Niemeier, Debbie A., Jennifer Morey, Joel P. Franklin, Thirayoot Limanond, K. Lakshminarayanan (1999) An Exploratory Study: A New Methodology for Estimating Unpaved Road Miles and Vehicle Activity on Unpaved Roads (Volume I). Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-99-02
The method previously employed by the California Air Resources Board (CARB) resulted in lower estimates of miles of unpaved road and higher estimates of annual VMT on unpaved roads than those yielded by the methods developed in this study. Using 1993 numbers, including both farm and non-farm roads, CARB estimates the miles of unpaved road in San Joaquin County (SJC) to be roughly 1022. The method presented in this study yields an estimate of approximately 1437 miles of unpaved roads. Using CARB's previous method, San Joaquin County's VMT was estimated to be approximately 3,732,667. The results obtained using the new methodology presented in this report result in a VMT estimate of approximately 2,653,527 per year.
The methods described in this document were developed using data from San Joaquin County. They were then applied to data from Fresno County in order to illustrate the methods' application in calculating both miles of unpaved road and predicting VMT in other California counties. Results from SJC from the proposed study methodology, show both harvest and non-harvest models to be significant at a 99 percent level of confidence. Although the models are statistically significant, it is recommended that a further, larger study be undertaken to test the veracity of these models and under which conditions they remain consistent. Specifically, we recommend more extensive survey data be collected from growers, further unpaved road traffic counts be collected, and additional counties with a diversity of crop types be included in the analysis.