I didn’t realize this until Hari (Hari Shetty of Jiffle) mentioned this to me first. We were passing next to a truck from 1-800-GOT-JUNK on El Camino Real. When I mentioned to Hari that this truck has been parked there since yesterday, Hari told me that I was wrong. He said it has been parked there for months. Later he explained to me that this probably their advertising model.
[Note the photo above is not the one of the truck that I mentioned. This is from Flickr]
Thinking about it further, it makes a lot of sense. There is no legal restriction on using parked trucks for advertising and it is definitely cheaper than buying advertising on billboards. Bonus – you also get the advantage of depreciating that asset.