Answer:
I am a protector. I sit on a bridge. One person can see right through me, while others wonder what I hide. What am I?
I have hands but cannot clap
When it comes to me you go on red and stop on green.
I help engines spin and pants stay up.
You write on me and secrets I can keep in places never seen. I spin like a top. Though stiff as a board, I'm often described like a mop. What am I?
What has wheels and flies but is not an aircraft?