Trautmann claimed the law - which takes effect May 1 - was unconstitutional. He argued the decals would serve as magnets for police to pull over young people and would allow others, including sex offenders, to easily identify and victimize teens.
But in his ruling issued Friday, Brennan said that "operating a motor vehicle is not a right, it's a privilege subject to state regulations."
Former Gov. Jon Corzine signed the measure, known as "Kyleigh's Law," in April 2009. It's named for Kyleigh D'Alessio of Long Valley, a 16-year-old honor student killed in a December 2006 crash.