Skid Accident Reduction Program (T 5040.17)

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3D"DOT =20

U.S. Department of Transportation
Federal = Highway=20 Administration=20

TECHNICAL ADVISORY=20

SKID ACCIDENT REDUCTION = PROGRAM

T 5040.17
December 23, 1980=20

Par.=20

Purpose=20
Background=20
Skid Accident Reduction Program=20
Pavement Design, Construction, and Maintenance=20
Wet Weather Accident Location Studies=20
Pavement Skid Resistance Testing Program

PURPOSE. To provide guidance for State and local highway = agencies=20 in conducting skid accident reduction programs.=20
BACKGROUND=20
1. This Technical Advisory provides a general overview of factors = that=20 should be considered as elements of any Skid Accident Reduction = Program.=20 This Technical Advisory supports current Federal Highway = Administration=20 (FHWA) policy and will be revised as appropriate to reflect changes = in=20 policy as they occur.=20
2. The existing requirement for skid resistance pavementsare = contained in=20 several documents including Highway Safety Program Standard No. 12, = Highway=20 Design Construction and Maintenance (23 CFR 1204.4), Federal Highway = Program=20 Manual (FHPM) 6-2-4-7, Skid Measurement Guidelines for the Skid = Accident=20 Reduction Program. Other sources of technical advice are cited in = the=20 appropriate sections of this Technical Advisory.=20
3. Highway Safety Program Standard 12 (HSPS No. 12) states that = every State=20 shall have a program of highway design, construction, and = maintenance to=20 improve highway safety. This program shall provide that "there are = standards=20 for pavement design and construction with specific provisions for = high skid=20 resistance qualities." The HSPS No. 12 also requires that each State = have a=20 "program for resurfacing or other surface treatment with emphasis on = correction of locations or sections of streets and highways with low = skid=20 resistance and high or potentially high accident rates susceptible = to=20 reduction by providing improved surfaces." In discharging the=20 responsibilities of FHWA, the Division Administrator should = determine the=20 acceptability of specification requirements and construction = practices for=20 placing, consolidating, and finishing both asphalt concrete and = portland=20 cement concrete pavements. Such determinations will rely on the = highway=20 agency to research, evaluate, and document the performance of the = various=20 aggregates, mix designs, and construction practices used.=20
4. Even though the use of studded tires is beyond the control of = most=20 highway agencies, their use can cause significant wear on the = pavement=20 surface texture. For example, grooves sawed in concrete pavements = have worn=20 completely down in as short a time as 2 years. States are encouraged = to ban=20 or restrict the use of studded tires.=20
5. Legislative actions in recent years support a general duty of = any=20 highway agency to "...maintain the roadway in a reasonable safe = condition.=20 This would involve, in essence inspection, anticipation of defects, = and=20 conformity with generally accepted standards and practices."* = (Engineering and Government Liability, David C. Oliver, FHWA, an = unpublished=20 paper presented to the American Road and Transportation Builders = Association=20 Local Officials Meeting, St. Louis, Missouri, August 23, 1978.) =
The practical result is that highway agencies should have = anorganized=20 system to identify and correct hazardous locations in a cost-effective = manner,=20 as well as a comprehensive pavement management program to design, = construct,=20 and maintain highways in conformance with reasonable standards. Such a = systematic process is the best way to execute the highway agency's = duty to=20 maintain a reasonable safe roadway.=20
SKID ACCIDENT REDUCTION PROGRAM. Each highway agency is = encouraged=20 to develop and mange a skid accident reduction program to reflect the=20 individual needs and conditions within the State. the purpose of a = skid=20 accident reduction program is to minimize wet weather skidding = accidents=20 through: identifying and correcting sections of roadway with high or=20 potentially high skid accident incidence; ensuring that new surfaces = have=20 adequate, durable skid resistance properties; and utilizing resources=20 available for accident reduction in a cost-effective manner. A program = comprised of at least the following three basic activities, if = faithfully=20 implemented, should enable the highway agency to comply with HSPS No. = 12.=20
1. The evaluation of pavement design, construction, and maintenance = practices through its pavement management program to ensure that = only=20 pavements with good skid resistance characteristics are used.=20
