DOWNTOWN BROOKLYN - The group Transportation Alternatives called on the NYPD Thursday to stop cops from parking on Brooklyn and city streets illegally.According to the group's study, the NYPD is the biggest offender among city and state officials who abuse parking privileges. Residents say police placards are frequently expired and cops park next to "no parking" and "no standing" signs.The NYPD counters that the department does not condone illegal parking. Officials say officers should have valid placards and illegally parked patrol cars are not exempt from being ticketed.