EFFECTS OF TRUCK WEIGHT ENFORCEMENT
Michael W. Goelzer, Fred R. Hanscom, and Kenneth H. McGhee
Transportation Research Corporation
2710 Lookout Road, Haymarket, VA, 20169, U. S. A.
AbstractThis paper describes a pavement management decision-making aid, a software package to predict the effect of truck weight enforcement on pavement life. The software program, Truck Weight Enforcement Effectiveness Tool (TWEET), was developed (under the United States' National Cooperative Highway Research Program) to assist highway agencies in determining the effectiveness of truck weight enforcement programs. The software takes into account the reduction of WIM-measured ESALs observed to be associated with truck weight enforcement programs.
The software user essentially conducts a pavement design-life enforcement-effects analysis. Default pavement design values for both flexible and rigid pavements are provided by the software to assist the user. Operation of the software requires Weigh-in-Motion (WIM) data collected under varying conditions of truck weight enforcement. The software determines pavement life differences based on observed truck axle weights.
1.0 INTRODUCTIONA key to managing pavements is the development of appropriate tools to aid decision-makers who are responsible for pavement design, construction and maintenance. A critical element in pavement wear is the effect of heavy trucks, and a significant consideration to pavement managers is the real impact of truck weight enforcement, i.e., the impact on weight violations to pavement service life.
The Truck Weight Enforcement Effectiveness Tool (TWEET) is a software application developed under NCHRP Project 20-34 and designed to aid users in determining the effectiveness of user-specified truck weight enforcement activities. It works by reading WIM data which has been collected under two user-designated enforcement conditions, and it allows the user to compare M.O.E. data from each condition so as to determine the more effective enforcement method.
2.0 EFFECTS OF AXLE WEIGHT ENFORCEMENT ON PAVEMENT LIFEOne of the basic premises of truck weight enforcement is that there will be a net increase in pavement life (reduction in the rate of pavement deterioration). The following discussion summarizes two methods of determining the increase in pavement life one could expect from reduced axle loadings accrued through enforcement activities. The methods makes use of an AASHTO design procedure (1) providing for the traffic input to design to be in terms of accumulated (or projected) 18,000 lb. equivalent single axle loads (ESALs).
In their approach, AASHTO uses the definition: "Load equivalency factors represent the ratio of the number of repetitions of any axle load and axle configuration (single, tandem, tridem) necessary to cause the same reduction in Present Serviceability Index (PSI) as one application of an 18-kip single axle load."(1). Thus, an axle load with an 18-kip equivalency of 2.5 could be considered to be 2.5 times more damaging than the 18-kip loading.
The general approach is to determine the cumulative ESALs a given pavement is capable of sustaining before it's serviceability is reduced to an unacceptable level, i.e., the design load capacity. Then, the traffic stream using that pavement is analyzed both before and after enforcement efforts are implemented to determine the effects of that enforcement on daily ESALs generated by the stream. Finally, the daily ESALs before and after enforcement are used to determine the estimated times (before and after enforcement) required to consume the load capacity.
AASHTO design procedures provide for the traffic input to design to be in terms of accumulated (or projected) 18,000 lb. equivalent single axle loads (ESALs). In their approach, AASHTO uses the definition: "Load equivalency factors represent the ratio of the number of repetitions of any axle load and axle configuration (single, tandem, tridem) necessary to cause the same reduction in Present Serviceability Index (PSI) as one application of an 18-kip single axle load."(1). Because of that definition, many designers view the equivalency factor of a given axle load to be a relative measure of pavement damage inflicted by that load.
The serviceability index (PSI or p) is a subjective measure of pavement condition on a 0 to 5 scale with 0 defined as unusable and 5 defined as perfect. While there are many variations, a typical new road will have an initial serviceability (p0 or PSI at time 0) of about 4.4 while the terminal or no longer acceptable serviceability (pt) generally ranges from 2.0 to 3.0.
Unfortunately, the analysis of traffic data from a pavement design standpoint is greatly complicated by the fact that the relationship between axle loads and ESALs (equivalency factor) is geometric rather than linear and the relationship is a function of pavement structural capacity as well the level-of-service at which the pavement is considered to have failed (the terminal PSI). Further, the relationships differ for flexible and rigid pavements. ESAL equations for both types of pavements and for single and tandem axle loads were derived from the AASHO Road Test (2). Relationships for tridem axles have been developed through other research to extend the Road Test results (3).