2. The detection and correction of locations with a high incidence = of wet=20 weather accidents utilizing (1) the State and local accident record = systems,=20 and (2) countermeasures for locations with high wet weather = incidences, to=20 ensure that existing highways are maintained in a safe condition.=20
3. The analysis of skid resistance characteristics of selected = roadway=20 sections to:=20
PAVEMENT DESIGN, CONSTRUCTION, AND MAINTENANCE=20
1. Pavement Design=20
2. Pavement Construction=20
3. Pavement Maintenance. The same procedures and quality = standards=20 used in construction should be used in the maintenance operations. =
WET WEATHER ACCIDENT LOCATION STUDIES. The purpose of this = type of=20 study is to identify locations with high incidence of wet weather = accidents,=20 determine corrective measures, and take appropriate actions in a = timely and=20 systematic manner. This activity should be conducted as part of the = highway=20 agency's safety improvement program and should make effective use of = the=20 agency's accident data file. Items to be considered for retrieval from = the=20 accident and traffic records are total accidents (rate), wet weather = accidents=20 (rate), and the wet/dry ratio.=20
1. Identification of Wet Weather Accident Sites=20
2. Field Review. A list of all sites ranked in order of WSF = or=20 another appropriate measure should be prepared as the basic list of=20 candidate sites for remedial treatments. The selected locations = should then=20 be skid tested and reviewed by a team representing various = disciplines such=20 as highway materials, design,construction, maintenance, traffic and = safety.=20 See Appendix B for skid testing procedures. The review team should = determine=20 probable reasons for the high incidence of accidents and recommend=20 corrective actions. Once the review team has recommended appropriate = corrective treatments, a priority list of projects can be prepared = based on=20 benefits and expected costs.=20
3. Priority Program. An assessment should be made of the = benefits=20 relative to the cost of providing remedial treatments for high = priority=20 projects. A number of highway agencies have their own methods for = conducting=20 benefit cost analyses of alternative remedial treatments. Some of = these=20 remedial methods are tied into traffic engineering or pavement = management=20 programs. A specific program for evaluating the benefits and cost of = alternative treatments is presented in reference 1, Appendix C.=20
4. Evaluation=20
PAVEMENT SKID RESISTANCE TESTING PROGRAM=20
1. General Description of Program. The actual testing of = pavement=20 friction provides basic data for use in the three activities = introduced in=20 paragraph 3. Figure 1 graphically presents the interrelation between = these=20 activities. The upper portion of Figure 1 provides an overview of = data to be=20 collected to serve the safety, construction, and maintenance = functions of=20 high-way organizations concerned with the skidding properties of = pavement=20 surfaces. The lower portion of Figure 1 indicates the various uses = of the=20 skid testing data, along with weather and accident data. Some of = these data=20 are evidence of the durability of particular surfaces, while other = data=20 provide a general overview of the skid resistance characteristics of = the=20 highway system.=20
  Figure=20 1
  Model Skid Accident Reduction Plan =20
2. Specific Data From Sample Sites. In conjunction with skid = resistance measurements, pavement wet time and accident records are=20 desirable for each roadway section in the sample. The highway = location=20 system should be used for correlating data from different sources. = An=20 example of specific data which is desirable at each sample site is = given in=20 Appendix D.=20
3. Sites with Low Skid Resistance. When sites with low skid=20 resistance are identified during the testing of system status, these = sites=20 should be analyzed for corrective action. This can be done through a = pavement management program, a high hazard elimination program, or = other=20 efforts. If the high hazard elimination program is used, the = analysis should=20 be in accordance with FHPM 8-2-3.

signed
R.D. Morgan
Associate Administrator for=20
Engineering and Traffic Operations=20

signed
Lorenzo Casanova
Associate Administrator = for Safety=20

Appendixes=20

EVALUATION OF WET PAVEMENT TIME AND ACCIDENT DATA=20

A.1 The quantity of rainfall (inches) recorded by weather = stations may=20 be used to calculate the percentage of pavement wet time. Wet pavement = time=20 (WPT) may be estimated from total annual rainfall in inches (AR) as = follows:*=20

