I am going to put something else in here, that I have only seen vuagely touched on by Magnatom , where he calls Mikey an experienced rider.
Speed.
I am a very fit, experienced cyclist who rides a painstakingly maintained and tuned roadbike. I ride very fast, and in a busy town / city situation this speed keeps overtaking cars at bay, lets me take a very dominant road position, and I am comfortable wearing my (what I think) is stylish black and red kit, relying on my lights alone.
I get noticed and feel very safe - safer in fact cycling through central London than I do driving through it.
Out here, where the speed limit is 60mph in most places, the roads are wide and treated like racetracks, the kind of position I hold in town is suicide. There is no other word to describe how terminally stupid cycling in the middle of the road is.
I think it is all about closing speed.
When riding alone on the busy roads out of town I wear a reflective yellow Sam Browne, it is unobtrusive and personal testing has led me to conclude that out of town it has a beneficial effect, especially since it contrasts so strongly with my black jersey.
In town there is no discernable difference.
When cycling in town with my gf, who is less experienced, and is weighed down with a ton of steel mtb, a HiVi jacket or Sam Browne makes a huge difference.
A cyclist riding fast in heavy traffic is easy for cagers to spot because they are in vision for a relatively long time as closing speed is low. The higher the closing speed, the more that HiVi makes a difference as it grabs attention from the background by being unusual.
I think this is where urban camo arguments fall down, as the speed of the vehicle is not taken into account. At low closing speed, a moving blob of HiVi is not any use, because compared to the approaching driver, it is moving little, and once registered becomes redundant as the whole cyclist is in view for the whole approach.
At high closing speed, the HiVi blob that registers makes perople think "What's that?" and serves as a reference that they are going to be approaching something that is either stationary / as good as staionary compared to the approaching vehicle.
So I think HiVi has a place for slow city riders, and any rider out on fast roads.