It would seem you did not read my whole post thoroughly nor the RCWs cited. RCW RCW 46.61.400 establishes the normative max speed limit for WA at 60mph, RCW 46.61.410 gives authority to the Secretary of Transportation to raise the speed limit after an engineering and safety study whose findings would support the change to a higher speed limit...hence the 65 and 70mph limits on parts of I-5 and I-90.Oh? Then why is it 65 and 70 on parts of I-5 and I-90 that are still in Washington?
Your argument is interesting though as a comparison to California law. In CA there is a basic speed law and Max speed law. The max speed law establishes 55mph for undivided highways and 65 for divided highways. CA law states that one cannot exceed these max limits unless a higher speed limit is posted (as they have some roads that are above 65mph). Under the basic speed law if one exceeds the posted limit but does not exceed the max speed limits for divided/undivided highways then they can succesfuly contest the ticket if they were otherwise driving safely for the time of day, the road surface, the weather, etc AND there was no study conducted in the last 7 years to support the posted speed limit. In contrast Washington law establishes maximum speed limits for highways, towns, etc, making no distinction between divided or undivided roadways. In CA if one exceeds the statutory max speed even in a zone with an arbitrarily posted speed limit (one with no supporting study) they are in violation of the max speed law (as opposed to violating the basic speed law) and cannot argue they were otherwise driving safely.
Again, there is nothing in the RCWs that requires WA state to conduct a study to set the max speed of a highway to 60mph other than the study used when the roadway was originally designed. The fact that a study may not have been conducted by the state to determine if the speed limit could be safely set higher for the section of road the OP's relative was stopped and cited is not material.
If you disagree then state your argument(s),