WPT =3D 3.45 ln (AR) - 5.07=20

*(This equation is based on a relationship developed by K.D. Hankins = in "The=20 Use of Rainfall Characteristics in Developing Methods for Reducing Wet = Weather=20 Accidents in Texas," Texas State Department of Highways and Public=20 Transportation Study No. 135-4, July 1975.)=20

Dry pavement time may be estimated by subtracting the amount of wet = time=20 and ice and snow periods from the total time in the period analyzed. = Data from=20 rainfall stations maintained by the National Oceanic and Atmospheric=20 Administration's Weather Service may be used for wet and dry pavement = time=20 estimates for various areas within a State.=20

Isohyetal maps may be used to develop site wet pavement times. If = ice and=20 snow cover pavements for a significant portion of the time, a map for = dry time=20 should be prepared as well. Figure A-1 provides an example of a wet = time map=20 drawn from isohyetal charts.

Figure=20 A-1
Percent of Time Pavement Is Wet 3D"Percent =20

A.2 Wet Safety Factor (WSF)=20

There are a number of ways to evaluate the relative safety of the = subject=20 location, one of which is the wet safety factor (WSF) approach.** =

**(The WSF is a generalized form of an index referred to as the "skid = trap=20 ratio" and recommended for use in NCHRP Report 37, Tentative = Skid-Resistance=20 Requirements for Main Rural Highways, " by H.W. Kummer and W.E. Meyer, = Highway=20 Research Board, Washington, D.C., 1967.)=20

For each wet weather accident location, a WSF may be developed. = This factor=20 is expressed as follows:=20

WSF =3D (DA)(PWT)/(WA)(PDT)=20

Where:=20

DA =3D number of dry weather accidents=20

WA =3D number of wet weather accidents=20

PDT =3D percent of dry pavement time=20

PWT =3D percent of wet pavement time

This factor is the reciprocal of the risk of having a wet pavement = accident=20 relative to having a dry pavement accident. On a specific roadway = section,=20 each of these variables must be developed for the same time period; = otherwise,=20 traffic exposure must be taken into account. Criteria may be developed = for=20 further consideration of pavement sections. A WSF less than 0.67 = suggests a=20 wet weather problem. This criteria is based upon the conservative = estimate of=20 the overall likelihood of a wet weather accident being 1 1/2 as great = as a dry=20 pavement accident. This estimate assumes that wet weather accidents at = the=20 site or road section under consideration are attributable entirely to = a=20 skidding problem. A low WSF in most cases is due to poor skid = resistance.=20 However, traffic engineering evaluations may reveal deficiencies in = sight=20 distance, road markings, inadequate drainage, etc. Auxiliary = information=20 obtained during the test program should provide indications of the = safety=20 problems.

SKID MEASUREMENT SYSTEM DESCRIPTION AND OPERATING PROCEDURES=20

B.1 DESCRIPTIONS OF SKID MEASUREMENT SYSTEM=20

The requirements of American Society for Testing and materials = (ASTM) E 274=20 states "The method utilizes a measurement representing the steady = state=20 friction force on a locked test wheel as it is dragged over a wetted = pavement=20 surface under constant load and at constant speed while its major = plane is=20 parallel to its direction of motion and perpendicular to the = pavement."=20

Although this specification may be met by a system involving only = one wheel=20 attached to a towing vehicle and although a few such systems are in = use, the=20 vast majority of skid measurement systems in use and expected to be in = use in=20 the near future consist of a towing vehicle and two-wheel trailer. On = many=20 systems either wheel may be locked during testing, but most commonly, = the left=20 is used.=20

The ASTM considers testing the left wheel track to be "normal." = However, a=20 differential in friction levels between the left and right wheel track = may=20 exist. When testing a site where a differential may exist, especially = a high=20 wet weather accident site, all lanes and wheel tracks should be = tested. If a=20 two-wheel trailer system is used, it is desirable to have the = capability of=20 testing with either wheel.=20

A skid measurement system must have a transducer associated with = each test=20 wheel which senses a force equal or directly related to the force = developed=20 between the sliding wheel and the pavement during test, electronic = signal=20 conditioning equipment to receive the transducer output signal and = modify it=20 as required, and suitable analog and/or digital readout equipment to = record=20 either the magnitude of the developed force or the calculated value of = the=20 resulting skid number (SN).=20

The system must include a facility for the transport of a supply of = water--usually 200 to 500 gallons--and the necessary apparatus to = deliver a=20 specified amount of water--4.0 gallons per minute per wetted inch of = pavement=20 at 40 miles per hour within specified limits in front of the test = wheel.=20

Finally, the system must include provision for measuring (and = preferably=20 for recording) the speed at which the test is conducted.

