It's part of the APCS (Advanced Pre-Crash Safety) system, which was an option in most markets. EU market LS600h equipped with APCS ought to be able to stop automatically and move in slow-moving traffic with adaptive cruise control; my car does this.
From the Launch Pack:
I've had the system activate once; I was going uphill, approaching a turn with a metal fence, and looking the other way to see if there are vehicles coming. I hadn't touched the brakes yet since I was going uphill and the speed was dropping anyway. I think it was the combination of the approaching fence, having not touched the brakes, and not looking straight ahead that caused the car to beep at me and gently apply the brakes. I've driven there hundreds of times in my life, and wasn't really in any danger, but I guess it's nice to know the car is looking out for me.