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Table of Contents

Page

Section 2.25. Weigh-In-Motion Systems Used for Vehicle Enforcement Screening –

Tentative Code ................................................................................................................. 2-119

A. Application .................................................................................................................................................2-119

A.1. General. ............................................................................................................................................2-119

A.2. Exception. .........................................................................................................................................2-119

A.3. Additional Code Requirements.........................................................................................................2-119

S. Specifications..............................................................................................................................................2-119

S.1. Design of Indicating and Recording Elements and of Recorded Representations. ...........................2-119

S.1.1. Ready Indication.................................................................................................................2-119

S.1.2. Value of System Division Units..........................................................................................2-119

S.1.3. Maximum Value of Division. .............................................................................................2-119

S.1.4. Value of Other Units of Measure........................................................................................2-119

S.1.5. Capacity Indication.............................................................................................................2-120

S.1.6. Identification of a Fault.......................................................................................................2-120

S.1.7. Recorded Representations...................................................................................................2-120

S.1.8. Value of the Indicated and Recorded System Division.......................................................2-120

S.2. System Design Requirements. ..........................................................................................................2-120

S.2.1. Violation Parameters...........................................................................................................2-120

S.3. Design of Weighing Elements. .........................................................................................................2-121

S.3.1. Multiple Load-Receiving Elements....................................................................................2-121

S.4. Design of Weighing Devices, Accuracy Class. ................................................................................2-121

S.4.1. Designation of Accuracy.....................................................................................................2-121

S.5. Marking Requirements......................................................................................................................2-121

S.5.1. Location of Marking Information .......................................................................................2-121

N. Notes............................................................................................................................................................2-121

N.1. Test Procedures.................................................................................................................................2-121

N.1.1. Selection of Test Vehicles. .................................................................................................2-121

N.1.2. Test Loads...........................................................................................................................2-122

N.1.3. Reference Scale...................................................................................................................2-122

N.1.4. Test Speeds. ........................................................................................................................2-122

N.1.5. Test Procedures...................................................................................................................2-122

T. Tolerances...................................................................................................................................................2-123

T.1. Principles. .........................................................................................................................................2-123

T.1.1. Design.................................................................................................................................2-123

T.2. Tolerance Values for Accuracy Class A. ..........................................................................................2-123

T.2.1. Tests Involving Digital Indications or Representations. .....................................................2-123

T.2.2. Tolerance Values for Dynamic Load Test ..........................................................................2-123

T.2.3. Tolerance Value for Vehicle Position Test .........................................................................2-123

T.2.4. Tolerance Value for Axle Spacing......................................................................................2-123

T.3. Influence Factors...............................................................................................................................2-123
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T.3.1. Temperature........................................................................................................................2-123

T.4. Radio Frequency Interference (RFI) and Other Electromagnetic Interference Susceptibility. .........2-124

UR. User Requirements ....................................................................................................................................2-124

UR.1. Selection Requirements ....................................................................................................................2-124

UR.1.1. General................................................................................................................................2-124

UR.2. User Location Conditions and Maintenance.....................................................................................2-124

UR.2.1. System Modification...........................................................................................................2-124

UR.2.2. Foundation, Supports, and Clearance..................................................................................2-124

UR.2.3. Access to Weighing Elements.............................................................................................2-124

UR.3. Maximum Load.................................................................................................................................2-124

Appendix D. Definitions......................................................................................................................................2-125

axle ..........................................................................................................................................................2-125

axle-group load. ...........................................................................................................................................2-125

axle load.......................................................................................................................................................2-125

axle spacing .................................................................................................................................................2-125

single-axle load............................................................................................................................................2-125

tandem-axle load..........................................................................................................................................2-125

triple-axle load.............................................................................................................................................2-125

weigh-in-motion (WIM). .............................................................................................................................2-125

weigh-in-motion screening scale .................................................................................................................2-125

wheel weight................................................................................................................................................2-125

WIM System................................................................................................................................................2-125
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Section 2.25. Weigh-In-Motion Systems

Used for Vehicle Enforcement Screening – Tentative Code

This tentative code has a trial or experimental status and is not intended to be enforced. The requirements are designed 

for study prior to the development and adoption of a final code. Officials wanting to conduct an official examination 

of a device or system are advised to see paragraph G-A.3. Special and Unclassified Equipment.

(Tentative Code Added 2015)

A. Application

A.1. General. – This code applies to systems used to weigh vehicles, while in motion, for the purpose of screening 

and sorting the vehicles based on the vehicle weight to determine if a static weighment is necessary.

A.2. Exception. – This code does not apply to weighing systems intended for the collection of statistical traffic data.

A.3. Additional Code Requirements. – In addition to the requirements of this code, weigh-in-motion screening 

systems shall meet the requirements of Section 1.10. General Code.

