Ford's Panther line of cars, which consists of the Crown Victoria, Lincoln Town Car and Grand Marquis, is designed with the fuel tank located outside the protection of the rear axle and within the car's "crush zone." No other passenger cars manufactured in North America have retained this fuel tank position because it has been considered too dangerous. At least five million of these unprotected vehicles are estimated to be in use on roads today. The problem is this: high speed rear collisions cause the back of the car to absorb the impact by collapsing. In these cars, the impact can rupture the tank or can drive surrounding parts or even items in the trunk into the tank, puncturing it. The fuel leaks and sparks created by the collision can ignite the fuel. Unfortunately, the force of impact often jams the cars' doors shut, trapping occupants. The safety shields cover mechanical parts that could serve as puncture sources. Ford also has offered trunk protectors to police departments to help keep trunk items, such as jacks, from becoming fuel tank puncturing projectiles in a collision. In 2002, Ford began installing fuel-tank shields in their Crown Victoria Police cars to protect them from explosions. In 2005, Ford began installing the same shield in the Lincoln Town Car limos. This is believed to be a direct result of the Howell case.