Modern living: everyone has a smartphone and everyone is doing something on their phone and forgetting about the world around them. That is the message of this poster in the subway: don't use your smartphone while walking about!
スマホ sumaho abbreviation for smartphone
注意 ちゅうい chū-i pay attention, careful!
危険 きけん kiken danger
aruki sumaho wa kiken desu
walking about with your smartphone is dangerous
巻き込み かきこみ kakikomi stumbling into someone
転落 てんらく tenraku plunging onto something
衝突 しょうとつ shōtotsu crashing into someone
All these things can happen and not only inconvenience others, or endangering their life, but you could also stumble about and end up on the railway track - just when the train comes. It would not be a smart end to the smartphone conversation.
Please, don't walk about with your smartphone in the station: it is dangerous for others, but also for yourself.