After they had first agreed to display a 3-inch "Be a Safe Driver" sign, homeowners in one Seattle neighborhood were likely to permit the installation of a huge, unattractive "Drive Carefully" sign in their front yards. This best illustrates the:

foot in the door phenomenon

Explanation:

Foot in the door phenomenon is a persuasion techniques that works / relies on the size of the request being made. Experiments on this persuasion technique show that if you wish to make someone do a big favour for you, you only have to get them to do a small favour first. The above example illustrates the foot in the door phenomenon where homeowners agree to a large, ugly, unattractive installation of a sign board because they had started off agreeing to have a small one installed first.