I don't have any freeway entrance merges on my commute. I hear they are trouble-spots.

In general, there are a few things that make it dangerous for me when I encounter them:

1) Dangerously rough pavement. I try to avoid this when possible.

2) Debris from wind storms, ice, etc. This can be strewn far out into the lane so I have to anticipate my moves well in advance.

3) Fast downhills. One of the things that makes biking safer in my opinion is the slower speeds I travel. 30 mph to 40 mph is fun, but a potentially serious crash, too. No margin for error, here.

4) Blind hills/corners. My horn and lights come in handy here. When an motorist approaches me on one of these from ahead or behind, I might beep to let them know I'm there. This also applies to busy intersections, sun setting behind me (blinding motorists to my presence,) etc.

I'm not as concerned about being hit by motorists while I'm upright as I am about crashing due to road conditions or my mistakes while I'm surrounded by traffic. That's how that lady in Boston was killed. She she was doored in a bike lane and she bounced into traffic, under the wheels of a bus. So debris, road imperfections and similar hazards (like dooring) concern me more than traffic itself.