S. Specifications

S.1. Design of Indicating and Recording Elements and of Recorded Representations.

S.1.1. Ready Indication. – The system shall provide a means of verifying that the system is operational and 

ready for use.

S.1.2. Value of System Division Units. – The value of a system division “d” expressed in a unit of weight 

shall be equal to:

(a) 1, 2, or 5; or

(b) a decimal multiple or submultiple of 1, 2, or 5.

Examples: divisions may be 10, 20, 50, 100; or 0.01, 0.02, 0.05; or 0.1, 0.2, 0.5, etc.

S.1.2.1. Units of Measure. – The system shall indicate weight values using only a single unit of measure. 

S.1.3. Maximum Value of Division. – The value of the system division “d” for a Class A, weight-in-motion 

system shall not be greater than 50 kg (100 lb).

S.1.4. Value of Other Units of Measure.

S.1.4.1. Speed. – Vehicle speeds shall be measured in miles per hour or kilometers per hour.

S.1.4.2. Axle-Spacing (Length). – The center-to-center distance between any two successive axles shall 

be measured in:

(a) meters and decimal submultiples of a meter;

(b) feet and inches; or

(c) feet and decimal submultiples of a foot.
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S.1.4.3. Vehicle Length. – If the system is capable of measuring the overall length of the vehicle, the 

length of the vehicle shall be measured in feet and/or inches, or meters.

S.1.5. Capacity Indication. – An indicating or recording element shall not display nor record any values 

greater than 105 % of the specified capacity of the load receiving element.

S.1.6. Identification of a Fault. – Fault conditions shall be presented to the operator in a clear and 

unambiguous means. The following fault conditions shall be identified:

(a) Vehicle speed is below the minimum or above the maximum speed as specified.

(b) The maximum number of vehicle axles as specified has been exceeded.

(c) A change in vehicle speed greater than that specified has been detected. 

S.1.7. Recorded Representations.

S.1.7.1. Values to be Recorded. – At a minimum, the following values shall be printed and/or stored 

electronically for each vehicle weighment:

(a) transaction identification number;

(b) lane identification (required if more than one lane at the site has the ability to weigh a vehicle in

motion);

(c) vehicle speed;

(d) number of axles;

(e) weight of each axle;

(f) identification and weight of axle groups;

(g) axle spacing;

(h) total vehicle weight;

(i) all fault conditions that occurred during the weighing of the vehicle;

(j) violations, as identified in paragraph S.2.1. Violation Parameters, which occurred during the 

weighing of the vehicle; and

(k) time and date.

S.1.8. Value of the Indicated and Recorded System Division. – The value of the system’s division “(d),” as 

recorded, shall be the same as the division value indicated.

S.2. System Design Requirements. 

S.2.1. Violation Parameters. – The instrument shall be capable of accepting user-entered violation parameters 

for the following items:

(a) single axle weight limit;

(b) axle group weight limit;
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(c) gross vehicle weight limit; and

(d) bridge formula maximum.

The instrument shall display and/or record violation conditions when these parameters have been exceeded.

S.3. Design of Weighing Elements.

S.3.1. Multiple Load-Receiving Elements. – An instrument with a single indicating or recording element, or 

a combination indicating-recording element, that is coupled to two or more load-receiving elements with 

independent weighing systems, shall be provided with means to prohibit the activation of any load-receiving 

element (or elements) not in use, and shall be provided with automatic means to indicate clearly and definitely 

which load receiving element (or elements) is in use.

S.4. Design of Weighing Devices, Accuracy Class.

S.4.1. Designation of Accuracy. – Weigh-in-motion systems meeting the requirements of this code shall be 

designated as accuracy Class A. 

Note: This does not preclude higher accuracy classes from being proposed and added to this Code in the future when it can 

be demonstrated that weigh-in-motion systems grouped within those accuracy classes can achieve the higher level of 

accuracy specified for those devices.

S.5. Marking Requirements. – In addition to the marking requirements in G-S.1. Identification (except G.S.1.(e)), 

the system shall be marked with the following information:

(a) accuracy class;

(b) value of the system division “d”;

(c) operational temperature limits;

(d) number of instrumented lanes (not required if only one lane is instrumented);

(e) minimum and maximum vehicle speed;

(f) maximum number of axles per vehicle;

(g) maximum change in vehicle speed during weighment; and 

(h) minimum and maximum load.

S.5.1. Location of Marking Information. – The marking information required in Section 1.10. General Code,

G-S.1. Identification and Section 2.25. Weigh-in-Motion Systems, S.5. Marking Requirements shall be visible 

after installation. The information shall be marked on the system or recalled from an information screen.

N. Notes

N.1. Test Procedures. 

