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Hughes (unpublished comment), February 2010

In North Carolina, the State Highway Patrol's MCSAP and Truck Size and Weight components of its Motor Carrier Enforcement Unit are attempting to develop a better understanding of operational freight issues and their impact on current and future enforcement efforts.

In a recent effort to evaluate the allocation of uniformed motor carrier enforcement (MCE) resources statewide, the Patrol looked at the relationship between CMV-involved crashes and a number of variables. Our current analysis confirms what we and our partners at the NCSU Institute for Transportation Research and Education (ITRE) have found before; that is, that the relative number of CMV-involved crashes in a geographic area (e.g., a multi-county troop) closely matches the relative population of the troop area. This was true for non-CMV crashes as well (see below).

Other variables that were looked at with respect to the distribution of crashes were (a) the percentage of land area represented by the counties within a troop and (b) the percentage of total state-maintained road miles within the geographic boundaries of each troop. These relationships are shown in the plots below.

The scatter plots show no relationship between the relative number of crashes (CMV or non-CMV) and the relative geographic size of the troops. While at first glance there appears to be an increasing relationship between the relative number of crashes in a troop and the relative number of state-maintained miles in the troop, the percentage of road miles provides a poor prediction (in terms of R2) of the extent of crashes in a troop (for both commercial and non-commercial vehicles).

In an effort to relate the population variable to 'freight' (in this future truck tonnage estimates) we looked at data in the recently released 2009 Truck Facts and Figures. Using the data provided in that document for future truck tonnage and data from US Census reports on US population projections, we observed the relationship in the chart below. The freight data and the population data are both plotted relative to their year 2000 values. We are having difficulty understanding the result; i.e., that truck freight (as measured by tonnage) is expected to increase at a faster rate than the rate of population growth.

We discovered this relationship while looking at data in the 2009 Facts and Figures publication from the standpoint of where data visualization might be used to enhance our understanding of basic freight issues, especially where such an understanding might influence sub-national stakeholders

(see the TRB Research Problem Statement on visualization and freight data at: http://rns.trb.org/printview.asp?ids=23011 ).