If you're lost in your phone, you'll be oblivious of those around you.
歩きスマホはケガやトラブルの原因になりますのでご遠慮ください。
Please refrain from focusing on your phone while walking as it's dangerous.
歩きスマホはやめよう。Stay off your phone while walking.
http://www.tokyometro.jp/corporate/csr/society/manner/index.html
