  We provide a model to understand how adverse weather conditions modify
traffic flow dynamic. We first prove that the microscopic Free Flow Speed of
the vehicles is changed and then provide a rule to model this change. For this,
we consider a thresholded linear model, corresponding to an application of a
MARS model to road trafficking. This model adapts itself locally to the whole
road network and provides accurate unbiased forecasted speed using live or
short term forecasted weather data information.
