nothing. at all.
they just sit there and drool, thinking about what they like more: curly fries or regular. "mmm. curly ones are so twisty and cool. like a little piggy tail! nom nom nom. but the regular ones are classic! i love the way i can just stick 'em right in my mouth, many at a time! nom nom nom"
honks from other cars or the realization that traffic is moving forward is usually what makes people finally take action. "huh, what? ok jeez people! so rude! people in this city suck. they can't drive. i hate slim fast i just want butter. i wish i could eat a big butter burger while i have to wait on these annoying lights to turn green."
as far as i can tell that's what's going down at green lights these days. it used to be that people moved the car forward. now they just daydream about curly fries and burgers made of butter until a mean person honks reminding them to, you know, go.