My first thought is that the distractions on your smartphone are more vapid and and they keep you from noticing the real world around you. The equivalent I can recall was reading books on the NYC subway on the way to High School or home. The book was chosen by me and did not beep or vibrate while in my pocket, and I was still aware of my surroundings. I'll take old school distractions any day. Phones vibrating and beeping are Pavlovian, and we are the dogs in the system.

Edit: I find reading HN like picking up the newspaper. I appreciate the simple layout, but I can make a comment and be active if I choose.