Always nice to see a debut puzzle on a Monday. Welcome to the club, Ms. Crinnion! A well constructed Monday puzzle is a challenge, so I hear. The theme has to be reasonably straightforward, and there can't be too much obscure trivia. With all that taken into account, I have to give our newb high marks.
The theme is well represented by the revealer at 64A: What you might do to pass on an Interstate ... or a phonetic hint to the starts of 18-, 28- and 50-Across (CHANGELANES). The concept here is you take a synonym for "lane" and use the homonym as the start of innocent-seeming phrases or names. Thus, "road" becomes RODESHOTGUN, "alley" becomes ALYRAISMAN, and "way" becomes WHEYPOWDER. I suspect it would be hard to find other examples of this trick.
Along with the first theme answer, which is driving related, I think MERGE counts as bonus theme material. I briefly had sUrGE at 68A: Sudden thrust forward (LUNGE), which probably came to mind because of the rest of the automotive answers.
I'd also like to take a moment to recognize Ms. Raiman for her excellent activism with regards to abuse and mistreatment of our young female athletes in this country. That such awful things happen to this extremely vulnerable group or people is yet one more stain on our standing as a supposed leader on human rights around the globe.
But let's get back to something a little lighter. Did you know that INDONESIA consists of over 17,000 islands making up 735,000 square miles? Now, Australia is 2.9 million square miles, and since it is officially part of the continent of Oceania, isn't it the largest "island nation?" I'll take debate on this question in the comments below.
I'll just take a moment to recognize OPULENT and BLOODYMARY as fine answers, and then acknowledge a small bit of hurt on reading 51D: Doctor, ideally (HEALER). There is so much more to being a physician than just "healing" people, folks!