This guy took a few days and a few dollars to put up a sign.
Caltrans took 8 years, undoubtedly thousands of dollars, stack and stack of paperwork, and a bunch of workers to do the same.
As to the time investment:
> He spent more than three months preparing, researching the color and shape of the signs, mixing his own custom paint to get the colors just right, and applying a bit of faux patina to finish it off. Ankrom even spent time tracking down the button-shaped reflectors used on other legitimate signs and convincing the supplier to give him some. Once he was satisfied that it looked authentic, he came up with a plan to install it.
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Caltrans has 15,000 miles of state highways that they have signage for. A single sign may only get reviewed once a decade because of that.
Doing single things in isolation is "easy" without going through the required public comments and such. Maintaining it for the entire state is much more challenging.