New York's Metropolitan Transportation Authority (MTA) is expanding a security awareness campaign by releasing a four-poster series of advertisements that encourage transit riders to notify police if they see suspicious packages, unattended bags or other dubious behavior.

According to the New York Post, the Madrid, Spain, rail bombings in March inspired the MTA to develop the poster campaign, after investigations revealed that many passengers had seen suspicious baggage on the trains before the explosions.

MTA officials have said that phone calls from passengers reporting suspicious packages have become the primary cause of subway delays in the past three years.

The posters will be placed inside subway cars and on the sides of buses, and brochures will be passed out on trains.