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11 The daily traffic distribution data was determined based on Nonpassenger vehicles (class 4 to class 13) are divided by the the traffic data collected in the field over the years. However, number of axles and the trailer units (15). Although a bus it was found that some sections did not have enough field (vehicle class 4) is a passenger vehicle, the term truck traffic data to determine the traffic characteristics, while others had applies to the load level and includes both trucks and buses complete historical traffic data. In order to consider the level since the proportion of buses in the traffic flow is relatively of collected traffic data, a hierarchical approach was adopted small (16). in the MEPDG and also is used in this project. The three lev- Because the light axle load groups, such as vehicle classes 1 els were defined based on the availability of collected traffic to 3, do not have significant effects on load related distresses, data and Weigh-In-Motion (WIM) data which is used to the traffic analysis in this study considered only the heavier determine the normalized axle load distribution for each axle load groups (i.e., classes 4 to 13). and vehicle types (14): Axle Load Distribution Factor Level 1: Very good knowledge of past and future traffic characteristics and site/segment specific WIM data; The axle load distribution is defined as the classification of Level 2: Modest knowledge of past and future traffic char- traffic loading in terms of the number of load applications by acteristics and regional default summaries WIM data; and each axle type (single, tandem, tridem, or quadrem) within a Level 3: Poor knowledge of past and future traffic charac- given range of axle load. The axle load distribution factor is teristics and national default summaries WIM data or only the percentage of the total axle applications in each load inter- Average Annual Daily Truck Traffic (AADTT) available. val by an axle type for a specific vehicle class (classes 4 to 13) (14, 17). The load ranges and intervals for each axle type are listed in Table 5. Categorization of Traffic Loads The determination of the axle load distribution requires In order to analyze traffic load effects for reflection crack- WIM data, which is the number of axles measured within ing, the annual number of axle loads for each vehicle class and each axle load range by axle types of each vehicle class. LTPP axle type were entered in the analysis process. The number of guidelines require that the vehicle axle weights should be col- axle loads was determined using the traffic load categorized lected using a WIM sensor by vehicle classes, type of axle, and based on the FHWA vehicle class, the axle type, and the num- axle load intervals. Using measured WIM data, the distribu- ber of tires (details of this process are found in Appendix C). tion is calculated by averaging the number of axles measured within each load interval of an axle type for a vehicle class divided by the total number of axles for all load intervals for Classification of Vehicles a given vehicle class. The normalized axle load distribution FHWA defines vehicles into 13 classes, as shown in Table 4, factors should total 100 for each axle type within each truck depending on whether they carry passengers or commodities. class. Table 6 presents an example of FHWA W-4 Truck Table 4. FHWA vehicle classification. Vehicle Schema Description Class 4 Buses 5 Two-axle, single-unit trucks 6 Three-axle, single-unit trucks 7 Four-axle or more than four-axle single-unit trucks 8 Four-axle or less than four-axle single trailer trucks 9 Five-axle single trailer trucks 10 Six-axle or more than six-axle single trailer trucks 11 Five-axle or less than five-axle multi-trailer trucks 12 Six-axle multi-trailer trucks 13 Seven-axle or more than seven-axle multi-trailer trucks

OCR for page 11
12 Table 5. Load intervals for each axle type. Axle Type Axle Load Interval Single Axles 3,000 ~ 40,000 lb at 1,000 lb intervals Tandem Axles 6,000 ~ 80,000 lb at 2,000 lb intervals Tridem Axles 12,000 ~ 102,000 lb at 3,000 lb intervals Quad Axles Table 6. Number of single axle loads for vehicle class 4 to 7 (LTPP Section 180901 in 2004). Axle Load Vehicle Class (lb) 4 5 6 7 3,000 0 53,818 183 11 4,000 10 54,606 558 52 5,000 42 39,113 993 139 6,000 175 20,289 1,099 168 7,000 988 24,555 2,426 252 8,000 10,687 22,491 5,617 298 9,000 9,713 13,719 8,154 365 10,000 10,156 12,839 12,423 879 11,000 6,011 7,127 8,945 1,516 12,000 5,875 6,413 7,725 2,913 13,000 3,409 3,511 3,257 2,464 14,000 2,947 3,128 2,289 2,710 15,000 1,640 1,756 975 1,740 16,000 1,239 1,513 725 1,419 17,000 679 834 285 664 18,000 446 800 235 423 19,000 212 424 104 159 20,000 181 360 73 111 21,000 106 261 44 70 22,000 51 131 22 46 23,000 41 135 6 26 24,000 21 85 4 9 25,000 24 90 3 12 26,000 11 43 1 7 27,000 4 33 1 2 28,000 1 12 3 1 29,000 4 25 0 1 30,000 3 13 0 0 31,000 1 16 2 0 32,000 2 8 0 0 33,000 0 5 0 0 34,000 0 2 1 0 35,000 0 0 0 0 36,000 0 0 0 0 37,000 0 2 0 0 38,000 0 0 0 0 39,000 0 0 0 0 40,000 0 0 0 0 Total 54,679 268,157 56,153 16,457