Just a rant. The first part is just for all these who say that riding fixed is bad for your knees.
No. It is not.
I have started riding fixed almost two years ago after skiing accident which partially torn an inner ligament of my right knee. At that time surgeons discovered serious instability in both of my knees (hardly a wonder after nearly two decades of martial arts practice and years of serious volleyball). Just before going skiing for the first time in two years I have the knees rechecked and the verdict was good to go - the only rehab I did was riding fixed, about 20 miles a day on the average.
Here is a part related to helmets: I almost never use helmet when riding, I always use helmet when skiing. On Tuesday, while skiing off pista in bad fog, I took a 4.5m drop (14' to you, using measurement systems based on human body) to horizontal ice/rock surface. My knees sustained the impact when my skis stopped dead on landing (I have a 2 by 1 inch gouge on the underside of my right ski to prove it) and when the bindings let go I just rolled forward.
I am OK with the right wrist, elbow and shoulder as well as my left hip hurting a bit - I do not usually practice ukemis falling from such heights. What I complain about is the helmet making a long, straight bruise on my forehead and crushing my googles which in turn cut the base of my nose - two stitches and some glue helped that.
I wonder if my head would miss the ground altogether - I know it would had I fallen from my standing height - were it not for an additional inch of a helmet in front and a whiplash effect?
Nevertheless, I will continue to use the helmet when skiing - the rock that gouged my ski would put a nice hole in my skull on a contact.