North Carolina forecasts for truck traffic
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2006-07-01
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Edition:Final Report; July 1, 2003- June 30, 2005
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Abstract:North Carolina has experienced significant increases in truck traffic on many of its highways. Yet, current NCDOT
project-level highway traffic forecasts do not appropriately capture anticipated truck traffic growth. NCDOT
methods forecast total traffic AADT based on historical trends and assume base year and future year truck
percentages are the same. Also, NCDOT determines the growth factor of historical AADT traffic data by looking
only at the first year and the last year ignoring intermediate year traffic data. This research recommends that traffic
for dual axle (Dual) trucks and tractor trailer (TTST) trucks be separately forecast using an average growth factor
(AGF) method that uses all available data and calculates growth factors year by year and averages them for the
historical trend. The results of this research propose statistical methods to separately estimate growth factors for
light vehicles (Cars), Duals and TTSTs. In this study Cars represent FHWA vehicle classes 1-3, Duals 4-7 and
TTSTs 8-13. The methods have modest data requirements based on NCDOT traffic counts and VTRIS on-line
traffic data. Statistical guidelines including mean AGFs and confidence intervals to help refine the estimates for
growth factors by vehicle class and highway type. Case study examples demonstrate the methods and guidelines for
Interstate, US, and NC highways. Other issues are explored including the effects of increased truck traffic on
highway level of service and pavement design.
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