The ESAL equivalency factor equations for flexible pavements are:
Equation (1): log10(wx/w18) = 4.79*log10(18+1)-4.79*log10(Lx+L2) + 4.33*log10L2 + Gt/bx - Gt/b18
Equation (2): Gt = log10[(4.2 -pt)/2.7]
Equation (3): bx = 0.40 + [0.081*(Lx + L2)^3.23]/[(SN + 1)^5.19*L2^3.23]
wx = number of loads of magnitude Lx required to reduce the PSI to pt,
w18= number of 18 kip loads required to reduce the PSI to pt,
Lx = load on one single axle or one tandem axle set (kips),
L2 = axle code (1 for single axle and 2 for tandem axle),
SN = pavement structural number (see Section 6 for examples of SN determination),
pt = terminal serviceability (on a 0 to 5 scale typical pt values are 2.0, 2.5, and 3.0), and
b18= value of bx when Lx = 18 and L2 = 1.
For rigid pavements, the equations are:
Equation (4): log10(wx/w18) = 4.62*log10(18+1)-4.62*log10(Lx+L2) + 3.28*log10L2 + Gt/bx - Gt/b18
Equation (5): Gt = log10[(4.5 - pt)/3.0]
Equation (6): bx = 1.00 + [3.63*(Lx + L2)^5.20]/[(D + 1)^8.46*L2^3.52]
where D is the slab thickness in inches.
3.0 SOFTWARE APPLICATIONThe software package, Truck Weight Enforcement Effectiveness Tool (TWEET), applies the above pavement design principles in determining the impact of user-specified truck weight enforcement activities. The program runs on any version of Windows. Its running time will depend upon a number of factors, i.e., vehicle sample size and host computer operating characteristics (e.g., the megahertz rating). The software has demonstrated outstanding efficiency, processing tens of thousands of truck weights in less than a minute.
This software presents the user with a variety of "dialog boxes", i.e., pop-up screens which enable the user to provide required input to run the software. The software is designed to be user friendly, e.g., in most cases the user will simply press the "Next" button to continue operation based on the program's output.
To start a truck weight enforcement analysis, the user would press the Start Analysis button in the program's main window (See figure 1 on the next page) as the program begins. Three discrete steps to the data analysis and interpretation process are as follows.
3.1 User InputThe program requires the user to enter such information as the applicable units the program is to use (i.e., English or Metric system), WIM data file format (if non-typical) , and legal weight limits. Default values are provided to assist the user.
3.2 CalculationsThe program performs necessary M.O.E. calculations based on WIM data contained the user's files. This calculation process is automatically performed by the program, and the user need not be concerned with this part of the program. During calculations, a graphical percentage-completion indicator is displayed to advise the user of the program's activity.
3.3 OutputCalculated Measures of Effectiveness (M.O.E.s) are displayed to the user. On-screen reports are displayed in a series of dialog boxes, each of which permit by the user to print out calculated results. The program will automatically display calculated values. Once the program has performed all the calculations, output can be viewed again by pressing the View Results button on the main window. Summary output can be printed out via the Print Results button.
4.0 PROGRAM OPERATIONStart the TWEET program, and the Main Window (see Figure 1on the next page ) dialog box will appear.
Figure 1. "Main Window" on M.O.E. Application Software
From the main window, press the button marked "Start Analysis." This will allow the user to start a truck weight analysis and enforcement effects. First the user will encounter a dialog labeled Select Units. See Figure 2 below.
Figure 2. "Select Units" Dialog Box
From this dialog box, the user selects the type of units to use (English or Metric) and presses "Next". This dialog box defaults to English units. Next, the user is asked to enter weight limits which the program is to use, via the Set Legal Weight Limits dialog box (see Figure 3). Current default values , easily modified by the user, are shown below.
Figure 3. The "Set Weight Limits" Dialog Box
Default values in the Set Legal Weight Limits dialog box have been set to the most commonly used weight limits. Modification of these defaults may be necessary depending upon prevailing legal regulations.
The user is then presented with the Select Truck Classification dialog box (See Figure 4). This dialog box allows selection of the user's choice of truck classification system. Choices are FHWA 13-Type or Custom. The 1995 FHWA Traffic Monitoring Guide 13-Type scheme is a standard 13-type vehicle classification system that should be adequate for most users. NOTE: At the time this software was developed, many states applied the FHWA Card-7 format. If data are in the FHWA Card-7 format, the user can click on the default standard 13-type classification option and the program will run normally.
