My response has been "the world is a safer place without me behind the wheel of a car" and "I feel a lot better mentally and physically as a result of riding a bicycle daily."
I've been doing it for two or three years and have lost like 30 lbs. Enough people are impressed by the weight loss and health benefits that I can argue they're in more danger as a result of going from sitting in a car, to sitting at a desk, to sitting in a car, to sitting on a couch every day.
Being free of the aggravation of sitting in traffic, responding to the price of gas and dealing with car maintenance has lowered my stress level a lot too.