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There is also the question of how much data to collect in a given month. The California data summarized in Figure 2.3 indicate that there is substantial variation in axle weights during weekends for single-unit trucks. Hence, monthly load spectra developed from a full week of data generally will be more accurate than load spectra developed from smaller amounts of data. However, this difference in accuracy is somewhat mitigated by TrafLoad's procedure (discussed earlier) for adjusting load spectra to account for the approximate effect of any miss- ing days of the week. 2.4 Level 2 WIM Sites and TWRGs Level 2 and 3 WIM sites are sites for which site-specific WIM data are not available. Pave- ment designs for such a site are developed using either a set of default load spectra devel- oped on a statewide basis or a default set developed for a particular TWRG to which the site has been assigned. The goal of assigning sites to TWRGs is to permit the use of load spectra that better describe the axle loads at member sites than would a statewide set of load spec- tra. The following subsections provide guidance for developing TWRGs to be used for this purpose. Figure 2.3 Daily ESAL Ratios ESAL Ratio 1.10 1.00 0.90 0.80 VC 5 VC 6-7 VC 8-13 0.70 1 2 3 4 5 6 7 Sunday Monday Tuesday Wednesday Thursday Friday Saturday Day of Week Source: Cambridge Systematics, Inc., Accuracy of Traffic Load Monitoring and Projections, Volume II: The Accuracy of ESALs Estimates, prepared for FHWA, February 2003, Figure 5.1. 2-17

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Background The basic goals in forming TWRGs for default values are to group roads at which axle loads are likely to be reasonably similar (e.g., axle loads are likely to be higher than average) and to assign roads whose axle loads are not expected to be similar to different TWRGs. The TWRGs should be defined so that every road or road segment for which load spectra may be of inter- est is unambiguously assigned to a specific TWRG. A key principle in forming TWRGs is that all weight limits should be essentially the same on all roads in the group. Thus, if some combinations are allowed to operate routinely at weights above 80,000 pounds on a set of designated roads, then the designated roads should be assigned to one or more TWRGs that are separate from the TWRGs to which other roads are assigned. Similarly, roads on which axle-weight limits vary seasonally (e.g., during spring thaw or during winter freeze) should be assigned to different TWRGs than roads on which axle-weight limits do not vary seasonally. A corollary to the above principle is that the WIM sites whose data are used for deriving load spectra for a given TWRG need not all be in the same state, provided that they are all subject to the same weight limits and that the trucks operating at the out-of-state WIM site(s) are believed to carry loads that are similar to those carried at other sites in the TWRG. One of the major influences on the load spectra for any site is the mix of empty, partial, and full loads of vehicles at the site. This influence is a significant factor affecting load spectra for combination trucks. Combination trucks on long trips (i.e., trips of more than 200 miles) are likely to be fully loaded. Past studies have indicated that only about 15 percent of combination trucks operat- ing on the rural Interstate system are empty. However, for short trips, most trucks operate loaded in one direction and empty in the other, while other trucks are used for pickup-and- delivery service, carrying partial loads for much of their trips. Since the percentage of combi- nation trucks making short trips rises in urban areas (particularly in large urban areas), the percentage of fully loaded trucks usually declines, and so do axle weights. Similarly, non- Interstate roads in rural areas also carry a mix of long-haul and short-haul traffic, so axle loads for combination trucks are also likely to be lower on these roads than on the rural Interstate system. The mix of empty and full loads can also result in significant differences in load spectra by direction. Directional differences are likely to be greatest on roads where most trucks are trav- eling to or from a particular site. For many such sites, nearly all these trucks are likely to oper- ate empty in one direction and full in the opposite direction. On the other hand, trucks oper- ating to or from a containerport usually operate loaded (and frequently very heavily loaded) in both directions. Roads that function primarily as access roads to a containerport are likely to warrant a TWRG of their own. Another major influence on the load spectra on any road is the weight of fully loaded vehicles operating on the road. If most of the trucks that use a particular set of roads carry commodi- ties that are likely to produce axle loads that are atypically high or low, that set of roads should be assigned to a separate TWRG. However, there is little reason to split a state into regional TWRGs if there are no regional variations in the commodities carried. 2-18

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Guidelines The above discussion leads to the following guidelines for developing a set of TWRGs to be used for load spectra defaults: 1. If there are any significant differences in the size and weight limits applied to vehicles on different roads in the state, partition all roads into two or more sets, each with uniform size and weight limits. 2. For each of these sets of roads, develop a separate set of TWRGs, and assign the roads to these TWRGs on the basis of Functional class, Region, and/or Direction. The second step should be performed using judgment, local knowledge of the trucks operat- ing in various parts of the state, and available WIM data.7 Some observations that may be use- ful in carrying out this step are the following: There is almost certainly value in distinguishing roads by functional system: urban, rural Interstate system, and rural other. If there are significant regional differences in the density of commodities carried (particu- larly on rural other roads), these differences may warrant either using a combination of regions and functional systems or using regions instead of functional systems. Similarly, if, within any region, there are significant differences between the density of com- modities carried on East-West roads and that carried on North-South roads, these differences may warrant using combinations of regions, functional systems, and road orientation. In the case of any TWRG that consists primarily or entirely of divided roads, if heavy (i.e., loaded) and light (i.e., empty) directions can be readily distinguished without using any WIM data,8 it is likely to be desirable to divide the TWRG into heavy and light directions. If practical, there should be between three and eight WIM sites in a TWRG. However, one or two WIM sites may be used for some small TWRGs. Three sites is the minimum number nec- essary to provide some confidence that all sites in the TWRG have reasonably similar load spectra. On the other hand, as the number of WIM sites in a TWRG grows, opportunities also grow for splitting the TWRG to produce smaller TWRGs, each with more uniform sets of load spectra. 7 Additional discussion of the formation of TWRGs is contained in FHWA's Traffic Monitoring Guide (May 2001, Section 5, Chapter 3). 8 The heavy/light distinction can be useful only if, for any project site for which there are no WIM data, local knowledge can be used to distinguish the heavy direction from the light direction. 2-19