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Proper friction measurements are needed to keep pavement free of ice during the winter season
The U.S. spent approximately $1.5 billion in direct costs and $5 billion in indirect costs on roadway snow and ice control in 1997 alone. The direct costs arose from such maintenance activities as plowing, salting and sanding road surfaces. The higher indirect costs stemmed from accidents, travel delays, lost economic opportunities, infrastructure degradation, environmental damage to vegetation and water supply sources and vehicle corrosion.
Since the main objective of a snow and ice control operation is to return the road surface to a safe state for the driving public within a reasonable period of time, developing new techniques that increase the efficiency and effectiveness of this operation could reduce costs, increase safety and improve mobility for the driving public.
Currently, the road surface conditions and safety are assessed visually, which is a subjective measure. In addition, the ice control operation is determined usually by the snowplow operators. Hence, visual assessment and chemical application rate are applied without considering other important factors, such as pavement temperature, dew temperature, precipitation rates and other information obtained through the roadway weather information system (RWIS). This current approach may result in a waste of resources, may aggravate the environmental problems associated with the applied chemicals and may adversely affect the pavement material properties.
Over the past several years, personnel at many highway agencies in Europe, Japan and the U.S. have come to believe that surface friction measurements may form the basis of improved winter maintenance operations and mobility. For example, friction measurements have been and continue to be successfully used in Finland as a decision support tool and quality assurance measure for winter maintenance activities.
If friction is to be a useful operational tool in winter maintenance in the U.S., the design of the friction indicator should include features that make it safe to take measurements that are repeatable, easy to interpret and of acceptable accuracy. However, the ongoing difficulties in obtaining repeatable friction measurements with robust and cost-effective devices have rendered the technology, for the most part, impractical.
Here are the scenarios
To assure the quality of winter maintenance operations, friction measurements could be used in three different ways: the threshold level approach, the contaminant classification approach and the spatial homogeneity approach.
Preliminary studies have shown that the three approaches are promising. However, further investigations are still needed before these approaches can be fully implemented. In addition, there is a fundamental need, with respect to friction in highway winter maintenance, to stay highly focused on operational needs and requirements. Specifically, the level of detail in friction measuring for highway winter maintenance may be of a fairly broad scale.
Friction measurements that lead to credible, qualitative descriptions of winter pavement state (poor, fair, good; or red, orange, green, for example) may be satisfactory for many operational winter maintenance activities. These friction measurements (as they translate into a measure of safe mobility) should be available to public road users in a simple way.
It has to be noted here that highway winter maintenance friction needs and requirements are somewhat different from those of the air transport runway winter friction community. In light of all these factors, NCHRP Project 6-14 was conducted to evaluate the feasibility of using friction indicators as tools for improving winter maintenance operations and mobility.
The different friction-measurement methods currently being considered for use during winter maintenance operations range from using simple observational methods to employing sophisticated devices.
The observational method of friction testing is widely used; a trained observer visits certain pre-determined sites on a road system, stops at those sites and leaves his or her vehicle to observe the road surface. These observations often involve “scuffing” a foot across the road surface to determine (solely by feel) how slippery the surface is.
However, there are several devices that can be used for measuring road surface friction. All practical techniques for friction measurement fall into one of five groups: deceleration devices, locked-wheel devices, side-force devices, fixed-slip devices and variable-slip devices.
In addition, the increasing availability of automatic anti-lock braking systems (ABS) and traction control systems (TCS) on new vehicles offers the possibility of using such vehicles to gather information on the friction of the road surface. The ABS only operates under braking conditions, while the TCS also operates when the vehicle is not braking. The TCS senses traction on the driving wheels of the vehicle, and this sensing occurs even when the vehicle is moving at a constant velocity. Thus, while both systems have the potential to measure friction, the TCS is able to measure friction more often and will have the potential to provide more data.
It should be noted that at least one ABS-based friction measuring system already exists: the AeroTechTelub MoRRS (Mobile Road Reporting System). This system, which was developed and marketed in Sweden, appears to be a more modern version of a deceleration-based friction-measuring device.
