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Special Purpose TWRGs
Consider the case in which pavement is being designed for a site at which truck weights are
believed to combine the weight characteristics of trucks operating at two Level 1 WIM sites
but that is not itself a Level 1 site. One option is to assign such a site to one of the standard
TWRGs (discussed above) and to use the load spectra developed for that TWRG as the load
spectra for the site in question.
Another option is to develop a special-purpose TWRG that is used just for this pavement
design. If the only Level 1 WIM sites assigned to this TWRG are the two sites mentioned
above, the load spectra developed for this TWRG will be formed as unweighted averages of
those developed for the two sites. (The current version of TrafLoad does not allow for using
weighted averages in developing load spectra for TWRGs.) In general, development and use
of such a special-purpose TWRG will require a separate run of TrafLoad, with the two
Level 1 WIM sites assigned to the special-purpose TWRG instead of the TWRG(s) to which
they are normally assigned. (The current version of TrafLoad allows sites to be assigned to
only one TWRG.)
2.5 Level 3 WIM Sites
Level 3 design can be used when little is understood about the axle-weight distributions of the
road segment being designed. Level 3 designs use the state's average axle-load distribution
curves. They can be applied to any design project in the state, but their use entails a relatively
high likelihood of at least modest load estimation error because few roads are truly average.
Thus, the pavement design is likely to be either over- or under-designed.
Consequently, states are encouraged to use Level 2 load spectra whenever the combination of
available data and engineering knowledge permits accurate assignment of a road to a TWRG
with a reasonable degree of confidence.
Pavement designs for Level 3 sites are developed using a set of statewide load spectra defaults
that are formed as a weighted average of the sets of load spectra developed for the TWRGs.
The weights used in this process are user specified. For averaging, relatively high weights
should be assigned to the more important TWRGs (in terms of size and/or degree of represen-
tativeness of conditions at Level 3 WIM sites) and lower weights to the less important TWRGs.
Because of the wide variety of loading conditions that occur from one road to another, the
error associated with statewide load spectra defaults will be large. To allow for the possibility
of large errors in the load spectra for Level 3 sites, the Pavement Design Guide software pro-
duces a very conservative pavement design. For roads with heavy loading conditions, such a
design will result in a small probability of premature failure. However, for other roads, pave-
ment will be over-designed (significantly so for roads with light loading conditions), causing
state funds to be used inefficiently.
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