5-axle
Class 6 – Truck/Tractor Trailers: 6-axle or more
Class 7 – Combination Trucks: 5-axle or less
Class 8 – Combination Trucks: 6-axle
Class 9 – Combination Trucks: 7-axle or more

The variable "Estimated Average Carried Weight" listed in the W-3 report is used for payload analysis by VTRIS vehicle group. To maintain accuracy, the data are checked for accuracy and validity record by record using the VTRIS software.

The average payload distribution analyses were carried out for all 50 states and the District of Columbia to fulfill the output requirements by each task as outlined in the SOW. In addition, payload analyses for national average were also carried out and reported as part of the deliverable. For VTRIS, the analysis was limited to 36 states that participated in the annual VTRIS data program. The payload analyses were carried out in the following nine areas:

  1. GVW classes without distance and commodity specifications
  2. GVW classes by 2-digit SCTG commodity level
  3. GVW classes by various traveled distance intervals
  4. GVW classes by various axle configurations
  5. GVW classes by various vehicle body types
  6. State-specific data on empty load
  7. State-specific data on truck loaded to size limit
  8. State-specific data on truck loaded to weight limit
  9. State-specific data on overloaded (weight) trucks

Since both VIUS and VTRIS report different vehicle classes, two sets of vehicle classes were used to accomplish the above task requirements. However, for comparison, VTRIS-equivalent vehicle classes were developed using the VIUS variable called "axle_config". The following criteria were used to develop comparable FHWA VTRIS vehicle classes using the VIUS data.

  • FHWA Class 1 – axle_config = 02
  • FHWA Class 2 – axle_config = 03
  • FHWA Class 3 – axle_config = 04 and 05
  • FHWA Class 4 – axle_config = 06 , 07 , 09, and 27
  • FHWA Class 5 – axle_config = 13, 15, 28, and 30
  • FHWA Class 6 – axle_config = 08, 11, 14,16, 17, 18, 19, 20, 29, 31, 31, and 32
  • FHWA Class 7 – axle_config = 37
  • FHWA Class 8 – axle_config = 38 and 41
  • FHWA Class 9 – axle_config = 39, 42 to 64

The national analysis on GVW payload, empty weight, and the overload weight limit illustrated in various tables and cumulative distribution graphs were developed using the following criteria and conditions:

  • The average payload distribution based on GVW vehicle group (Table 1 and Figures 1-7) and FHWA vehicle class (Table 3) are developed from the VIUS. In VTRIS, the W–3 report consists of the payload variable "Estimated Average Carried Load" based on the FHWA vehicle, which is similar to the VIUS payload (Table 5).
  • The average empty weight from VIUS is calculated for both the GVW (Table 2) and FHWA vehicle groups (Table 4) based on the variable WEIGHT EMPTY. The default empty weight reported in VTRIS W-3 is directly taken for the analysis (Table 6).
  • The average payload distribution from VIUS based on the 2-digit SCTG commodity code is calculated using the VIUS variable PRODUCT_PRINCPL. Table 9 and Figure 17 illustrate the national distribution of average payloads by commodity carried. For state-specific, this distribution was also carried out for both the GVW (Table 10) and the FHWA vehicle class (Table 13). VTRIS data do not support the product handled and therefore no analysis was conducted.
  • The average payload distribution of VIUS data based on the distance traveled is performed using the variable TRIP_PRIMARY. The analysis is carried out for VIUS GVW group (Table 11) and VIUS FHWA vehicle class (Table 14). The payload distribution based on the distance traveled cannot be calculated from a VTRIS data source due to the fact that VTRIS does not support the travel distance criteria. The analysis used the following VIUS travel distance intervals:
    1. Off-the-road
    2. Less than 50 miles
    3. 51 to 100 miles
    4. 101 to 200 miles
    5. 201 to 500 miles
    6. 501 miles or more
  • The VIUS identifies 28 vehicle body types. In a similar payload analysis for the FAF-2 traffic analysis project by Battelle, these 28 body types were further grouped to 11 major body types that represent most all possible commodity combinations used by various vehicle body configurations. These include:
    • Platform
    • Van
    • Auto Transport
    • Dump Truck
    • Grain Bodies
    • Garbage Truck
    • Livestock
    • Pole and Logging
    • Tank Truck for Dry Bulk
    • Tank Truck for Liquid Gases
    For analysis consistency, Battelle used these similar groupings. The VIUS 2002 variable for this analysis is BODYTYPE.  No VTRIS/WIM analysis was feasible for this task. The analysis is carried out for the GVW vehicle group (Table 12) and FHWA vehicle class (Table 15) from VIUS data source.
  • State Truck Size and Weight (TS&W) regulations are specific to vehicle and axle configuration. No specific regulation exists for size limit that is specific to a particular gross vehicle weight class. Therefore, Battelle developed state-specific data on trucks loaded to size limit by FHWA vehicle configuration and also the GVW classes. Cumulative frequency distribution curves were developed to allow for quantifying the level of overloading by a particular GVW or FHWA class under a governing weight regime (Figures 18 and 19). This cumulative gross weight distribution curve should allow the estimation of the percentage of overloaded and overweight vehicles for a known TS&W size regime in a given state. VTRIS W7 reports the percentage of overweight vehicles by FHWA vehicle class. VTRIS software restricts the gross weight limit values to 5 digits and hence W7 reports for those states with more than 100,000 lbs limitation cannot be generated for analysis.

State Specific analysis results are provided as a separate electronic database in Excel format.

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