Stopping Car Bombs in their Tracks

Inside vehicular perimeter security systems and strategies

From embassies and universities to transportation hubs and used car lots, a wide variety of organizations protect themselves from errant drivers and truck bomb threats with barriers, bollards, barricades and crash gates.

Think back to April 2010, in Peshawar, Pakistan. A Taliban bomber rammed his car into a police checkpost, killing five policemen; however, Peshawar police chief Liaquat Ali told Reuters that the attacker was apparently trying to make his way into the city but decided to set off his explosives when he was stopped by pedestrian gates. 

If one compares the destruction car and truck bombs have caused to the event in Peshawar, it is easy to discern that a large factor in saving lives from vehicle bombers is to successfully stop the attacking vehicle far enough away from a facility to avoid the high pressure shock wave of a bomb blast.

The U.S. Embassy in Jordan not only uses barriers to protect its compound from charging vehicles, but the barriers also create a sally-port which tightly controls traffic into the embassy. The first barricade is lowered to let in a car, while the barrier in front of the car stays up. The one in back then raises and the car is sandwiched between them. Once searched and permitted to pass, the second barricade lowers and the car is allowed to enter the embassy.

The bottom line is that terrorists typically do not go where they see barricades, so placing them in vulnerable spots reduces security risks dramatically — even if the need is only short-term.

Temporary barriers can protect facilities during events, such as a college football game or presidential visit, until permanent barriers are installed, and where physical conditions preclude permanent solutions, such as the State Department did to protect the embassy on Paris’ city streets.

 A High School Physics Reminder

To evaluate security risk for a given facility, pay attention to the weights and velocities of vehicles that could be used to penetrate the facility. A vehicle moving towards a barricade has a certain kinetic energy, which is the major measure of how much "hitting power" it possesses. Mathematically, kinetic energy is derived from the vehicle velocity and its weight (mass). On impact, some of this energy is converted to heat, sound and permanent deformation of the vehicle. The barricade must absorb the remainder of this energy if the vehicle is to be stopped.

The amount of remaining energy varies on many factors — primarily the velocity of the vehicle at the moment of impact. The amount of kinetic energy changes as the square of its velocity. For example, a vehicle moving at 50 mph has 25 times as much kinetic energy as it would at 10 mph. Thus, an armored car weighing 30 times as much as a Toyota Corolla and moving at 10 mph would have less hitting power than the Toyota moving at 60 mph.

Because of this, every effort must be made to force a vehicle to slow down before it reaches the barricade. The most frequently used technique is to require a sharp turn immediately in front of the barrier. When vehicle speed is reduced by 50 percent, the "hitting power" is reduced by four times. If the speed is reduced by 65 percent, the force of impact will be reduced by nine times.

Upon designing a way to slow down vehicle approach, precautions should also be taken so the attacking car cannot make a "corner cutting shot" at a barricade. Often, only a light post defines a turning point and a speeding car can take it out and not even hesitate. Knolls and other impediments should be considered. If the approach to the facility is long, it is best to create curves along the access roads as a natural obstacle to speeding cars or trucks.

Overcoming Common Design Deficiencies

No area is more critical to the vehicle barrier selection process than testing. Without adequate testing, there is no assurance that the barrier will resist the threat. Testing is normally by an independent testing company or government agency, such as the Department of State (DOS) and the military. Comprehensive reports of test results are issued and are available from the testing agency or manufacturer.

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