B.2 FIELD OPERATING PROCEDURES=20

B.2.1 Field Force Verification=20

It is generally impractical to perform force plate calibrations at = frequent=20 intervals while the measurement system is in the field. Facilities = should,=20 however, be available to permit the operator to ascertain that = significant=20 changes have not occurred in the force measurement subsystem since the = most=20 recent force plate calibration.=20

If the measurement system uses a torque transducer and is adaptable = to=20 mounting a torque arm, the verification can be accomplished within a=20 reasonable time and effort. This device, consisting of an arm capable = of being=20 bolted to the test wheel in a horizontal position and of supporting = known=20 weights located at specified distances from the center of the test = wheel, may=20 be used to test the torque transducer to predetermined values of = torque.=20 Typically, the test wheel of the inventory system is raised off the = ground,=20 the torque arm is attached to the test wheel and held in a horizontal=20 position, the brake of test wheel locked, and a series of known = weights are=20 suspended on the torque arm. This procedure will induce a series of = known=20 strains on the transducer, resulting in a series of output signals = through the=20 signal conditioning equipment. The magnitude of these signals should = then be=20 compared to the magnitude of signals produced through use of the same=20 technique immediately after the most recent force plate calibration.=20 Adjustment of signal conditioning equipment gain setting may be made = to offset=20 small force measurement subsystem variations which could occur.=20

Verification should be repeated periodically.

B.2.2 Test Tire and Wheel Preparation, Control of Tire=20 pressure=20

Tire Specification=20

Unless otherwise specified, all tests shall be performed with tires = meeting=20 the requirements of ASTM E 501, Standard Tire for Pavement Skid = Resistance=20 Tests, and all pertinent sections of that specification as well as = ASTM E 274=20 should be observed in their use.=20

Tire Mounting and Break-in Procedure=20

The tire should be mounted on a Tire and Rim Association 6JJ rim. = The rim=20 should have been examined to determine that it has suffered no damage = or=20 misalignment in prior use. After mounting, and before break-in, the = tire and=20 wheel should be balanced. The tire should be subjected to a break-in = of 200=20 miles use before being used for testing. This break-in may be = accomplished by=20 using the tire on the skid trailer wheel which is not used for = testing. If the=20 tire must be remounted before test use, it should be rebalanced after=20 remounting.=20

Tire Warm-Up Procedure=20

The test tire should be inflated to 24 + 0.5 pounds per square inch = measured at ambient temperature. After tire pressure measurement and=20 adjustment, the tire should be subjected to a 5-mile warm up, = traveling at=20 conventional highway speeds, before tests are performed. The 5-mile = warm-up=20 should be repeated on any occasion when the measurement system is = parked for a=20 periodof 15 minutes or more.=20

Tire Wear and Replacement Procedure=20

The standard pavement test tire has a series of visual wear guide = sipes=20 (small circular holes) cast into each of the outer ribs of the tire. = The test=20 tire should be withdrawn from testing use when wear has progressed to = a point=20 at which the wear guide sipes are no longer visible. During routine = testing,=20 test tires should be examined at least twice daily (and more = frequently as=20 tire nears unacceptable wear level) to determined that wear has not = progressed=20 beyond acceptable limits.=20

Additionally, after any series of tests on pavements having very = high skid=20 numbers (in excess of SN=3D70) or in the event of a deliberate or = inadvertent=20 dry skid, the test tire should be examined for the development of a = flat spot.=20 If a significant flat spot or spots develop on a test tire, it should = be=20 withdrawn from test use due to the tendency of the test wheel to seek = out and=20 return to such a flat spot in subsequent lockups.

