Business Law: Contracts/Agreements

In some ways contracts can be very easy to enter into. In fact, we often enter into agreements simply by our actions. 

For example, many businesses place small notices near their entryways warning that by entering the building you consent to video recording or by parking in a specific lot you agree that the lot owner is not liable should anything happen to your car (even if you paid to park there). 

Are these agreements valid? 

Should individuals be held to agreements even if they did not know about them? 

Use specific details about contract formation and performance in your response.