As 25hz wrote " Recumbents produce a very unique profile that doesn't fit any of the mental conscious or unconscious profiles....that people have."
Bingo. I can't speak for recumbent bikes, but I feel this is spot-on regarding recumbent trikes. I commute in a city among millions (literally) of cyclists, and with chronic traffic congestion. Having riden here for 4 years on both uprights and a tadpole, I can say without hesitation that I get noticed far more by motorists when riding the trike. Cars, buses & trucks that wouldn't hesitate to pull out in front of an upright will nearly always give me right of way. I don't think this is so much a conscious decision, as it's typically made in a split second, but rather a reaction to something foreign, something different that raises a red flag in their subconscious.
Other significant safety issues to bear in mind regarding recumbent trikes are:
1) trikes are inherently more stable. You can't fall over and you have to be doing something really wild to roll one. Even then, you are much closer to the ground than on an upright. Impact with the ground, all other things being equal, will be lower than on an upright.
2) Most fatal cycling injuries invole injury to the head/cervical spine, which are typically the first point of impact when one falls from, or is knocked off of an upright, or otherwise collides with another vehicle or object. Recumbent riders are in a feet-first posture; accordingly, the feet or knees are likely to be the first body part to make inpact in a collision. The more fragile head and cervical spine areas are the most distant body areas from the most likely point of impact in a collision.
In four years living here in Beijing, I've had 2 fairly significant bike accidents, one in which I was hit from behind by a car driving (illegally) in a bicycle lane, and the other when a bike I was overtaking suddenly and without signalling decided to swerve to his left (into me) pushing me into a curb. Both of these accidents happened while riding uprights.