B.2.3 Watering Subsystem Procedures=20

Daily Procedures=20

Prior to the beginning of each day's activity, the crew should = perform at=20 least the following functions with respect to the water subsystem:=20

Determine that the water nozzle (nozzles) when in the testing = position=20 assumes the proper angle with respect to the pavement (ASTM E 274 = requires=20 an angle of 25 + 5 degrees).=20
If the measurement system has provision for raising and lowering = the=20 nozzle between tests, determine that the mechanism is working = properly and=20 that the nozzle assumes a fully lowered position during the test = sequence.=20
Determine that the nozzle, when in the test position, will = discharge=20 water directly in front of and centered on the test wheel.=20
Examine the nozzle outlet orifice to determine that it is free = from=20 damage or distortion.

The above inspections should be repeated during a day's testing in = the=20 event of operation on very rough highways (or in the event of any = off-highway=20 travel) which may have caused damageto the nozzle or adversely = affected its=20 orientation.=20

Water Trace Width Check=20

Periodically the crew should make a measurement of the water trace = width as=20 a gross measure of overall water subsystem performance. This may be=20 accomplished by driving the measurement system over a pavement at a = selected=20 convenient speed (the same speed should be used on all occasions), = initiating=20 water flow without locking the test wheel brakes, and measuring the = width of=20 the resulting water trace on the pavement. The trace width measurement = should=20 be made as quickly as possible after passage of the inventory system=20 (preferably within 30 seconds). This would require that one member of = the crew=20 drive and operate the measurement system while the other member is = positioned=20 off the side of the pavement at the location at which the measurement = is to be=20 made. Best results are achieved if this procedure is performed on a = relatively=20 smooth pavement surface (low macrotexture).

B.2.4 Instrumentation Calibration Verification=20

Provision should be made to allow for verification of the signal=20 conditioning instrumentation calibration (to account for the effects = of zero=20 and gain drifts).=20

General Requirements for Calibration Signal=20

The minimum acceptable facility for verification of conditioning=20 instrumentation is a calibration signal subsystem. The calibration = signal=20 should be provided from such a source and in such a manner that there = is=20 little likelihood of variation in the calibration signal itself. This=20 assurance then permits theoperator to make adjustments in the = measurement=20 subsystem gain to offset the frequent small deviations which occur due = to=20 changes in ambient temperature and other operating parameters.=20

Force Measurement Calibration Signal=20

The most straightforward technique for providing a force = measurement=20 calibration signal is to make provisions for switching a high quality = shunting=20 resistor of known value in parallel with one arm of the force = transducer=20 strain gauge bridge. This induces an imbalance in the bridge = equivalent to the=20 application of a known force to the transducer. The resultant signal = is=20 sufficient to verify, or provide means of adjustment for, all elements = of the=20 force measurement system forward of the transducer itself.=20

Frequency of Use=20

Instrumentation calibration verification through use of calibration = signals=20 should be accomplished at the beginning of each day's operation after=20 equipment warm up, at intervals of no more than 2 hours when the = system is in=20 continuous use, and upon the renewal of operation throughout the day = after any=20 period during which the signal conditioning equipment has been turned = off or=20 the unit has been allowed to stand without use for 30 minutes or more. =

B.2.5 Check List=20

A check list should be available to the crew and should be used = prior to=20 the beginning of daily operations and on any occasion during the day = when=20 testing is suspended for 30 minutes or more or when instrumentation = has been=20 turned off. The check list varies from system to system due to = differences=20 between the systems, but should provide for at least the following = checks:=20

all power subsystems on and providing proper levels of power=20
all signal conditioning subsystems on for adequate time to reach = stable=20 operation (typically 10 to 30 minutes)=20
all recording systems on and functioning properly=20
instrument calibration (described above) performed=20
tire pressure checked and adjusted if necessary=20
test tire checked for wear=20
water nozzles checked for position and condition=20
water tank adequately filled=20
fuel supply adequate=20
safety chains and all other connections between trailer and = towing=20 vehicle properly connected, positioned, and protected if necessary=20
trailer jacks (if available) in retracted position=20
all auxiliary equipment (air-compressors, lights, etc.) = functioning=20 properly

B.3 USE OF STATIC AND DYNAMIC CALIBRATION PROCEDURES=20

B.3.1 Purpose of Field Test Center=20

At the present time the highest order of calibration and evaluation = available for a State skid measurement system is that provided through = the=20 Field Test Center established under contract by the Federal Highway=20 Administration (FHWA). Arrangements to receive the services of the = Field Test=20 Center may be initiated by a State through submittal of a request for = such=20 services to the local FHWA division office.

