This is indeed Akranesvegur. Before the Hvalfjörður tunnel was built, there was a ferry between Akranes and Reykjavík. The  Icelandic Road and Coastal Administration owns and maintains the road to link the old ferry port to highway 1. The number 509 is the number used by the Administration    but the streets also have names e.g. Faxabraut is part of road 509.