Abby, this is a great primer for someone who hasn’t read the literature or seen the movie, or wasn’t around in the late 60s.
The most plausible explanation given the setting and that Mary Jo left her keys & pocketbook behind is that:
a) she & Ted had no intention of catching the ferry or driving off the island,
b) he was looking to get away from the party briefly and to find the most remote spot on the island so that he could get some.
The accident happened in the heat of horniness, regardless of who was driving. Ted made the same sudden decision that hit-and-run drivers make time & time again, that his victim would become collateral damage and he would flee the scene and salvage his reputation by obfuscating the facts …