Think of who else does street work like this, and the signals it may give to your target audience. I would worry that such an approach would be mistaken for Scientology street recruiting (compare it to "Discover your true potential" and other such come-ons for their introductory "personality test"), or similar street work by some other religious group, and might be avoided by exactly those people you might wish to reach.
In the evangelical days of my youth, this sort of thing is exactly what we were encouraged to do: get strangers talking about a "deep" subject that provided a natural transition to just how bleak things are without god, the fulfillment of salvation, and so on and so forth.
Existential risks are the risks of human extinction. A global catastrophe will happen most likely because of the new technologies such as biotech, nanotech, and AI, along with several other risks: runaway global warming, and nuclear war. Sir Martin Rees estimates these risks to have a fifty percent probability in the 21st century.
We must raise the awareness of impending doom and make the first ever street action against the possibility of human extinction. Our efforts could help to prevent these global catastrophes from taking place. I suggest we meet in Union square, San Francisco, September 27, 2014 at 2:00 PM in order to make a short and intense photo session with the following slogans:
Stop Existential Risks!
No Human Extinction!
AI must be Friendly!
No Doomsday Weapons!
Ebola must die!
Prevent Global Catastrophe!
These slogans will be printed in advance, but more banners are welcome. I have previous experience with organizing actions for immortality and funding of life extension near Googleplex, the White house in DC, and Burning Man, and I know this street action, taking place on September 27th, is both legal and a fun way to express our points of view.
Organized by Alexey Turchin and Longevity Party.
Update: Photos from the action.