A research found that signs banning smoking may not have as much of an impact on secondhand smoke concentrations as the presence of ashtrays or ashtray equivalents.

They found that outdoor or indoor signs that banned smoking did not correlate with levels of secondhand smoke in areas where signs were posted. However, the presence of ashtrays, or ashtray equivalents, such as candleholders, was strongly associated with a higher concentration of secondhand smoke. Based on their results, the authors conclude "While the public may be supportive of smoke-free legislation, adherence may decline rapidly if enforcement is limited or non-existent. Moreover, enforcement agencies should also focus on the comprehensive removal of ashtray equivalents that could act as a cue for smoking within a venue."
Source-Eurekalert