Also, it's impossible to talk about freeways without mentioning I-19, which is signed in kilometers because it seemed at the time like the US was ready to go me

Also, it's impossible to talk about freeways without mentioning I-19, which is signed in kilometers because it seemed at the time like the US was ready to go metric, and the Bay Area's I-380 which is not quite two miles long. It would have been longer had the original plans not been cancelled, but not much longer. Even then it's longer than I-180 which is only one mile, but I like the example if I-380 more because it's so heavily trafficked and it's quite clear at the interchange with I-280 where it almost went on further. I-19's metric signage and I-280 connect to an odd common thread, though: California, perhaps because of its size and historic isolation, has historically had its own way of doing things. This extends to freeways, where California retained its historic system of marking highways (post-miles) instead of adopting mile-posts as the federal government dictated. The difference there is slight, but there's a bigger difference: California did not, initially, number its exits. Instead they were known only by name. By the '80s they had started numbering all exits but in areas with older infrastructure, such as 101 through San Francisco, most exist are still unnumbered until something forces them to replace the signs. The decision not to number exits is perhaps partially because of the post-mile system in which mile numbers are somewhat less linear than mileposts, but probably more due to California's large set of US Highways which were upgraded to freeway condition (such as 101) and had not historically been numbered. Today this would be unusual, a US Highway updated to freeway infrastructure is more often designated as a freeway (see this happening with US-93 near Las Vegas becoming I-11), but California did it in a number of major places. The "California Difference" runs deep. Traffic signal controllers in California are built to a different standard than in the rest of the country, for example (except a few states that later adopted the California system). Mostly it's first-mover advantage.