I never knew the story behind it but apparently they have the same thing there with miles to Sacramento!
https://www.capradio.org/articles/2016/05/06/two-coasts-two-...
From https://en.wikipedia.org/wiki/Cove_Fort
> In 2004, the Federal Highway Administration was testing a new typeface, Clearview, designed to have improved readability at night with headlight illumination. One test sign was placed at Baltimore, Maryland – the eastern terminus of Interstate 70 – that listed Cove Fort as a control city with a distance of 2,200 mi (3,500 km). One employee stated with the number of queries the department received about Cove Fort, the test was a success. The sign became so popular that after the test was over, federal authorities made arrangements with Maryland authorities to keep the sign permanently installed. The sign prompted a series of stories about Cove Fort to be published in the Baltimore area. Since that time, a small effort has been made by people in both states to lobby the Utah Department of Transportation to reciprocate by placing a sign at Cove Fort listing the distance to Baltimore.
Lends a bit of flavor, though San Diego, CA may have more of them than most cities.
Last Exit: 3,073m
bounds testing is important!
EDIT: I think I just reiterated what you just said. though you were more succinct.
You might be able to do something with a chain or restaurant that is on both ends, a little coin they can take from one to the other if they're driving all the way.
Like how the Swedes have a prank whereby on a bachelor party they glue a fake beard to the groom... but they make the beard out of all the groomsmens shaved pubes...