What is the safest type of pedal for all-weather city cycling?
There is a lot of discussion about this topic (i.e. a lot of anecdotes) because pedals are a very personal matter (cf. LondonCyclist or Zach Gallardo). However, I think that you're over-thinking this in some way: The "safest" pedal is the pedal you're most used to and comfortable with. 

Consider, for example, a hypothetical case where clipless pedals have been "empirically proven" to be The Safest Pedal: You then buy some fancy clipless pedals and have no idea how to clip in and out and then you fall at a stop light and some car behind you runs over your leg.

Of course, this answer wouldn't be complete without me giving my own two cents (i.e. anecdote):


Since biking in the city (esp. "all-weather" cycling) involves dealing with a lot of unexpected obstacles/dangers/stops/etc., you should be able to quickly and confidently dismount your bike and re-mount with little hassle. Perhaps if you've got really good reflexes and you're very used to your own clipless setup then doing this with clipless pedals is no issue at all. However, there still may be some unexpected event which happens just a bit too quickly for you to properly unclip, and that split second which makes the difference between properly unclipping and falling may lead to problems like those described in the preceding paragraph.
Likewise, you should have good contact with the pedals. If you wear leather-soled shoes, then please make sure you have rubberized pedals: Slipping on your pedals can be very painful and this will happen if you're wearing leather soles and your pedals get wet.