My mistake. The inference came from the IHM Legend which shows that a Vehicle Block must be barrier=block or motor_vehicle=no. The implication being that barrier=block precludes a vehicle from passing so no "motor_vehicle=no" is necessary. However, in IHM, "barrier=debris" by itself ALSO gives the Vehicle Block, so I inferred that barrier=debris is similar to barrier=block and precludes passage by car.