It does, in fact, make it safer. There is less traffic at night. Those over 17 are still out causing most of the trouble. They also said that curfews help the police to identify gang members and come in contact with them at an earlier stage, help to curb young peoples' activities before they become more violent, and help the police to seize the guns and drugs of gang members, thus impairing their ability to fight. Finally, the curfew helps to educate parents to the signs of gang membership and activity.
Gang activity stops after curfew hours begin.
It curbs activities before they get to a more violent level. In other words, if you catch youths early it is more likely they can become valuable members of society. I have never seen a gang member who wasn't a truant first. Curbing truancy curbs gang violence. Seizing these things lowers gangs' ability to fight.