I was thinking about the Japanese 初心者マーク (shoshinsha) which new drivers are required to display on their car while driving:

And here is a pic of one on a car:

And I starting wondering if there is a French "rookie driver" insignia but I don't think one exists. Basically I was just thinking of French things for this theme since we all know how much they love anything from France.
But, we do have a Japanese car, so we could embrace the Japanese mark. And what is better yet, is this mark, the もみじマーク (momiji):

Drivers 65 years or older are required to display this logo while driving in Japan! So what do you think? We'll use the grandma/grandpa slow driver Japanese mark on the car!! hahahahaha