N.1.1. Selection of Test Vehicles. – All dynamic testing associated with the procedures described in each of 

the subparagraphs of N.1.5 shall be performed with a minimum of two test vehicles. 
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(a) The first test vehicle may be a two-axle, six-tire, single-unit truck; that is, a vehicle with two axles with 

the rear axle having dual wheels. The vehicle shall have a maximum gross vehicle weight of 10 000 lb.

(b) The second test vehicle shall be a five-axle, single-trailer truck with a maximum gross vehicle weight 

of 80 000 lb.

Note: Consideration should be made for testing the systems using vehicles which are typical to the system’s daily 

operation.

N.1.1.1. Weighing of Test Vehicles. – All test vehicles shall be weighed on a reference scale before being 

used to conduct the dynamic tests.

N.1.1.2. Determining Reference Weights for Axle, Axle Groups, and Gross Vehicle Weight. – The 

reference weights shall be the average weight value of a minimum of three static weighments of all single 

axles, axle groups, and gross vehicle weight.

Note: The axles within an axle group are not considered single axles. 

N.1.2. Test Loads. 

N.1.2.1. Static Test Loads. – All static test loads shall use certified test weights.

N.1.2.2. Dynamic Test Loads. – Test vehicles used for dynamic testing shall be loaded to 85 % to 95 % 

of their legal maximum Gross Vehicle Weight. The “load” shall be non-shifting and shall be positioned to 

present as close as possible, an equal side-to-side load.

N.1.3. Reference Scale. – Each reference vehicle shall be weighed statically on a multiple platform vehicle 

scale comprised of three individual weighing/load-receiving elements, each an independent scale. The three 

individual weighing/load receiving elements shall be of such dimension and spacing to facilitate: 

the single-draft weighing of all reference test vehicles;

(a) the simultaneous weighing of each single axle and axle group of the reference test vehicles on different 

individual elements of the scale; and

(b) gross vehicle weight determined by summing the values of the different reference axle and reference 

axle groups of a test vehicle. 

The scale shall be tested immediately prior to using it to establish reference test loads and in no case more than 

24 hours prior. To qualify for use as a suitable reference scale, it must meet NIST Handbook 44, Class III L 

maintenance tolerances. 

N.1.3.1. Location of a Reference Scale. – The location of the reference scale must be considered since 

vehicle weights will change due to fuel consumption.

N.1.4. Test Speeds. – All dynamic tests shall be conducted within 20 % below or at the posted speed limit.

N.1.5. Test Procedures. 

N.1.5.1. Dynamic Load Test. – The dynamic test shall be conducted using the test vehicles defined in 

N.1.1. Selection of Test Vehicles. The test shall consist of a minimum of 20 runs for each test vehicle at the 

speed as stated in N.1.4. Test Speeds.
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At the conclusion of the dynamic test, there will be a minimum of 20 weight readings for each single axle, 

axle group, and gross vehicle weight of the test vehicle. The tolerance for each weight reading shall be based 

on the percentage values specified in Table T.2.2. Tolerances for Accuracy Class A.

N.1.5.2. Vehicle Position Test. – During the conduct of the dynamic testing, ensure the vehicle stays 

within the defined roadway along the width of the sensor. The test shall be conducted with 10 runs with the 

vehicle centered along the width of the sensor; 5 runs with the vehicle on the right side along the width of 

the sensor; and 5 runs with the vehicle on the left side along the width of the sensor. Only gross vehicle 

weight is used for this test and the tolerance for each weighment shall be based on the tolerance value 

specified in T.2.3. Tolerance Value for Vehicle Position Test.

N.1.5.3. Axle Spacing Test. – The axle spacing test is a review of the displayed and/or recorded axle 

spacing distance of the test vehicles. The tolerance value for each distance shall be based on the tolerance 

value specified in T.2.4. Tolerance Value for Axle Spacing.

T. Tolerances

T.1. Principles.

T.1.1. Design. – The tolerance for a weigh-in-motion system is a performance requirement independent of the 

design principle used. 

T.2. Tolerance Values for Accuracy Class A.

T.2.1. Tests Involving Digital Indications or Representations. – To the tolerances that would otherwise be 

applied in paragraphs T.2.2. Tolerance Value for Dynamic Load Test and T.2.3. Tolerance Value for Vehicle 

Position Test, there shall be added an amount equal to one-half the value of the scale division to account for the 

uncertainty of digital rounding.

T.2.2. Tolerance Values for Dynamic Load Test. – The tolerance values applicable during dynamic load 

testing are as specified in Table T.2.2. 

Table T.2.2.

Tolerances for Accuracy Class A

Load Description* Tolerance as a Percentage of Applied Test Load

Axle Load ± 20 %

Axle Group Load ± 15 %

Gross Vehicle Weight ± 10 %

* No more than 5 % of the weighments in each of the load description subgroups shown in this table shall 

exceed the applicable tolerance.

