Estimating Wet-Pavement Exposure with Historical Precipitation Data in California

Statistics indicate that rain and wet pavement have more significant impacts on road safety than snow and ice. The effect of Wet Percent Time was found to be an important variable in wet accidents. Caltrans sponsored a two-year research project in 2006 to develop an updated Wet Percent Time table, as the current table was developed with data more than 30 years old. Historical hourly precipitation data of California reported by rain gauges were obtained from the five network data sources including CDEC, CIMIS, MESOWEST, NCDC, and the National Weather Service. Based on the density and distribution of available hourly precipitation data stations, an 11-year period (01/01/1995-09/30/2005) was chosen for this project. When available archived precipitation data were downloaded from various upstream weather data providers and uploaded to the WTI database, we followed three processes to check and improve the quality of the data: Reprocessing, Quality Control, and Missing data In-filling. To assure and quantify the quality of the data used in this study, the statistical summaries were generated and the results were reviewed. Overall there were 1296 out of 1718 stations available from CDEC, CIMIS and MESOWEST for constructing the California Wet Percent Time table from 1995 to 2005, whereas 422 problematic stations were removed from the WTI database for either short duration of available data or large percent of erroneous and missing data. An improved method using the Zonal Statistics provided in the ArcGIS Spatial Analyst was tested and chosen for this project to produce a more accurate County-Average Wet Percent Time table.