B.3.2 Criteria for When to Use the Field Test Center=20

Each measurement system should be submitted for calibration and = evaluation=20 at the Center as soon as possible after its introduction into service. = It=20 should be resubmitted for calibration and evaluation whenever:=20

significant repair or modification has been accomplished by the = owning=20 agency which might reasonably be expected to affect test results, or =
whenever it has experienced sufficient use such that normal wear = in the=20 various subsystems might be expected to have affected their = operation.=20
The second consideration suggests that each measurement system = should be=20 resubmitted at least every 2 years.

B.3.3 Calibration Services Provided by Field Test = Center=20

The static and dynamic calibration services provided by the Field = Test=20 Center include the following:=20

Horizontal and Vertical Force Calibration. This provides = for=20 evaluation of the accuracy, linearity and hysteresis of the = measurement=20 system force transducers and signal conditioning equipment through = use of an=20 air bearing force plate maintained by the Center, and periodically=20 calibrated by the National Bureau of Standards.=20
Flow Rate Evaluation and Adjustment if Required. This = includes=20 determination that the water delivery subsystem of the measurement = system=20 provides a quantity of water (dependent upon trace width) in front = of the=20 test tire which meets ASTM E 274 requirements at speeds between 20 = and 60=20 miles per hour.=20
Static Evaluation of Water Distribution. This provides an = evaluation of the uniformity with which the total water flow is = distributed=20 across the trace width and adjustment, if necessary, to assure that = the=20 water is in fact delivered uniformly and in line with the test tire. =
Force Plate or Load Cells. The visitors force plate used = for=20 routine checks of the force measurement subsystem can be calibrated = while at=20 the Center.=20
Speed Calibration. The speed measurement (and recording = if=20 available) subsystem is evaluated, calibrated and, where necessary = and=20 possible, adjusted to produce accurate speed measurement values over = the=20 range of 20 to 60 miles per hour.=20
Tire Pressure Gauge Calibration. This provides assurance = that=20 tire pressures in the test wheels and in the speedmeasuring fifth = wheel (if=20 used) can be accurately measured and set.=20
Dynamic Correlation. Two such correlations are conducted: = The=20 first with the measurement system in the "as arrived" condition and = the=20 second after all of the foregoing evaluations have been conducted = and=20 indicated adjustments accomplished. The first correlation results in = the=20 development of mathematical relationships between the measurement = system and=20 the Area Reference Skid Measurement System that permit data = collected by the=20 measurement system, prior to its visit to the Center, to be adjusted = to a=20 common base provided by the use of the Area Reference System. The = second=20 correlation permits the development of similar relationships which = may be=20 used to relate the results of subsequent testing to the Area = Reference=20 System base. The data from the second correlation also provide an = estimate=20 of the system measurement variance.

B.4 MAINTAINING SYSTEM INTEGRITY BETWEEN FIELD TEST CENTER=20 CALIBRATIONS=20

Two basic types of procedures are available for determining that=20 significant changes have not occurred in the measurement system since = its most=20 recent evaluation and calibration at the Center. These involve = techniques for=20 evaluating important subsystem performance and techniques for = evaluating=20 performance of the total system.

