Advertise on my car. Is this illegal?

[Kayang Thao]

I live in California.
I posted a sign on my car
"My business website"
Buy more, spend less.
A new online discount store.


it's a sticker. I didn't read the regulations before doing, so.
I thought it wasn't restricted for a sticker on your car!!!

SO! I have driven my car around town for a month now. Yes, some police does happen to drive behind me and I drive behind them. Side by side in the free way.
The police didn't happen to pull me over.

Today, my parents drive my car to a city about 5 hour drive due to a funeral.
They came home MAD and upset with me putting my business on my car.
I asked why...
they explained; " When we pulled over from the free way to a truck stop park and use the bathrooms & stretched our legs... a police car drove and parked next to your car. The police men did not say anything, but checked out your stickers. Wrote something on a note book, and left."

Will I get a ticket? I'm worried. We took off the stickers.

I just did some research and a blog says it is restricted by some states.

I live in California, United States.