Police questioning during a routine traffic stop is not usually considered to be an "illegal interrogation." An illegal interrogation is when the police conduct a custodial interrogation without having first informed the suspect of his or her Miranda rights (the right to remain silent, the right to have an attorney present during the questioning, etc.).
Even though you are "detained" by the police during a routine traffic stop, and not free to go, the detention is brief and the encounter occurs in public. The Supreme Court has ruled that this kind of detention does not amount to a custodial interrogation to which Miranda rights attach.