An unobtrusive on-grade barrier. One embodiment comprises a concrete-lined trench over which a biased hinged plate is affixed to an end wall of the trench. The biased hinge holds the free end of the plate against a tab affixed to the other end wall, assuring the plate does not rise above grade. The plate is supported for vehicle passage by a sliding mechanism energized by an actuator controlled by a controller that may be automated or operated by security personnel. The barrier may be configured so that vehicles may approach from either end. Upon authorizing access to the driver, the vehicle is permitted to pass over the supported plate. If the driver is not authorized access, the support is slid out of the way and any vehicle attempting to pass slides into the trench. Embodiments may include means for clearing the trench of a trapped vehicle.