When BLM tries to close a trail to vehicles without fencing it off completely they usually dig a trench across it.
The berm goes on one side so the vehicle would be more likely to high-center as the wheels drop into the trench.
Seen many of these, also notice that wet weather and other factors may collapse them or fill them in.
It should also be noted that most BLM land closures are NOT posted per NRS requirements.
I'm not sure of the exact distance apart but I found out many years ago that a legal "Posting" requires signs on ALL sides of the property, with signs close enough together to not be missed.
This came up when I posted MY property to discourage illegal wood cutters, who simply cut down the trees the signs were nailed to first.
I've also found myself seeing "Posted" signs in my mirror when exiting the outback.
Lazy/cheap or
fraudulent posting is common, usually only along the most likely access route.
Come into the area from any other direction and you are unlikely to ever see any "Posted" signs.
ECO-NUTS and anti-Hunter types are fond of putting up Posted signs of varied types on public land that is NOT posted.
Seems they think it is THEIR land, not "OUR" land.