Herbert Nelson Jr. says he was 18 the first time the police pulled him over. The ticket, for speeding, felt small in the moment. But more traffic tickets piled up, as did warrants for failing to appear in court or pay his fines. So Mr. Nelson, now 26, has found himself repeatedly jailed in this St. Louis suburb and in the maze of municipalities nearby — part of a pattern that black residents here complain has grown all too common....