Over on Google Earth Community, sysop PenguinOpus informs us that “The road layers have been updated in a number of ways. Have a look in the US, in Canada, in Puerto Rico, and in the US Virgin Islands to see some of the enhancements.”
I don’t immediately notice a difference in appearance (but then, I’ve never looked very closely at the roads). There definitely seems to be a difference in the manner that the roads layer shows up, however, and this change probably matches the improvements for Europe back in December, which aimed to make Google Earth zippier for older systems.
My system went from zippy to… zippy, so if Google Earth got faster for anybody with a slowish computer, do tell.