Yes, I agree with you that cars stickers remain a very effective form of advertising, especially when they are made strategically. Precisely because cars are in constant motion, a good design manages to become a kind of "mobile fence" that impacts different places and contexts. Not only catches the attention of pedestrians, but also of other drivers and passengers that are in traffic or parking. It is a resource that, with a single investment, can give visibility for months or even years.
The important thing, in my opinion, is that the sticker meets two basic requirements: to be visually attractive and be clear with the message. Many times I have seen cars with stickers that are too loaded with text or with very small typefaces, and there they lose efficacy because they are not legible to the naked eye. Instead, a striking logo, a short slogan or even a well -outstanding contact number usually work much better.