Road Weather Management Program
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Best Practices for Road Weather Management Version 2.0


Driver Response to Rainfall on an Urban Expressway


This paper models the effects of light rainfall on urban freeway operations in order to improve our understanding of both road safety and speed-flow-occupancy relationships during suboptimal weather. The paper also addresses three broader issues: What is the form of the relationship between various traffic variables during rain relative to dry conditions? Are the safety implications of driver adjustments during the rain different for day versus night? How should speed variation be measured in ecological studies? Both the volume-occupancy and speed-volume relationships are affected by rainfall. In particular speeds are reduced and there is a stronger dependency of speed on volumes in the rain. In nighttime uncongested conditions, speeds are reduced and time gaps are increased in the rain, though only minimally. In daytime, when traffic volumes are typically high, speeds are reduced substantially during rainfall and, because of the interaction between traffic variables, volumes also decrease. Physical time gap also increases marginally while 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.


85th Transportation Research Board (TRB) Annual Meeting, University of Waterloo (Canada). For an electronic copy of this resource, please direct your request to

Date: 2006


Unrau, Andrey


Freeway management
Pavement condition

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