If you want to know when the next train is with the app:
1. Where's my phone? It's not in my pocket, so maybe I left it on the table over there. Ugh, but I'm sitting down comfortably with a blanket and a book.
2. Fine, I'll stand up and walk over to get my phone.
3. Hrm, my phone isn't on the table. Maybe the kitchen?
4. Walk to the kitchen.
5. Good, here's my phone.
6. Unlock phone.
7. Unlock phone, this time with the right unlock key.
8. Scroll to find app.
9. Tap to launch it.
10. Read data.
11. Wait, is that up-to-date?
12. Tap to reload data from server.
13. Wait for data to reload from server.
14. Read data.
With this sign:
1. Turn head to the left.
2. Read data.
It's all about making it incredibly simple to do the task. Whenever someone posts a "Show HN" here, there's a dozen people complaining they can't try out the site without signing up, so they bounce. Yes, giving a way to try out a site with a guest account is frivolous -- just sign up, dammit! -- but it works better. And so does this sign: the data is just there for you to read.