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4.0 Data Handling
This chapter discusses the data collection and handling needed to create the datasets used by
TrafLoad and the relationship of this data handling to a state's traffic data-collection program.
Highway agencies currently collect, manipulate, store, and report traffic data. No fundamen-
tal change in this existing data flow is required to meet the traffic data requirements of the
Pavement Design Guide.
· Data must still be collected from the field, preferably using modern, calibrated, automated
data-collection equipment.
· Data are downloaded from the devices used to collect data from the field and analyzed in
the office. This analysis process includes checks for data quality, a summarization step,
and a storage process that allows for later use.
· These summaries are then extracted and manipulated as needed to produce the traffic load
estimates required by the Pavement Design Guide.
These activities are discussed in the first two sections of this chapter, and related administrative
and institutional issues are discussed in Section 4.3. Extensive information on data-collection
equipment is presented in a companion volume.1
4.1 Data Collection
The Pavement Design Guide mechanistic design software does not require collection of new
types of traffic load data. The TrafLoad data analysis system that feeds the Pavement Design
Guide software uses the traditional measures of volume, vehicle classification, and truck axle
weights to compute the traffic loading inputs needed. All of these measures are currently col-
lected to one degree or another by every state highway agency and are discussed in FHWA's
Traffic Monitoring Guide.
While all state highway agencies already collect data, it is likely that the number, timing, and
location of counts that highway agencies perform will change in order to produce better traffic
loading estimates. In addition, some state highway agencies may have to create new summary
1
Cambridge Systematics, Inc., and Washington State Transportation Center, Equipment for Collecting
Traffic Load Data, prepared under NCHRP Project 1-39, June 2003 available online at http://trb.org/
news/blurb_detail.asp?id=4403.
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output reports and data files from the data they are already collecting in order to input traffic
loading estimates into the new pavement design software.
These changes in count location and duration are purely voluntary. However, the availability
and quality of data collected by each state will have a direct impact on the accuracy of traffic
load inputs to the pavement design process and consequently on the reliability of the pave-
ment designs developed with the new software.
The basic data-collection design for providing traffic load data fits within the general traffic
data-collection guidance provided by the FHWA in the 2001 Traffic Monitoring Guide. A key
point is that a large portion of the traffic data collection required for estimating traffic loads
should be collected as part of the routine traffic data-collection program. Thus, pavement
design engineers need to work closely with those engineers who select, schedule, perform, and
analyze the traffic data being collected. This increased level of communication will ensure that
traffic load estimates can be collected cost effectively and that the summary statistics needed
by the pavement designers are readily available and easily loaded into the pavement design
software.
Each state highway agency should have a traffic count program that, at a minimum, collects
· Short-duration volume counts,
· Continuous volume counts,
· Short-duration classification counts,
· Continuous classification counts, and
· Weigh-in-motion (WIM) measurements (i.e., truck weighing).
Because pavement depth is not significantly impacted by the volume of light-duty vehicles,
mechanistic design is primarily concerned with the number and weight of trucks and buses
using the roadway in question. Volume data collection is not discussed in this report. Uses of
classification and weight data are discussed in earlier chapters, and the collection and han-
dling of these data are discussed in this chapter.
Short-duration classification count program elements are designed to provide site-specific vol-
ume (by VC) measurements that determine the total number of axle loads on a given roadway
segment. Continuous-count elements provide measures of temporal variation needed to con-
vert short-duration counts into unbiased measures of average annual conditions. WIM mea-
surements provide data on the weights of each axle group.
These data-collection program elements provide all of the information needed for producing
the traffic loading estimates required by TrafLoad. Table 4.1 summarizes the data require-
ments of TrafLoad and identifies the traffic data-collection elements that provide the raw data
needed to meet these requirements. Table 4.2 describes the data that each of these elements
contributes to the pavement design process. Both tables identify distinctions between the three
levels of classification data discussed in Chapter 3.0 and between the three levels of weight
data discussed in Chapter 2.0.
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