[Slonopotam845]

From>> NewScientist.com

The US army is worried about suicide bombers targeting its facilities with truck and car bombs. The usual way of combating this is by placing large, heavy obstacles at entrances to prevent such vehicles gaining access, but gives the impression of an area under siege.

Now the army has funded inventor Charles Marsh to come up with an unobtrusive way of stopping unauthorised vehicles. His idea is to dig a truck-sized trench across the access road, and cover it with an aluminium plate strong enough to support the vehicle. In ordinary circumstances, the trench is entirely hidden, but the plate is hinged on one end.

Should a vehicle attempt to cross without authorisation, the plate drops and the vehicle drives into the trench to await appropriate action from security personnel. The invention can also include a hydraulic lift to later raise the vehicle out of the trench.Read the full hidden barrier patent application.
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Clever, and like most clever inventions, blazingly obvious once explained.

[NeroASERCH]

It's the bloody middle ages.

[Lt_Apple]

Two things that define an intelligent forward thinking society:

1. Never pull up train or tram lines.
2. Never forget moats.

[Paul Bunyan]

Activation is the default or so I'd imagine. Plus retrieving the live occupant(s) for questioning is better than hauling their dead bodies of to the morgue.