Well, If I'm feeling curt, I usually just say: "Well, car drivers don't want to hit me either."

If they feel inclined to push the issue, then I usually respond with information overload. I extoll the virtues of speed limits, coming to a complete stop at stop signs, obeying all the traffic laws, how the brain/eyes do scanning & fill in the blanks...You are a good driver aren't you?

Then there's the bit about how as fast as cars are, they really aren't any faster from point A to point B for most trips. Partly because of traffic controls, but mostly because of other cars.

Then there's the bit about the danger to everybody else, (the reason for licensing standards, afterall.) & the $500/month obligation just to keep a car around just doesn't make a lot of sense financially, to me. I do have a car, but it's a dozen years old & paid for. "How's that extra shift at work working out for ya?"

I never need to find a parking spot & I can ride all the way to the time clock at work. Trip time is the same no matter which method I choose.

My health really isn't part of the discussion. I really don't think that is a factor in my bike riding. So, I don't bring it up. But the Mr. Metabolic Syndrome at work still gives me a look of disgust, so I must be doing something right.

Usually when I'm done, they are sorry they've asked.