This is a very important gadget I use everytime I need to take the subway trains. Before you go around the city, it is best to plan first your whereabouts. A detailed plan of where you are going and what's the best transportation you want to use to suit your comfort. If you hate traffic and parking issues, the best option is of course the subway trains. In New York, the subways are properly labeled and almost every corner you will see big maps. There are even maps inside the train and announcements for possible transfers to trains and buses.
I would like to suggest to use this gadget especially during the weekends. There are repairs conducted which may change the routes of the trains. So, for smooth flow of your travel, this gadget offers a big help.