> a sign indicating that too large vehicles must turn, which most drivers won't read/see in time

When the sign activates, it also causes the traffic light to turn red. In theory, that provides plenty of time to notice and respond to the sign; in practice, collisions still happen because people just run the red light.

If someone's sufficiently determined to do something stupid and reckless, there's only so much you can do to discourage them.

> instead of having some sort of 'bumpers' ahead of the bridge for those vehicles to bump into first

That's what the big yellow beam is; it was installed to protect the bridge. If something was installed farther away (i.e. before the intersection) it would get in the way of large vehicles that were intending to turn before the bridge.