B.4.1 Techniques to Evaluate Subsystem Performance=20

As a minimum, the owner of each measurement system should maintain = and=20 periodically make use of facilities for evaluating the force, water, = and speed=20 measurement subsystem of the inventory system.=20

Evaluation of Force Subsystem=20

The force subsystem should be evaluated through use of a force = plate. An=20 air-bearing force plate is recommended since its action is such as to=20 essentially eliminate the effect of friction in the plate itself. If = an=20 air-bearing force plate is not available, any of several commercial = mechanical=20 force plates may be used. If a mechanical device is used, precautions = should=20 be taken to assure that all moving parts (particularly load = application screws=20 and spherical or roller bearings) are well lubricated and that the = lubricant=20 is periodically removed and replaced.=20

To conduct an evaluation, the test wheel of the measurement system = should=20 be centered on the force plate, the test wheel brake locked, and known = frictional forces introduced to the tire-force plate interface through = appropriate motion of the force plate. Frictional forces should be = both=20 increased and decreased in a stepwise manner to allow for detection of = possible hysteresis effects. The indicated force readout values for = the system=20 should then be plotted against known force input values. The resulting = plotted=20 calibration line should be evaluated for nonlinearity and hysteresis=20 characteristics. Also actual readout values for known force inputs = should be=20 compared with those readout values determined from tests conducted = with the=20 same equipment after the most recent Center evaluation.=20

Evaluation of Water Subsystem=20

The most effective evaluation of the water subsystem to discern = variations=20 in performance is that of flow. Flow rate may be evaluated by raising = the rear=20 wheels of the towing vehicle, running the vehicle at an indicated = speed of 40=20 miles per hour (or any other desired speed), collecting the water = pumped=20 through the system and out the nozzle during a measured time period, = and=20 calculating the flow rate in gallons. This procedure should be = repeated at two=20 or more speeds to evaluate linearity of the water delivery subsystem = with test=20 speed.=20

The Pennsylvania State University has developed a water rate flow = tank=20 which is circular in cross section and of such size that it fits = easily into a=20 standard manhole. The tank has athreaded opening in the bottom for = drainage=20 and a stop-plug with a long handle which permits the plug to be = removed and=20 replaced from the top of the tank after it is hanging in the manhole. = It also=20 has a scale calibrated in gallons on the inside of the tank. This tank = may be=20 suspended in a standard manhole, the measurement system positioned so = that the=20 nozzle will discharge directly into the tank, the rear wheel of the = towing=20 vehicle raised, and total flow measured at any desired speed. The only = additional equipment required is a stopwatch.=20

Evaluation of Speed Measurement Subsystem=20

The speed measurement subsystem should be evaluated by operating = the=20 measurement system at various test speeds over a measured mile course. = If the=20 basic speed measure is done through the use of the tow vehicle = speedometer or=20 through a tachometer-generator driven by the tow vehicle or by a fifth = wheel,=20 then the vehicle should be driven over the measured mile course at a = selected=20 speed and the time of transit measured with a stopwatch. The actual = speed,=20 calculated from the distance and the elapsed time, is then compared to = the=20 indicated speed.=20

If speed measurement is based upon a pulse generator driven by a = fifth=20 wheel, the accuracy of the speed measurement is directly dependent = upon the=20 accuracy of the fifth wheel for distance measurement. To evaluate this = subsystem, the fifth wheel tire pressure is adjusted until the = distance=20 indicated agrees with the known distance traversed (the assumption = being made=20 here is that the electronic package which converts the pulses to = velocity is=20 functioning properly).=20

If tapeswitch event detectors, placed 200 feet apart, and an = interval timer=20 (+0.01 second resolution) are available to measurethe time required by = the=20 inventory system to travel 200 feet, a very accurate speed measurement = is=20 obtained to check against the indicated value.=20

Time Between Subsystem Evaluations=20

The force, water and speed measurement subsystems of the = measurement system=20 should be checked by the methods described above at intervals no = greater than=20 3 months.