Figure 4. "Select Truck Classification Scheme" Dialog Box
The user can apply a custom classification scheme by clicking on the Custom check box. The user will be prompted to enter the name for each of up to 15 different truck classifications in a series of dialog boxes.
The File Conversion dialog box (See Figure 5) is designed to assist agencies whose data format does not conform to either the 1995 FHWA Traffic Monitoring Guide
Figure 5. The "Data File Conversion" Dialog Box
13-Type scheme or Card-7 classification formats. If the user's data are not in either one of these formats, the Convert button is applied to display the TWEET Conversion Utility dialog box.
The TWEET Conversion Utility (See Figure 6) main window provides a fast, efficient way to convert data files from other formats to the 1995 FHWA Truck Weight Record format. The following is an overview of the features of the main windows:
Figure 6. The "TWEET Data Conversion Utility" Dialog Box
Input File field: This field allows the entry of the name of an input file (in some foreign format) to be converted to the FHWA 1995 Truck Weight Record. If the name of the file is not known, the Browse button allows for a file to be selected from the available files on the system's hard disk or floppy disks.
Output File field: This field allows the entry of the name of the output file to be created which will contain the data in the input file, formatted in the FHWA 1995 Truck Weight Record format. If the name of an already existing file is entered in this field, that file will be permanently overwritten.
Input Format field: This field allows for the selection of an input file format. There are two choices of possible input file formats:
Card 7 - Files of this popular format cannot be automatically converted to FHWA 1995 Truck Weight Record format files.
Custom - This option allows for the entry of a custom data file format through the Custom Data Format dialog.
Output Format field - This field, which is not changeable by the user, shows that all output files processed by the TWEET Data File Conversion Utility will be formatted as FHWA 1995 Truck Weight Record files.
Convert button - Once all of the four fields above have been set correctly (except for the Output Format field, which is set by the program itself) pressing this button will cause the file conversion to be performed.
Advanced button - This button allows for the use of a custom file filter in the case that the regular conversion program cannot handle a particular custom file format. A custom filter is a specific routine written by a user that would plug in to the TWEET Conversion Utility to convert its own unique, non-standard file format.
The user then defines observed enforcement conditions providing input to the Enforcement Condition 1 of 2 dialog box (See Figure 7).
Figure 7. The "Enforcement Condition" Dialog Box
Now the program asks the user to enter information about enforcement conditions which the user is going to study. For each condition, the user will be asked to enter the following information:
Name The user may give the condition any name. It is advisable, although not necessary to use a unique name.
Location This field allows the user to enter the location from which the data was collected. This field may be left blank.
Start Date This field asks the user to enter the starting date of the study; it too may be left blank.
End Date This field asks the user to enter the ending date of the study; it too may be left blank.
The above information corresponds to enforcement conditions to be observed in the field. The user will also provide similar information to the above in response to the Enforcement Condition 2 of 2 dialog box .
For each designated enforcement condition, a Number of Files for Condition dialog box asks for the identification of WIM data files which pertain to each condition. Up to four files can be utilized for each condition. Following this step, the File 1 of 1 for Condition 1 dialog box (See Figure 8) asks the user to select a particular data file for the current condition.
The program will then ask the user to select (or name) the data files pertaining to each condition. The user is presented with a series of dialogs requesting the path of each data file labeled "Select Data File x for Enforcement Condition y". "x" represents sequentially numbered data files and y is the number of the enforcement condition for which you are selecting data files.
Figure 8. The "Data File for Enforcement Condition" Dialog Box
The Pavement Analysis dialog box (See Figure 9) appears next. The user now is given the option to conduct a pavement design-life enforcement-effects analysis. The program asks for specific (and detailed) pavement design data. Because of the complexity of the pavement design-life analysis, the user has the option of skipping the pavement analysis by simply clicking the 'skip pavement analysis' option.
Assuming that the user wants the pavement design-life analysis, he (or she) first selects the applicable pavement material, either Flexible or Rigid. Note that depending on whether Flexible or Rigid pavement is selected there will be a different set of variables in the Pavement Characteristics box at the bottom of the dialog. This box will prompt the user for appropriate pavement design parameters. A comprehensive "Help" screen associated with the Pavement Analysis Dialog boxes explains the design theory, including the AASHTO design equations, underlying the computations shown here.
Figure 9. The "Pavement Analysis" Dialog Box
Flexible pavements are discussed first. Default values are shown on the screen for the following parameters.
