I've found that hand signalling matters, particularly in places where you wouldn't ordinarily think of signalling, but it can still be helpful.

For example, the most dangerous part of my commute a number of years ago was crossing the onramps of a freeway I had to pass under. People who were taking the onramps to get from the street to the highway frequently seemed either to not know or not care that I was proceeding straight along the road... across the ramp. What I ended up trying was hand signaling as if crossing the ramp was a lane change, from the right side of the ramp to the left side. People stopped nearly running me over when I did that.

I think what hand signalling does is engage a different part of their brain. When just checking what was around, their eyes might skip over me and then their brains fill in the gap without knowing I'm there. But when I signal that creates movement and communication, which engages a second part of their brain, allowing them to realize I'm there.