One of the oldest continuously used roads in the world, following the same route traders and pilgrims have used for over 3,000 years, from Amman down through Madaba, Karak, Dana, and Showbak to Petra. It's less a single destination than a spine connecting most of Jordan's Christian and Crusader-era sites — the alternative to the faster but far less interesting Desert Highway.
The full King's Highway run from Amman to Petra is about 5-6 hours of driving without stops (270 km), but realistically takes a full day or two once you factor in Karak, Wadi Mujib, and Dana along the way — versus roughly 3 hours on the Desert Highway with nothing in between.
Spring and autumn, mainly because most stops along the way (Dana's hike, Karak's ramparts) are more pleasant then — winter can also bring fog and slower mountain driving on this route.
This route is worth it specifically because of what's along it, not the road itself — if you're short on time and just need to get to Petra, the Desert Highway is faster. Budget a full day minimum if you want to actually stop at Karak, Wadi Mujib, and Dana rather than just drive through.