Four broad scenarios for the use of friction measurements were developed during the NCHRP 6-14 project. The actions taken under each scenario will be greatly affected by climatic conditions, traffic levels and road characteristics. The four scenarios are numbered in order of escalating technological and implementation complexity.
Scenario 1: Friction measurements by a winter maintenance vehicle
In this scenario, point or continuous friction measurements are used to provide information to support the winter maintenance decision maker. Specifically, a winter maintenance patrol vehicle is dispatched to travel over portions of the road system during and after winter maintenance activities have been performed, making either periodic or continuous measurements of roadway friction. The friction information may be presented to the patrol vehicle operator in simple, qualitative terms (i.e., a green, yellow or red pavement friction condition).
Little or no effort is made to record, transmit or archive the friction information for either electronic transmission or later consideration. If some or all of these measurements do not meet certain friction criteria, then the winter maintenance decision maker may recall the maintenance fleet and re-treat those sections of the road that do not meet the criteria. This need is communicated through the radio frequency channels used by winter maintenance operations; portions of roadway that need additional winter maintenance effort are identified verbally by milepost. Snow and ice control material spread rates are manually set prior to departing on winter maintenance routes.
Scenario 2: Friction measurements by snowplow/spreader vehicles
Friction measurements on individual winter maintenance snowplow/ spreader vehicles would control one or more of the winter maintenance functions of those vehicles, such as the application rates of snow and ice control materials and down pressure on forward and under-body snowplows.
In this scenario, all winter maintenance snowplow/spreader vehicles are equipped with an independent friction measuring capacity and have in-cab control capacity of snowplow and spreader functions beyond simply up/down or on/off.
Due to the demanding task environment under which snowplow/spreader operators must work, it is unlikely that this operator group will have an opportunity to safely and effectively take regular friction measurements with any device that requires hard braking of the snowplow/spreader vehicle.
Scenario 3: Recorded, archived friction measurements by patrol or snowplow/spreader vehicles
As in Scenarios 1 and 2, either a winter maintenance patrol vehicle or a snowplow/spreader vehicle measures the friction. However, this scenario is enhanced because the friction measurements are recorded for future consideration. In addition to the manual or automatic (electronic) entry of the friction information, the patrol or snowplow/spreader vehicle operator must enter the milepost location of the friction data record.
Alternatively, the vehicle may be equipped with GPS or another type of automatic vehicle-location technology, and the location of a given friction measurement is automatically recorded, along with the measurement value itself. Again, these friction measurements cannot be reasonably acquired from devices that require hard braking of snowplow/spreader vehicles.
In this scenario, friction measurement records are used in post-winter maintenance periods as a quality assessment and assurance technology for winter maintenance activities.
The records of friction measurements and locations also can be used to develop spatially averaged maps of the pavement friction state during winter maintenance periods, similar to the pavement thermal state maps developed for winter maintenance purposes.
These maps of average pavement friction can influence route-of-travel selection and travel planning. This non-real-time information can be communicated to the public as web-based and printed material.
Scenario 4: Recorded, archived and real-time transmitted friction measurements by winter maintenance patrol or snowplow/spreader vehicles
Building on the activities in Scenarios 1, 2 and 3, the records of friction measurements and locations are transmitted in near-real-time from the snowplow/spreader vehicles to a central location, where the information is processed by cell phone and radio.
Friction data and location volumes, and the regular intervals at which these data must be transmitted, preclude using the voice communication radio frequencies that are presently in use. Some of these require investments in auxiliary technology including RWIS, over-the-road techniques such as anti-icing, technologically and staff intensive traffic operations centers and advanced traveler information systems. This information can be used to dispatch snowplow/spreader vehicles to existing and anticipated trouble spots.
Also, chemical snow and ice material application rates for different periods during a storm can be changed in near-real-time as the storm intensifies or winds down.
Winter road friction information will help road users in pre-trip planning and en-route travel decision support. En-route public use of winter friction communications would include public agency or commercial radio and dial-in cell phone advisories, as well as wireless web-based postings.
Traffic control devices, such as variable message signs that provide motorist warnings and advisories, can be used.
