Mayor Rudolph Giuliani is urging commuters to use public transportation to travel into and out of New York City, citing horrendous traffic tie-ups created by an overload of motorists. In the wake of the World Trade Center terrorist attacks, subway ridership has been lighter than normal, Giuliani said. Some regular riders said they fear that the subway could be targeted for a terrorist attack. Traffic was so bad in some areas that 15-minute commutes were extended to two hours. One frustrated motorist who shifted to subway travel this week told the New York Daily News, "I don't expect to start driving again until mid-October."