As stated by labor law poster, all employers who have more than 2 employees need to have safety posters posted in their workplace to warn their employees to maintain their health and safety.
These posters are most seen on workplaces that impose great potential risks or danger if there are no warnings. For example, at a construction site, we may see the poster "turn off the compressor when not required". Safety posters are also used widely in smaller workplaces. You can see the poster "no smoking" at a petrol station or "Flammable gases; Keep fire away" at the kitchen of a restaurant.
Safety posters are required by State Law. The state and federal labor law posters require certain safety posters to be put up in certain workplace. The added requirement is that these posters need to be placed in the most visible area so that they are within the sights of all employees while they are at work. The posters can be about potential dangers and the solutions when certain accidents happen. Sometimes they are just the warnings and about the "dos" and "don'ts".
All employers should comply with the labor law posters because if they fail to do so and this is detected by an inspector, they will have to face with heavy fines. They still be fined heavily although they argued that their employees were informed clearly before about the potential dangers while working but these employees still accepted to work for them.
Many states mandate that the safety posters should be in both English and Spanish. Not only useful for employees, those posters protect the rights of employers as well. For example, an employer has displayed a safety poster stating clearly that every worker at his construction site needs to wear a hard hat to protect their hat and the poster is put within the sights of all workers. So if one of them fails to comply with the rule, the employer will not have to bear any responsibilities for that worker's injuries. On the contrary, without the presence of this poster, the employer will have to take responsibilities for anything happening to his workers although that is his worker's fault of not wearing hard hat.
Not only necessary at workplace, these safety oriented posters are put in commercial settings as well. They are used to warn clients as well as customers of risks and dangers. For example, if you take your kids to a theme park, you will see lots of them being put up everywhere. These posters warn park goers about which games are suitable for which types of people and they are display at the most visible area. You can also see lots of safety posters if you are keen on using public transportation.