Soft stop.

To prevent big lorries from crashing into the Sydney Harbour Tunnel, the Australians came up with a warning sign you cannot possibly oversee. If all flashing lights fail to alert the driver, a giant STOP sign will appear out of nowhere, blocking the tunnel entrance. But no worries, it is only a projection on a wall of water. We wonder however, if this spectacle might not cause trucks to get extra-close?!
autoblog.com