Analysis of Impact of Adverse Weather on Freeway Free-Flow Speed in Spain

Abstract

Weather conditions affect traffic flow characteristics, including speed and capacity. Several studies have stated that rain, snow, wind speed, and visibility loss cause reductions in speed and capacity. Understanding these relationships is important to managing the traffic flow appropriately. This paper presents new research that evaluates the free-flow speed reduction caused by inclement weather conditions, including rain, snow, wind speed, and visibility loss. Fifteen freeway locations in northwestern Spain were selected for the study. Data were collected in 15-min intervals by weather and traffic stations for almost 3 years, from 2006 to 2008. Individual correlations between the weather and traffic variables were examined to select the most important weather variables and to identify speed trends and thresholds. All climate conditions were divided into four groups: no precipitation and temperatures above 0°C, no precipitation and temperatures below 0°C, rain, and snow conditions. A multiple nonlinear regression analysis was performed with the final variables. Results showed that rain and snow both caused a reduction in speed, with a more dramatic reduction during snow conditions. Wind speed over 8 m/s affected traffic speed, while the effect of visibility loss presented a logarithmic form. It was also determined that the location caused the variables to affect the speed differently, so further research should utilize a greater number of sites.