There is no "legal limit" for xanax. It is a question of your ability to safely drive. While alcohol over .08% is per se illegal, controlled substances like Alprazolam (xanax) have no set limit.
There are two types of impaired driving.
DUI- which is where the persons abilities are substantially impaired
DUI is a misdemeanor
DWAI- driving with abilities impaired only means driving with slight impairment. and this is a traffic infraction
I am not a medical expert, so i cant say how that amount of alprazolam may effect you. If it is prescribed by your doctor, i am sure if he thought it would impair you, he would tell you. In my opinion this dosage would not be sufficient to impair your abilities, however it may depend on how long you have been taking the medication, the side effects are generally stronger initially until you develop a tolerance for it. Xanax is benzodiazepine, Therese are drugs which have effects if, anti-anxiety, sleep aids, panic disorder ect. If you were very tired and were driving, the xanax could contribute to this because one of the effects of xanax is to cause drowsiness, it is not as strong in causing drowsiness as other benzodiazepine, such as temazepam, or lorazepam, but it does have some effect of drowsiness. If you were in an accident because you fell asleep at the wheel is the only way i could see you being at risk, but to be sure ask your doctor or pharmacist, as I am not a medical expert.