I disagree. A light honk is beyond annoying to a cyclist and the driver is often distracted by trying to find his horn and may drift into the cyclist.
Personally, I have a mirror attached to my helmet. I see clearly all oncoming vehicles. I also use my ears as much as my eyes.
This cyclist, as he was training for a triathlon had probably protected himself as much as possible with colors, helmet etc.
As a member of the college Campus Crusade for Christ ministry, his family can take comfort in his eternity with the Lord.
No, a light honk on the horn distracts them and makes it more likely that they will swerve into your path. Trust me.
It's even worse if they don't have a rear-view mirror and they think they have to turn suddenly and see why the hell someone's honking at them.
Just slow slightly, move to the left until you can clear them by at least three feet (at city speeds; more if you're going faster), then drive past them in a nice straight line.