Standoff distance is a security measure that focuses on preventing unscreened people and vehicles from approaching within a certain distance of a building, car, or other shelter where a violent criminal is sheltered, hostages are under armed threat from kidnappers, where a bomb is believed to have been placed, or where other unspecified dangers may be lurking. It is a measure of distance informally used by agents handling the situation to protect their own persons from physical injury or death while the situation is resolved.
When an armed and violent criminal is sheltered in a location not easily reachable by a tranquilizer round or disabling shot - or extreme prejudice 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. Sniper coverage is used often in these situations, and standard procedure for officers or operatives (or citizens taking part in a citizen's arrest) is to call for heavily armored backup while maintaining cover themselves. Therapeutic or diplomatic techniques may be used to talk down the suspects or identified threats and assailants.
In a hostage situation, the primary goal is the recovery of the hostages, who are usually held under threat of violence or other prolonged physical harm (starvation, poisoning, bleeding, illness) from kidnappers; thus the situation is treated similarly to situations with other armed attackers under cover, but with even more care. Again, snipers are often employed to attempt to provide leverage against the hostage-takers; but unless all kidnappers can be neutralized simultaneously, generally extreme prejudice is not used as freely unless all kidnappers may be eliminated at once, due to the danger of other kidnappers killing the hostages, as in the 1972 Munich example. This is not true in lone wolf situations, which are often neutralized by armor-piercing or wall-piercing sniper if talk resolution is impossible.) However, in all situations the preferred situation is to talk the kidnappers into releasing the hostages for ransom or otherwise talking them down using therapeutic or diplomatic techniques, to protect the safety of the hostages.
Standoff distance is also intended to deter terrorists from using car bombs by making it more difficult for them to cause catastrophic damage. In the wake of the Oklahoma City bombing, many high-risk federal buildings began enforcing standoff distances. It is based on the concept that a blast shock load is essentially a high-pressure front that moves out radially and decays very quickly - because blast falloff is thus often more exponential than linear (indeed radial - any given explosive must cover a circular-shaped area but blast power generally only increases linearly with explosive strength), any standoff distance helps increases survival chances for passersby and minimizes danger, though shrapnel mitigates this effect if present. 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. Jersey barriers and concrete planters have also been used to maintain separation between screened and unscreened traffic. Certain infrastructure at risk of terrorist attack, such as bridges, may not be well-suited to standoff distances since their purpose is for traffic to travel along them.