Expert opinions were obtained using two questionnaires to determine the feasibility of using different friction measurement techniques to support winter maintenance operations and mobility and to evaluate the proposed scenarios.
The first questionnaire was sent to winter maintenance and field supervisors to solicit their opinions on the feasibility and perceived benefits of using friction measurements and solicited information on the efforts needed to ease the implementation of these scenarios as well as the possible technological impediments to their implementation.
The second questionnaire was prepared to solicit opinions from knowledgeable national and international sources on the feasibility of the proposed scenarios and to identify the most promising friction-measurement technologies for each scenario.
All for four
It was clear that the winter maintenance operators and field supervisor respondents believe that the use of friction measurements would improve winter maintenance operations. However, some of the problems with the friction measuring equipment used by these agencies included lack of reliability and calibration requirements.
The information collected by low-cost, reliable friction measuring devices, complemented by other data such as pavement temperature, surface conditions, weather conditions and air temperature, could be useful to allocate snow-fighting resources in real-time. Devices that require hard braking may not be used, citing safety considerations.
The opinions were divided on the use of the simple friction indication in aiding in the selection of appropriate application rates for ice control materials or down-pressure on underbody snowplows. Furthermore, the respondents did not see a potential benefit in using friction records for developing spatially average maps of surface friction during snow-fighting periods, or posting this information on the web or other media outlets for user information.
The knowledgeable national and international sources were asked to provide a percentage weight for each criterion considered for the evaluation of the proposed scenarios. It was suggested that the potential to enhance winter maintenance operations is the most important criterion, followed closely by the potential to enhance safety and mobility, which are given approximately the same weight. Then implementation feasibility and practicality both received roughly the same weight. Prior domestic and international experiences were given the lowest weights.
The input shows that Scenario 4 provided the highest potential to enhance winter maintenance operations, mobility and roadway safety, and Scenario 3 provided the lowest potential; this is particularly true among North American respondents. This could be attributed to the fact that Scenario 3 could be considered a first step before advancing to Scenario 4. The main difference between the two is that Scenario 4 requires coordination with related agencies (RWIS data for example), which is achievable in North America.
For friction-measuring technologies, repeatability, reliability and effectiveness were the most highly regarded characteristics. According to the data analysis, the TCS/ABS is the most promising technology, followed closely by deceleration and slip devices.
In summary, individuals with adequate familiarity of friction issues believed that using friction measurements to improve winter maintenance operations and mobility is feasible; TCS appears to be the only way to eliminate the extra wheel used in current friction measuring devices, and their use to predict road surface condition has a great potential for enhancing winter maintenance operations; developing a model to predict road surface condition using climate-, traffic- and pavement-related data is feasible; and significant work needs to be conducted in the area of human response to better understand drivers’ reactions to winter maintenance and to determine the best method of communicating roadway friction to drivers in a simple way.
Using friction measurements to provide information to support winter maintenance decision making qualitatively appears to be promising and practical for improving winter maintenance operations, safety and mobility.
In this scenario, a winter maintenance supervisor’s vehicle is dispatched to travel over portions of the road system during and after winter maintenance and to make either periodic or continuous measurements of roadway friction, and the operator of this patrol vehicle is given the collected friction information in simple, qualitative terms (e.g., a green, yellow or red surface friction condition). This scenario is thought to have the highest potential for successful implementation. If some or all of these measurements do not meet approved friction levels-of-service, then the decision maker can call the maintenance fleet to re-treat those road sections.
Additionally, appropriate application rates for ice control materials for the monitored conditions can be relayed to the entire winter maintenance fleet so that individual operators can use this information to set the spread rates of their ice control material.
Near-real-time transmittal of friction measurements and locations from the winter maintenance patrol or snowplow/spreader vehicles to a central office for processing was thought to be the most promising scenario for enhancing winter maintenance operations, mobility and safety, yet it requires further technology development and integration prior to an operational trial.
In this scenario, RWIS data are used for dispatching snowplow/spreader vehicles to existing and anticipated trouble spots and for selecting ice control material application rates for different storm periods. The information (or a summary) can be made available to both public and commercial road users with warnings about sections of roadway where surface friction may be inadequate for safe mobility.