If you're biking at the appropriate speed for being near pedestrians, you don't need to alert of your presence because you are no danger.
You only need to alert your presence if you're going faster than what is safe to do, which is being a dick and pedestrians are right to be mad at you.
I watched it for the first time recently. He just kept making the dumbest decisions every time. He's supposed to be a super genius, yet I would've chosen most of the decisions better.
The only explanation is that he's bored and wants to make things difficult, but that is not what's portrayed in the anime at all. It's just what I came up with to explain his horrible decisions.