If the arrest was outside the statutory it is not considered at all. Since it is for a driving infraction it is not likely to be considered when deciding on moral character. It should not have any affect.
Issuance of a "yellow letter" is a normal course of action by USCIS when applicant discloses an arrest even if you have previously submitted all court certified records. Most likely you will be fine and the single arrest followed by a dismissal won't affect you eligibility for naturalization.