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group. TrafLoad uses the traffic ratios obtained at a Level 1A site to convert short-duration clas-
sification counts obtained at any associated Level 1B site to estimates of AADTi by lane. (See
Part 4, Step CF, available online at http://trb.org/news/blurb_detail.asp?id=4403.)
As in the case of monthly traffic ratios, the monthly/DOW traffic ratios applied to short-duration
counts obtained at any Level 1B site should be "current year" traffic ratios; i.e., they should be
developed from data that are collected over a 12-month period that includes the month dur-
ing which the short-duration count is collected.
The research team makes two observations about this factoring procedure. The first is that the
conversion process involves dividing by the MADW for each Type 1 VC group. Hence, the
accuracy of the resulting estimates of AADTi is affected by the similarity of the seasonal and
DOW volume patterns for the VCs within each VC group but not by the similarity (or dis-
similarity) of these patterns between VC groups. The second observation is that the adjust-
ment procedure uses traffic ratios obtained from a single Level 1A site, a site that should have
seasonal and DOW volume patterns that are very similar to those at the corresponding
Level 1B site(s). For this reason, the resulting estimates of AADTi should be substantially bet-
ter than those that can be produced at Level 2 sites.
3.4 Level 2 Classification Sites
Level 2 classification sites are classification sites that do not qualify as Level 1 classification
sites. That is, less than 12 months of current data are available for these sites, and they are not
associated with another site on the same road for which 12 months of current data are avail-
able. A few of these sites are continuous classification sites at which data are missing for one
or more months. However, most of these sites are ones at which classification counts are col-
lected as part of a state's short-duration classification-count program. Counts collected as part
of this program fall into three categories:
1. Coverage counts that are collected periodically (e.g., once every third year) at a relatively
fixed set of sites to provide general information about truck volumes and how these vol-
umes are changing over time;
2. Expected project counts, collected at sites at which highway projects are anticipated, to pro-
vide data for use in the planning and design process; and
3. Project-specific counts, collected either to provide additional information about sites at
which expected project counts have already been collected or to provide data to be used
for projects that had not been anticipated.
Most short-duration counts collected for pavement design projects are likely to fall into the
second of these categories. That is, they are likely to be collected to support pavement design
projects that are anticipated to occur in the near future. These potential projects should be
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identified by highway planners as soon as practical. Planning and programming tools avail-
able for this purpose include pavement management systems.
The early identification of pavement projects and scheduling of classification counts requires
coordination among the data-collection staff, the pavement design staff, and other agency staff
involved in the programming and prioritization process. While this level of communication is
not easy, it has several advantages. It allows the traffic engineering office to schedule needed
counts so that they can be collected efficiently. It ensures that data are available to designers
when needed, thus speeding up the design process. And finally, it provides an opportunity to
collect extra counts to be used in the design of major projects. As discussed below, estimates
of AADTi developed from three or four 7-day counts collected over the course of a year are
likely to produce appreciably better estimates of AADTi than similar estimates developed
from a single 48-hour classification count.
A key to this approach is to be generous when estimating possible pavement design locations.
Traffic volume and classification counts at most locations are considered to be reliable for at
least 2 years. Thus, even if an expected pavement design project does not make this year's
design list, it will likely make next year's list, and traffic data will already be collected and
available for that location.
Even with good communication between pavement and traffic engineering staff, it may not
be possible to collect all the traffic data required as part of the routine data-collection effort.
Accordingly, allowance should always be made for a possible need for project-specific counts
to supplement the expected project counts.
Short-duration classification counts are usually collected using automatic vehicle classifiers
(AVCs). However, at some urban sites, manual classification may be preferred. Because man-
ual counts usually cover only part of a day, estimates of AADTi derived from manual counts
are not likely to be as good as estimates derived from accurate classification counts obtained
with AVCs for periods of 48 hours or more. Accordingly, these two types of short-duration
counts are distinguished from each other by calling AVC sites Level 2A sites and calling man-
ual classification sites Level 2B sites. The two following subsections contain brief discussions
of the collection and analysis of counts at these two types of classification sites.
AVC Sites (Level 2A)
Level 2A classification sites are sites at which automatic vehicle classifiers (AVCs) are used to
obtain one or more classification counts over the course of a year. Accurate AVC counts usu-
ally require that vehicles are traveling at constant speed with adequate spacing between vehi-
cles, conditions that may be difficult to meet in urban areas. Each AVC count should cover a
period of at least 48 weekday hours (though TrafLoad's Level 2A procedure is capable of esti-
mating AADTi from 24-hour classification counts). Improved estimates of AADTi will be pro-
duced if count duration is extended or if multiple classification counts are collected over the
course of a year.
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Section 3.3 included a discussion of TrafLoad's use of data from continuous classification-
count sites (Level 1A sites) to develop sets of monthly and DOW traffic ratios. These traffic
ratios are used by TrafLoad to convert the short-duration counts collected at Level 2A sites to
estimates of AADTi. For any Level 2A site, the traffic ratios used are those developed for the
seasonal and DOW factor groups to which the site belongs.
As observed in Section 3.3, monthly traffic ratios that are derived from current year data work
better for this purpose than monthly traffic ratios derived from historic data. For this reason,
when providing TrafLoad with a set of Level 2 classification counts to be factored, the user
should (if possible) also provide TrafLoad with Level 1A counts for a 12-month period that
includes the month(s) during which the Level 2 counts were collected.
The quality of the estimates of AADTi that are produced will tend to vary with the degree to
which the seasonal and DOW patterns in truck volumes at the Level 2A site match the vol-
ume patterns in the seasonal and DOW factor groups to which the site has been assigned. Use
of 7-day counts reduces or eliminates the need for DOW factoring, and use of multiple counts
over the course of a year reduces the need for seasonal factoring. Thus, 7-day counts and mul-
tiple counts are strategies for increasing the amount of data collected in order to improve the
quality of AADTi estimates.
Hourly distribution factors for each Level 2A site are developed by TrafLoad from the counts
collected at the site. TrafLoad sets the monthly traffic distribution factors for each Level 2A
site equal to the monthly traffic ratios for the seasonal factor group to which the site has been
assigned.
Manual Classification-Count Sites (Level 2B)
In order to classify vehicles reliably on the basis of axle-spacing criteria, AVCs must be located
where vehicles are neither accelerating nor decelerating and where the spacing between vehi-
cles is sufficient to allow consecutive vehicles to be readily distinguished. Because these con-
ditions are difficult to meet in urban areas, urban classification counts frequently are collected
manually. (Alternatively, classification on urban streets and roads may be limited to length
classification.) Manual classification counts are usually collected only during daylight hours,
usually for a period of 6 to 12 consecutive hours. Conversion of these partial-day counts to
estimates of AADTi is a two-step process:
1. Each set of partial-day classification counts is converted to a set of estimates of volume by
VC for the day on which the counts were collected.
2. The procedures discussed in the preceding section are used to convert these estimates of
24-hour volume by VC to estimates of AADTi.
Procedures for performing the first of these two steps are discussed below. This step adds
some additional error to the resulting estimates of AADTi (over and above the error intro-
duced by the factoring procedures discussed above). Accordingly, sites at which classification
counts are obtained manually are described as Level 2B sites.
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