The Sidney Harbor Tunnel had a similar problem. They came up with an elaborate but successful solution.[1] There are detectors and huge flashing warning signs that light up. Then, there's the last-chance sign. At the entrance to the tunnel, pumps force a curtain of water straight down across the entire roadway. Then a powerful laser system projects a giant octagonal red STOP sign onto the water curtain, filling the entire height of the tunnel opening. That's in addition to the six red traffic lights, flashing red Xs above each lane, and a big STOP with a flashing frame above the tunnel mouth.
That was expensive, but after someone wedged an oversized truck into the tunnel, blocking it for three weeks, it was worth it. Gets tripped about 18 times a year.