The effects of light rainfall on urban freeway operations are modeled to improve understanding of road safety and speed–flow–occupancy relationships in suboptimal weather. Three broader issues also are addressed. What is the form of the relationship among various traffic variables under rainy versus dry conditions? Are the safety implications of driver adjustments under rainy conditions different for the day and night? How should speed variation be measured in ecological studies? Volume–occupancy and speed–volume relationships are affected by rainfall: specifically, speeds are reduced and speed is more strongly dependent on volume. Under nighttime, rainy, uncongested conditions, speeds are reduced and time gaps are increased, but only minimally. Under daytime rainfall conditions, when traffic volumes are typically high, speeds are reduced substantially, and because of the interaction between traffic variables, volumes also decrease. The physical time gap increases marginally as well, whereas speed variability within the traffic stream is reduced. For congested daytime conditions, light rainfall is not associated with any changes in volume or time gap, but speeds are reduced. Finally, safety-related information on speed deviation can be derived from traffic loop data by calculating the variability of travel speeds within small time units. (Author/publisher)
Abstract