In many states it is not explicitly illegal to put flyers on car windshields in public places. However, it is almost always illegal to place advertising materials on vehicles parked on private property. Because of this, it is important to examine local laws and city ordinances to ensure marketing techniques are legitimate.
It is almost never legal to place any type of advertisement on vehicles parked in grocery store car parks, on driveways or on other types of private land. Additionally, littering laws can make it very tough to place flyers on vehicles parked in public place. While the flyer-placing act isn't illegal in itself, the flyer is legally seen as litter, making it illegal from another viewpoint.