While these data are the most detailed and reliable source of information on road collisions and casualties, they do not provide a complete record of all such incidents.
Er mai'r data hyn yw'r ffynhonnell wybodaeth fwyaf manwl a dibynadwy am wrthdrawiadau ffyrdd a marwolaethau ac anafiadau, nid ydynt yn cynnwys gwrthdrawiadau ffyrdd: