The problem is that they are legally doing nothing wrong. Everything is done according to the rules, so they can't be held accountable for not following them. After all, they are taking all reasonable precautions, what more could be expected of them?

The fact that the situation on the ground isn't safe in practice is irrelevant to the law. Legally the hedge is doing everything, so the blame falls on the driver. At best a "tragic accident" will result in a "recommendation" to whatever board is responsible for the rules to review them.

@Bombcar is correct. Once they've been legally notified of the potential issue, they have increased exposure to civil liability. Their lawyers and insurance company will strongly encourage them to just fix it (assuming it's not a huge cost to trim back the stupid hedge). A registered letter can create enough impetus to overcome organizational inertia. I've seen it happen.

All that applies for criminal cases, but if a civil lawsuit is started and evidence is presented to the jury that the parties being sued had been warned repeatedly that it would eventually occur, it can be quite spicy.

Which is why if you want to be a bastard, you send it to the owners, the city, and both their insurance agencies.