With a law barring smokers from lighting up in nearly all city-run open spaces in San Francisco scheduled to take effect next Friday, "No Smoking" signs have yet to be posted, raising questions about how well the ban will be enforced. Six months ago, the Board of Supervisors passed what is believed to be the most comprehensive outdoor smoking ban in the country. The ban covers parks, squares, gardens and playing fields under city jurisdiction. First offenders could be slapped with a $100 fine issued by a police officer or member of the city's park patrol. At the time the...