Headlights set off motion detector

I saw a another thread where people noticed a sudden increase in sensitivity. This is NOT related. I've had this issue for the past year, ever since I installed the doorbell. I live in a cul-de-sac, and in the cool evenings, a car turning around in the cul-de-sac will set off my motion detector on the door bell. I have it set for 'people only', but that doesn't make a difference. The Motion Sensitivity is set to about a third of the way from the left, so not terribly sensitive. Yet after dark, every time a car comes through, I get chimes. The odd thing is that it isn't the car itself that sets it off...it is the light from the headlights sweeping across the doorbell/side of the garage. any ideas?