I don't know where you get your information but I lived in that area for 15 years (until 2004) and there is no such signs on I-5 or Merlin-Galice RD. Also, Merlin-Galice RD does not turn in Bear Camp Rd, you must turn off Merlin-Galice Rd just before Galice to take the route to Gold Beach. It is after turning off Merlin-Galice where the warning signs would be. And they may be difficult to see at night or if coverd with snow.
Not sure why they took that route, it is difficult in the summer and depending on storms is normally impossible in the winter. From Grants Pass it is the shortest route to Gold Beach in miles if not time. Also if you do a direction search (Grants Pass to Gold Beach) on MapQuest, Google Earth, or MSN maps it will send you the wrong way that the Kim's attempted. Only Yahoo maps directs you through the prefered route from Grants Pass to Gold Beach via hwy 199.
But they hadn't ever gone all the way in to Grants Pass. They left the Interstate at the Merlin exit.
PS - I make that point about Grants Pass, because I feel as though they wouldn't have taken their tragic turn if they *had* gone on in to Grants Pass - into a "town" - where they might've stopped, maybe even gotten gas or primarily, just some coffee for the journey - and *someone* would've talked to them about where they were headed.