Answer:
Although studies are being conducted, the cellphone industry is lobbying heavily against this type of research and statistic collecting. In addition, VERY FEW people are going to voluntarily admit to the police or their insurance company that their inattention to their driving caused an accident.
Suffice it to say that your own personal observation should lead you to the inescapable conclusion that a significant percentage of inattention accidents are the result of texting and cellphone usage.