B.4.2 Techniques to Evaluate Total System Performance=20

Use of Measurement System Sample Variance as Performance = Measure=20

A portion of the information furnished, as a result of an = evaluation at the=20 Center, is the pooled sample standard deviation of the measurement = system for=20 repeated test at three test speeds on five special test surfaces. If = the=20 sample standard deviation at the desired speed is squared, the = resulting=20 value, SD²_t is an estimate of the skid = measurement=20 system variance. Subsequent to the Center evaluation, the crew should=20 periodically select a pavement location having a skid number of = approximately=20 30 to 40 and run 20 repeat tests at the desired speed over the same = location.=20 From the results of these latter tests a new estimate,=20 SD²_E, can be calculated. If the ratio=20 SD²_E/SD²_t does not exceed = 2.0, the=20 chances are 19 in 20 that the system standard deviation has not = doubled over=20 that established during its visit to the Center. (If the system has = not been=20 to a Center to obtain an estimate of SD²_t, its = crew=20 should select a pavement location having a skid number of = approximately 30 to=20 40, run repeat tests at each desired speed over the same location, and = calculate the sample standard deviation at each such speed.)=20

As an alternative, the above procedure could be performed making = only 10=20 repeat tests on the selected pavement. In this case, the ratio of=20 SD²_E/SD²_t should not = exceed 2.2.=20 The chances are then four in five that the system standard deviation = has not=20 doubled over that previously established.=20

The above procedure should be performed at time intervals no = greater than 3=20 months.=20

Short Term Checks of System Performance=20

The agency operating the measurement system should select several = pavements=20 located close to the site at which the system is normally garaged and = perform=20 repeated tests on the surfaces at quite frequent intervals, preferably = weekly.=20 Measured values of skid resistance on these surfaces will obviously = change as=20 thesurfaces change from traffic wear, environmental, and/or seasonal=20 variations. However, these changes should occur in an orderly and = predictable=20 fashion and any abrupt change would be an indication of possible = erratic=20 performance of the measurement system. A continually updated record of = the=20 results of such tests should be maintained and examined after each = updating=20 for evidence of such erratic performance.

REFERENCES=20

The following is a selected list of references which may be helpful = in=20 implementing the program described in this Technical Advisory. This list = is not=20 intended to be a bibliography of all documents available in this field:=20

* Means that these studies are available through the National = Technical=20 Information Service, 5285 Port Royal Road, Springfield, Virginia 22161.=20

*1. Effectiveness of Alternative Skid Reduction Measures,Benefit Cost = Model,=20 Report No. FHWA-RD-79-12, Volume II, November 1978, Federal Highway=20 Administration.=20

*2. Accident Research Manual, FHWA-RD-80-016, February 1980, Federal = Highway=20 Administration.=20

*3. Evaluation of Minor Improvements (Part 8), Grooved Pavement = (Supplemental=20 Report) CA-DOT-TR-2152-11-75-01, September 1975, R.N. Smith and L.E. = Elliott,=20 Office of Traffic, California Department of Transportation.=20

4. Evaluation of Minor Improvements (Part 9), Open Graded Asphalt = Concrete=20 Overlays, January 1972, James I. Karr, Transportation.=20

5. Variations in Skid Resistance Over Time, FHWA-VA-80-33, February = 1980,=20 S.N. Runkle, David C. Mahone, Virginia Highways and Transportation = Research=20 Council.=20

6. Seasonal Variations in the Skid Resistance pavements in Kentucky, = Research=20 Report 532, November 1979, James L. Burchett, Roland L. Rizenbergs, = Kentucky=20 Department of Transportation.=20

SPECIFIC DATA TO BE REPORTED FOR SAMPLE SITES=20

The following data should be collected in testing samples locations:=20

D.1 Skid numbers (SN) should be taken for major classes of = roads=20 stratified by traffic volume and geographical location.=20

D.2 Auxiliary data which should be included in order to = establish=20 distribution of skid numbers may include the following:=20

(a) Location of site or roadway section=20

(b) Responsible jurisdictional unit and route number or other = designator=20

(d) Surface type (e.g., bituminous, open-graded, concrete, tine = finish,=20 etc.)=20

(e) Average annual daily traffic (use traffic count data if = available)=20

(f) Length of roadway section=20

(g) Lane where skid measurements are made=20

(h) Date of skid measurements=20

(i) Number of tests made in section=20

(j) Average SN=20

(k) Range of SN measurements=20

(l) Presence of atypical geometric or feature=20

(m) Evidence of skidding (e.g., skid marks, scarred posts, etc.) =

FHWA Home | = Directives | = Technic= al=20 Advisories | Feedback=20 3DFHWA

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