It refers to I-5, WA, OR, CA. Remember that CA, OR have a 55 mph speed limit on Semi's. This will limit your miles per day, assuming you run legal. Also, DOT is rather strict here since it's so easy to catch you doing too many miles. WA state is a little better at 60 mph limit, but from Olympia to the Canadian border is prone to traffic jams.
Finally, for the most part it's a level run. But where it gets into the higher elevations, snow is common. The states and the drivers have very little idea how to handle the white stuff. You don't really have trouble handling your rig, just watching out for those who don't know what they are doing.
Each area of the nation has it's own challenges. These are what I've noticed in the I-5 corridor.