SN Pavement Structural Number. TWEET offers the option of computing this variable based on input values provided by the user.
po Initial Serviceability Index
pt Terminal Serviceability Index
MR Effective Roadbed Soil Resilient Modulus (psi)
ZR Standard Normal Deviate corresponding to design reliability
SO Standard Deviation associated with pavement performance prediction
Because the pavement's Structural Number (SN) is so important, TWEET provides three ways for the user to enter this value. First, he may accept the commonly-applied default value show here. Second, he may apply his own value for SN, if it is known. Third, if the user knows the material composition of the pavement, TWEET can automatically calculate the SN value. In this case, the user clicks on 'Calculate SN', and the Automatic Calculation of SN dialog box appears as shown in Figure 10.
Figure 10. The "Automatic Calculation of SN" Dialog Box
The above dialog box allows the user to select the appropriate surface, base, and sub-base characteristics, i.e., pavement layer thickness (in inches), and strength coefficient. According to the specified material type, the program will suggest the most appropriate default Strength Coefficient. Pavement materials and design personnel who run this software have the option of overriding default values, depending upon their own knowledge of pavement materials and design procedures along with specific pavement characteristics associated with the enforcement location
In the event the user had selected Rigid Pavement, the Pavement Analysis (Rigid Pavement) dialog box would appear as shown in Figure 11.
Figure 11. "Pavement Analysis" Dialog Box
The above dialog box provides the following design values for user application.
k Modulus of Subgrade Reaction (psi/in.)
E PCC Elastic Modulus (psi)
D Slab Thickness (inches)
sc' Estimated Mean PCC Modulus of Rupture (psi)
p0 Initial Serviceability Index
pt Terminal Serviceability Index
As was the case with the Flexible Pavement Characteristics box, the most likely default design values have been provided in the case of Rigid Pavements. The user has the option of manually entering values specific to the highway study site.
5.0 VIEWING RESULTS OF CALCULATIONSThe program will now perform calculations. Unless data files are extraordinarily large, these calculations should take no more than a few seconds. An animated graphic Status dialog box (see Figure 12 below) will appear to advise of the program's progress on the computational process, as the truck moves from right to left on the roadway section.
Figure 12. "Calculation Status" Dialog Box
Once the calculations have finished, the user will be presented with a series of "output" dialog boxes which display calculated values based on input data.
The first M.O.E. output dialog box, Severity of Violations (see Figure 13), also reports summary information, i.e., enforcement condition, highway type, total vehicle, and truck sample. The violator numbers and average overweight values are indicated.
Figure 13. "Severity of Violations" Dialog Box
The Calculated Percentages of Overweight Trucks dialog (see Figure 14) displays the calculated percentages of overweight trucks in the sample. It lists four calculations based on the data files:
Percentage of trucks over the legal gross weight limit This number is merely the percentage of all trucks whose gross weigh exceeded the legal limit as set by the user in the Legal Weight Limits dialog.
Percentage of trucks over the single axle weight limit This is the percentage of trucks exceeding the weight limit for a single axle.
Percentage of trucks over the tandem axle weight limit This is the percentage of trucks exceeding the weight limit for a set of tandem axles. Tandem axles are defined as a set of axles which are within a certain distance of each other, usually 6ft (or metric equivalent). The sum of the weights of each axle which is within this distance from the other axles is called the tandem axle weight.
Percentage of trucks violating the Bridge Formula This is the percentage of all trucks in the sample which violated the Bridge Gross Weight Formula. The presence of a violation is determined by using an equation, the Bridge Formula, which relates the axle spacing and axle weights.
Figure 14. "Calculated Percentage of Overweight Trucks" Dialog Box
The Violation Data by Truck Classification dialog box (Figure 15) indicates violators, by truck number and percentage, for each class of truck. Simply click on the truck classification list to the screen's left, and the statistics appear on the right side of the screen.
Figure 15. "Violation Data by Truck Classification" Dialog Box
This Violation Data by Truck Classification dialog displays violation information as broken down by truck classification. (NOTE: The data disk included with the Draft Report contains all Type 9 trucks.) This information is useful in determining which types of trucks exhibit the largest (or least) violations and which are the most (or least) common on the road, etc. The dialog consists of two parts:
Truck Classification List Box This box lists all of the truck classifications which were input by the user during the beginning of the analysis, or if the default was selected, the FHWA 13-type classifications. Click on one of these classifications with the mouse or select it with the arrow keys on the keyboard. The data corresponding to the selected type of truck will appear at the right in the Violation Data section.