T.2.3. Tolerance Value for Vehicle Position Test. – The tolerance value applied to each gross vehicle 

weighment is ± 10 % of the applied test load.

T.2.4. Tolerance Value for Axle Spacing. – The tolerance value applied to each axle spacing measurement 

shall be ± 0.15 m (0.5 ft).

T.3. Influence Factors. – The following factor is applicable to tests conducted under controlled conditions only.

T.3.1. Temperature. – Systems shall satisfy the tolerance requirements under all operating temperature unless 

a limited operating temperature range is specified by the manufacturer.
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T.4. Radio Frequency Interference (RFI) and Other Electromagnetic Interference Susceptibility. – The 

difference between the weight indication due to the disturbance and the weight indication without the disturbance shall 

not exceed the tolerance value as stated in Table T.2.2. Tolerances for Accuracy Class A.

UR. User Requirements

UR.1. Selection Requirements. – Equipment shall be suitable for the service in which it is used with respect to 

elements of its design, including but not limited to, its capacity, number of scale divisions, value of the scale division,

or verification scale division and minimum capacity. 

UR.1.1. General. – The typical class or type of device for particular weighing applications is shown in Table 1. 

Typical Class or Type of Device for Weighing Applications.

Table 1.

Typical Class or Type of Device for Weighing Applications

Class Weighing Application

A Screening and sorting of vehicles based on axle, axle group, and gross vehicle weight.

Note: A WIM system with a higher accuracy class than that specified as “typical” may be used.

UR.2. User Location Conditions and Maintenance. – The system shall be installed and maintained as defined in 

the manufacturer’s recommendation. 

UR.2.1. System Modification. – The dimensions (e.g., length, width, thickness, etc.) of the load receiving 

element of a system shall not be changed beyond the manufacturer’s specifications, nor shall the capacity of a 

scale be increased beyond its design capacity by replacing or modifying the original primary indicating or 

recording element with one of a higher capacity, except when the modification has been approved by a competent 

engineering authority, preferably that of the engineering department of the manufacturer of the system, and by 

the weights and measures authority having jurisdiction over the system.

UR.2.2. Foundation, Supports, and Clearance. – The foundation and supports shall be such as to provide 

strength, rigidity, and permanence of all components. 

On load-receiving elements, which use moving parts for determining the load value, clearance shall be provided 

around all live parts to the extent that no contacts may result when the load-receiving element is empty, nor 

throughout the weighing range of the system. 

UR.2.3. Access to Weighing Elements. – If necessary, adequate provision shall be made for inspection and 

maintenance of the weighing elements.

UR.3. Maximum Load. – A system shall not be used to weigh a load of more than the marked maximum load of 

the system.
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Appendix D. Definitions

The specific code to which the definition applies is shown in the [brackets] at the end of the definition. Definitions 

for the General Code [1.10] apply to all codes in NIST Handbook 44.

A

axle. – The axis oriented transversely to the nominal direction of vehicle motion, and extending the full width of the 

vehicle, about which the wheel(s) at both ends rotate. [2.25]

axle-group load. – The sum of all tire loads of the wheels on a group of adjacent axles; a portion of the gross-vehicle 

weight. [2.25]

axle load. – The sum of all tire loads of the wheels on an axle; a portion of the gross-vehicle weight. [2.25]

axle spacing. – The distance between the centers of any two axles. When specifying axle spacing, the axels used also 

need to be identified. [2.25]

S

single-axle load. – The load transmitted to the road surface by the tires lying on the same longitudinal axis (that axis 

transverse to the movement of the vehicle and about which the wheels rotate). [2.25]

T

tandem-axle load. – The load transmitted to the road surface by the tires of two single-axles lying on the same 

longitudinal axis (that axis transverse to the movement of the vehicle and about which the wheels rotate). [2.25]

triple-axle load. – The load transmitted to the road surface by the tires of three single-axles lying on the same 

longitudinal axis (that axis transverse to the movement of the vehicle and about which the wheels rotate). [2.25]

W

weigh-in-motion (WIM). – A process of estimating a moving vehicle’s gross weight and the portion of that weight 

that is carried by each wheel, axle, or axle group, or combination thereof, by measurement and analysis of dynamic 

vehicle tire forces. [2.25]

weigh-in-motion screening scale. – A weigh-in-motion system used to identify potentially overweight vehicles. 

[2.25]

wheel weight. – The weight value of any single or set of wheels on one side of a vehicle on a single axle. [2.25]

WIM System. – A set of sensors and supporting instruments that measure the presence of a moving vehicle and the 

related dynamic tire forces at specified locations with respect to time; estimate tire loads; calculate speed, axle spacing, 

vehicle class according to axle arrangement, and other parameters concerning the vehicle; and process, display, store, 

and transmit this information. This standard applies only to highway vehicles. [2.25]
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