"Fehlt das ß auf der Tastatur eines Computers oder einer Schreibmaschine, schreibt man dafür ss."
Translation: "If the ß is missing on a computer keyboard or typewriter, you have to write 'ss' instead".
[0] http://www.duden.de/sprachwissen/rechtschreibregeln/doppel-s...
They just sound the same to my hears, so I got around it by memorizing the respective use cases.
OTOH it could also be just spending priorities of updating street signs vs. other things.
I knew nothing about the phasing-out of ß until reading the comments on this article.
In Switzerland, "ss" is standard (Duden says: "In der Schweiz kann das ß generell durch ss ersetzt werden <§ 25 E2>.") I haven't seen "Strasse" on signs in Germany, but there might be regional variation (since street signs are a local-government responsibility)