A DUI is a misdemeanor, not a traffic violation. (A traffic violation is like speeding or driving without your headlights on, etc. A violation for which you would NOT face jailtime.) You must disclose the DUI if the question is worded as you indicate.
If you do not disclose the DUI, you could be discharged, which you obviously want to avoid.
Also, if the application includes a statement that the application is truthful to the best of your knowledge and you sign that statement without disclosing the DUI, you could be charged with perjury.
My guess is the employer would be more offended if the employer discovered you lied to them than they will be about a 15-year old DUI.