Traffic & The First Rainy Day.

When I was in Italy I observed how every first day of rain after many sunny days always had the effect of dramatically increase traffic. This was especially evident after long sunny periods in summer; after all that sun, the first rainy day would have the good effect of producing that sweet aroma of dust blown off the road by the rain drops, and the bad effect of increasing traffic in a unbelievable manner.
At the time I explained this phenomenon thinking that, during sunny days, Italian people drive their scooters or motorcycles, and when the rain comes they have to take the car to stay dry. Since many people drive two-wheelers, the explanation was somewhat logical. Another additional explanation was that car accidents are more frequent due to slippery roads. Since car accidents tend to slow down traffic, that was also a good explanation. The reason accidents are more frequent the first rainy day is that roads, since they have not been washed by the rain for a few days, tend to be covered in dust; when it finally rains, that dust produces a slippery coating that makes driving hazardous.
When I lived in Switzerland for a few months, I observed the same thing. The first day of rain after a few days of sun, produced a great increase of traffic. In Switzerland I don’t recall many motorcycles and I never saw any traffic jams caused by car accidents.
When I moved to Seattle I noticed the same phenomenon. Again, people don’t tend to drive motorcycles all that much around here. Some people do ride bikes but, since it rains most of the time anyway, they don’t seem to care if they get wet.
This morning I witnessed this phenomenon again. It had been sunny for several days and today it rains. The traffic is horrible, but there were no accidents; just lots of cars.
I have no explanation. Can somebody tell me why the first day of rain, after long stretches of sun, causes the traffic to get so bad everywhere in the world?