Actually, in 1998 NAVTEQ (then called Navtech) proposed transitioning to a vendor-independent format called SDAL. Looking at the list of "SDAL Partners", it didn't get very far, and the last "News" item is from 1999.

A quick Google search for "SDAL format spec" provides the following PDF link, bypassing the thser registration and restrictive license agreement:

SDAL Spec 1.7 PDF

Every once in a while someone posts a "how do I read a SDAL format map" question, so there must have been at least one released.