Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Using my own eyes as a test (driving around New Forest, tree laden roads with lots of shadow even in the day) yes high vis is much easier for me to see than a rider wearing all black.
Yes I can see the rider dressed in black but not until I am nearer and it helps if I can see the rider well in advance so if it helps me it will help less observant drivers too. I also notice different behaviour from drivers - but that is too much anecdotal.

And to keep on helmet topic, I don't wear a helmet.

What about in London where there are street lights, lights from cars and traffic lights, do you have any evidence that hivis helps is that situation?