Violation Data This part of the dialog lists violation data for the currently selected truck classification. First, the Total Number of Trucks field displays the number of trucks of the selected type which were in the sample (regardless of whether they were violators). Second, the Number of Violators field lists the number of trucks of the selected type which violated the weight limits. Third, the Percentage Violating field lists the percentage of trucks of the selected type which were violators (this percentage is simply the Number of Violators divided by the Total Number of Trucks). Finally, this part of the dialog indicates the total violator proportion presented by truck classification. In the figure above, while only 37 percent1 of Type 9 trucks were violators, this sample comprised 87 percent of the violators due to their high representation in the overall traffic stream.
The following two dialog boxes are not illustrated.
The Breakdown of Violations by Day-of-Week dialog displays the percentage of violations occurring on each day of the week. The dialog simply lists each day of the week, and next to it lists the percentage of all violations which occurred on that day. Note that only gross, single-axle and tandem-axle violations are counted toward the percentages for each day (Bridge Formula Violations are not counted).
The Breakdown of Violations by Time-of-Day dialog displays the percentage of violations occurring at different hours of the day. Because it would be overly complex to display the percentage of violations occurring at each of the 24 hours of the day, the five hours with the most violations are listed. If it is necessary to know what percentage of violations occurred at every hour of the day, the Print option will be of use. The printed copy of the data, unlike the on-screen display, does display the percentage of violations for each hour of the day. Like the Breakdown of Violations By Day of Week dialog, only gross, single-axle and tandem-axle violations are counted toward the percentages for each hour.
The ESAL Data dialog box indicates average ESAL calculations using the FHWA Traffic Monitoring Guide procedure according to the number of axles. This dialog also indicates computed Excess ESAL violations by truck axle-count.
Six dialog boxes follow contain the same M.O.E. calculations as noted above for Enforcement Condition #2.
Now, TWEET goes into its 'What does it all mean?' mode! The Comparison of Enforcement Conditions dialog indicates to the user whether or not the observed M.O.E. differences are significant.
The Comparison of Enforcement Conditions dialog box (see Figure 16) contains results of applied statistical significance tests to the computed M.O.E.s and indicates to the user whether or not the observed differences are significant. Separate tests of statistical significance are applied to M.O.E.s depending upon whether the measure was calculated as a mean (i.e., average gross weight violation) or a proportion (i.e., proportion of gross weight violators). Significance tests are applied at the .05 level of statistical confidence.
Figure 16. "Comparison of Enforcement Conditions" Dialog Box
Next, a Sampling Guide dialog box (not shown) provides an aid to determine how many sites will needed to be surveyed in order to detect regional changes for designated M.O.E.s given specified levels of statistical confidence.
The final dialog box (Figure 17) presents results of the Pavement Effects Analysis.
Figure 17. The "Pavement Effects Analysis" Dialog Box
Results contained in the Pavement Effects Analysis dialog are based on the calculated pavement design-life effect, associated with differential enforcement-related ESAL loading conditions. Had the user opted to include the pavement design-life effect computation, this screen would be displayed. Displayed information consists of the calculated pavement ESAL capacity, the estimated pavement life under both observed enforcement conditions, and estimated percentage pavement-life change due to the observed ESAL-loading difference associated with the enforcement activity.
6.0 PRINTING OUT RESULTSEach output dialog box incorporates a Print button for the purpose of printing results shown. However, if the user would like to print all of the results from the analysis rather than pressing the print button in every single output dialog box, the Print Results button is pressed once the main window appears. This window (which incorporates the Start Analysis button for a new data computation) will appear following the output dialog boxes. The Print button in this dialog box will print all of the calculated results from the last analysis for both enforcement conditions
7.0 ACKNOWLEDGMENTResearch reported in this paper was conducted under NCHRP Project 20-34, Developing Measures of Effectiveness for Truck Weight Enforcement Activities, under contract to the Transportation Research Corporation.
1. American Association of State Highway and Transportation Officials, AASHTO Guide for Design of Pavement Structures, Washington, DC, 1993.
2. American Association of State Highway and Transportation Officials, AASHTO Interim Guide for Design of Pavement Structures 1972, (Chapter III Revised, 1981), Washington, DC, 1972, 1981.
3. Treybig, H. J. and H. L. Von Quintus, Equivalency Factor Analysis and Prediction for Triple Axles, Report No. BR-2/1, Austin Research Engineers for Texas Research and Development Foundation, Austin, TX, 1976.
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