Standoff distance is a security measure that focuses on preventing unscreened vehicles from approaching within a certain distance of a building. It is intended to deter 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. 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.