The whole wave thing is just an acknowledgement and a sign of respect to Porsche that has been established throughout the generations of P-car ownership and I think if someone doesn't wave that it can seem offensive. I don't think a harmless wave going to kill you unless your other hand is not on the steering wheel and you are canyon carving
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It shows that us the Porsche owners value each other and each other's appreciation and enthusiasm for fine automobiles. I learned the Porsche wave thing my first week of ownership as I was driving to work and I thought it was the coolest thing. Otherwise one might as well go buy a Toyota Camry and blend in the background.