• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.6
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• pp.2427-2435
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• 2013

Design hourly volume and the K-factor, first proposed by FHWA in the 1950s, is based on the 30th hourly traffic volume during a year (out of 8,760 hours). It was used when surveying the traffic volume was laborious in the past and is still being used now although it leaves some to be desired for practical applications. More reasonable K-factor for better design, based on theoretical evidence, is needed. This paper proposes the knee searching method based on simple linear regression to find out the inflection point of the volume ranking curve that describe the annual 8,760 hourly traffic volumes. The method was applied to the Chungcheong province's national highway, and the results were compared to the existing guidelines' values of K-factors. Identified design hourly traffic volumes ranked between 43rd to 694th, which is much lower than the 30th volume, meaning that some overdesign examples are inevitable if the conventional $30^{th}$ volume is used.