Does this poll only apply to Roman citizens? If so, literacy was widespread: even members of the Roman legions, who were almost always drawn from the poorer ranks of society, had some degree of literacy. It is not just the Vindolanda tablets that reveal this, but also various discharge papers that have been found (I cannot remember where, but my memory banks will probably let me know later). The fact that the Romans so openly attached 'propaganda' inscriptions to their monuments would also suggest that the urban poor could read. For instance, why else would Augustus have to such an effort to place the text of the Res Gestae all over the Empire? Was to create awareness amongst the nobility? They already were aware of the fact that the Res Gestae was a prime example of Imperial propaganda. Therefore, the text would have been for the benefit of the ordinary citizens.
If the poll were referring to all the peoples of the Empire, I would have to say that the vast majority of Rome's subjects (at least in Gaul and Britain) probably could not read. In Britain, the archaeology suggests that during Rome's occupation, life on rural farms (after the initial shock of invasion and rebellion anyway) seemingly continued as normal. These inhabitants probably did not have the means to speak Latin, let alone read it. The only provincials in Britain (if we are to believe Tacitus) who had the means to have some degree of Latin literacy were the local nobility who opted for social advancement in their local civitas: in the Western Empire, the ability to write in Latin was a catalyst for social advancement.
For more information on the rural poor of Roman Britain, consult Salway.