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9CHAPTER 4 PERFORMANCE-BASED LEVEL OF SERVICE The definition of terms, concepts, and a suggested Pavement Snow and Ice Condition (PSIC) rating system were described in Chapter 2 in the section entitled Performance Measuring of Level of Service (LOS). When defining LOS goals, two time frames relative to a winter weather event need to be considered: ⢠Within-winter weather event and ⢠After-end-of-winter weather event. Higher LOS are associated with âbetterâ within-event pave- ment ice conditions and more rapid achievement of âbetterâ or âbareâ pavement conditions after the event ends. THE âDESIGNâ WINTER WEATHER EVENT A winter weather event having a snowfall rate of âXâ inches per hour should be chosen as a basis for determining what level of service can be provided with existing resources or deter- mining the necessary resources to provide a desired level of service. âXâ should be a rate that is only exceeded in âYâ per- cent of snowfall records in an average year (from climatolog- ical records). âYâ of about 20 percent can be selected (7). WITHIN-WINTER WEATHER EVENT LEVEL OF SERVICE A within-winter weather event has two intertwined compo- nentsâthe amount of loose snow/ice/slush that is allowed to accumulate between plowing cycles and the condition of the ice/pavement interface in terms of bond and packed snow/ice. The amount of loose snow allowed to accumulate on the roadway between plowing cycles is usually the driving force for plowing resource requirements. Plowing operations are limited to one lane at a time while material spreading opera- tions can treat more than one lane at a time. Once the allow- able amount of loose snow/ice is established, the necessary equipment resource can be determined. First, the local plow- ing production rate in terms of lane miles per hour (including reloading and deadheading) has to be determined. This, in conjunction with design snowfall rate, yields the cycle time required to meet the âaccumulationâ goal. Sufficient equip- ment has to be provided to achieve the desired cycle time(s). The second issue is the condition of the snow/ice pave- ment interface in terms of bond or packed snow/ice. This is largely a function of pavement temperature, the type of mate- rials treatment being provided, materials application rate, and cycle time. Generally, cycle times shorter than 1.5 hours using straight chemicals and plowing will allow a high within-event level of service. AFTER-END-OF-WINTER WEATHER EVENT LEVEL OF SERVICE The after-end-of-event LOS is usually expressed as a time to achieve particular pavement surface conditions in terms of ice or snow coverage, or PSIC level. ESTABLISHING LEVEL OF SERVICE GOALS The first step in the process of establishing LOS goals is to prioritize the roadway system and particular locations within the system into some type of LOS system. This is typically a numeric, alphabetic, or color system. The next step is to assign both within-winter weather event and after-end-of-winter weather event LOS goals. These goals may be described in terms of a PSIC as shown in Table 6 or a variety of other descriptors. The final step is a reality check to ensure that suf- ficient capability to meet the goals for the âdesignâ condi- tions is available. Table 6 is an example of LOS goals for a small roadway system.
10 Highway segment Within-event LOS After-end-of-event LOS Route Mile post LOS classa PSIC Maximum snow accumulation (in.) PSIC Hours after end-of-event 15 2-25 2 3 1.5 1 2.0 8 175-186 4 5 2.0 2 6.0 16 37-51 3 4 1.5 2 3.0 5 256-271 3 4 1.5 2 3.0 2 0-4 2 3 1.5 1 2.0 10 277-291 4 5 2.0 2 6.0 10 291-315 3 4 1.5 2 3.0 4 26-32 1 2 1.0 1 1.5 4 32-50 3 4 1.5 2 3.0 Main Street â 1 2 1.0 1 1.5 Baxer Bridge â 1 1 0.0 1 0.0 a â1â is the highest; 4 is the lowest LOS class. TABLE 6 Example of level of service assignment