The rules of the road rights of way help determine fault. Often fault for an accident is pretty clear. Often it is not. There may be differing accounts of what happened. ‘I had the light.’ ‘No, it was green for me.’ Also, the right of way is not always an either/or proposition.
The disfavored driver (the one lacking the right of way) bears "the primary duty to avoid a collision." 
But, the “rule of the road right of way is relative rather than absolute" and "does not absolve" a favored driver "from the duty to exercise due care." 
The favored driver (the one with the right of way) may assume other drivers will yield unless a “point of notice” is reached.  The point of notice is a point “at which a reasonable person exercising reasonable care would realize that the disfavored driver is not going to yield.”  In other words, if the driver with the right of way should have realized the disfavored driver was not going to yield, the driver with the right of way might be partly at fault.
Fault is comparative, meaning one driver could be completely at fault, or both drivers might be partly at fault, depending on the circumstances and, if there is a dispute about what happened, who is found more credible.
 Sanchez at 597.
 Cox v. Kirch, 12 Wn.2d 678, 682, 123 P.2d 328 (1942); Sanchez v. Haddix, 95 Wn.2d 593, 627 P.2d 1312 (1981).
 Channel v. Mills, 77 Wn.App. 268, 278-79, 890 P.2d 535 (1995); Whitchurch v. McBride, 63 Wn.App. 272, 276, 818 P.2d 622 (1991).
 Whitchurch, 63 Wn.App. at 276-77.