Stacey D. Stewart, CEO of MADD, tells TMZ that JT is seriously lucky his night out in Sag Harbor, NY didn't end tragically ... with himself or someone else getting hurt. But, that being said, she's imploring the singer to use his celebrity to condemn impaired driving.

At the moment, it doesn't look like that will happen, not for awhile -- not until the case is settled, and JT's high-powered lawyer is, at least, saying they're ready to fight tooth and nail.

JT was cuffed and booked after allegedly rolling through a stop sign following a get-together with pals ... cops claimed had bloodshot/glassy eyes, a strong odor of alcohol on his breath and a failed field sobriety test.

While Justin's been silent about what happened leading up to his arrest, Stewart says she'll be "extremely disappointed" if he eventually issues a weak mea culpa and gets a slap on the wrist, criminally.

The MADD honcho calls JT's massive fame and relevance a silver lining ... in that it gives him a tremendous chance to save lives by educating the public on the dangers of impaired driving.

As you may know, this is Justin's first offense, which means he'll most likely strike a plea deal. Stewart thinks that's unacceptable, but also not unexpected -- she's fully aware first-time offenders -- especially those not in an accident -- get minimal criminal consequences or get off the hook entirely.