Smoking and Restaurants.

So I just got back from a Chinese restaurant for pickup. While waiting for my food to be made I stood outside and had a cigarette. Now I know to be at least 20 feet away from doors and open windows while smoking in California; but, I noticed something funny. The sign on the door said you had to be 20 feet away to smoke; but, the ashtray was right next to the door. Look around next time you go out, you will find businesses tend to keep an ashtray next to the outside door; but, you are not allowed to smoke within 20 feet. Is that entrapment?

How did they come up with 20 feet anyways? Any real industrial hygienist will tell you that cigarette smoke is too dispersed to have any effect past one foot. What science was used to come up with 20 feet? The answer is none.