Why are public service announcements(psa) effctive?

Definition: these are slogans that are 1 or 2 minutes long (if a TV or radio ad) or large single page posters that try to influence people about social issues. They are free (without cost). I think they are almost always government initiated.

Examples:
No Kid Hungry narrated by Jeff Bridges.
Stop Cyber Bullying
Save Lives: Stop Red Light running.
Don't Drink and Drive

They are effective because:
1. They are free. They can be entered into the media many times. Not only that but they reach across the country because many radio / TV stations are networked.
2. They can be like chain letters. They can cause people to talk about the issues.
3. Organizations are built around these slogans. Do you have MADD in the United States? That stands for Mothers Against Drunk Drivers. Their slogan is Don't Drink and Drive. It's very well known up here. Mothers who are against Drunk Driving are very adamant and very outspoken. They are also single minded.
4. They are effective because they are meant to make you feel guilty if you don't agree with the slogan. Imagine standing up to a mother who has lost a child and saying "It's not so bad. There are worse things. No damage no harm done."

Bolder words good luck