[a] Standoff distance is used when a violent criminal is in a fortified position, when hostages are under armed threat from kidnappers, when a bomb is believed to have been placed, or when other unspecified dangers may be lurking.
When an armed and violent criminal is sheltered in a location not easily reachable by a tranquilizer round or disabling shot - or lethal ammunition, if authorized by mission leaders - police, military, and counterterrorism officers maintain distance (often out of the direct line of sight and behind cover) while often using a megaphone to call for backup, the arrest of the subject, or to take him/her into custody.
In the wake of active shooter scenarios, some law enforcement agencies have switched to moving in on the suspects, to prevent the gunmen from harming civilians.
This is not true in lone wolf situations, where the hostage taker is often shot by a snipers with armor-piercing or wall-piercing ammunition if talk or negotiation resolution is impossible.
A shock wave consists of highly compressed air traveling radially outward from the source at supersonic velocities.
As the shock wave expands, pressures decrease rapidly and, when it meets a surface that is in line-of-sight of the explosion, it is reflected and amplified.
Diffraction effects, caused by corners of a building or structure, may act to confine the air-blast, the airborne shock wave that results from the detonation of the explosives, prolonging its duration.
[3] Standoff distance is also intended to deter terrorists from using car bombs by making it more difficult for them to cause catastrophic damage.
Hydraulic roadblocks (sometimes wedge-shaped), or bollards can be raised to block approaching vehicles; these can be designed to prevent even a heavy, fast-moving truck from getting through.