Nicolas Cage is racing his silver Mercedes through the narrow streets of London when beer kegs suddenly start dropping from the truck in front of him. But he can't swerve to avoid the exploding kegs. In fact, he can't control anything in the car at all. It's not a nightmare, it's movie magic: Though Cage sits in the driver's seat, the actual driving is being done by a stuntman in a metal rig atop the car for a chase scene in National Treasure: Book of Secrets. It's a classic car-chase tool, says producer Jerry Bruckheimer, who has learned a thing or two about the art of the chase after producing dozens of action films and crime shows. "They add excitement and we try to amp it up," he said. "Everybody lives in a car, except maybe New Yorkers, and a lot of things we do in these car chases I'm sure audiences would love to do." Not that those things come easy. Or cheap. Car chases take months of planning and can cost millions to shoot. The concept for the chase is usually born early in the story's development, said Bruckheimer, whose credits include Gone in 60 Seconds, Bad Boys and Days of Thunder. It then takes a team of stunt coordinators, location scouts, production designers, writers, producers and the project's director to make the chase happen. While artists build a computer animation of the sequence to tell the team which cars are crashing when, mechanics work on modifying the vehicles to ready them for repeated rumplings. "Rather than taking a naked car, a new car, and just smashing into it, it will withstand 10 crashes" once it's been reinforced, said Steve Dent, a stunt coordinator for 25 years who oversaw the chase in National Treasure. Door panels are removed and steel is added to the car's frame. Roll cages are built in. Sliding rigs are attached to the wheels to create a drifting effect, he said, "like it's got skates underneath it." And top-driving rigs are constructed so the actors at the wheel are really just acting and not wielding any control over the car. "It's much safer for them," Dent said. The cars featured in the chase aren't the only ones modified. Camera rigs are also built onto a "good, strong - normally a Chevy - truck," he said, along with high-speed tracking vehicles and a specially outfitted camera bike driven by "a world-champion motocross guy." Newer cars aren't ideal chase-scene candidates because of their onboard computers, Dent said. "You go into a drift and it will shut down on you. For safety it's brilliant, but it's a pain when you're trying to make a film with them." PREPPING THE vehicles can take months. Shooting can, too, depending on the location. It took nine weeks to film the National Treasure chase scene because the crew only had access to the London streets on weekends, said director Jon Turteltaub. All told, that chase included seven cameras, 80 stunt people, the destruction of 40 to 50 cars and many hours of editing. "It's really in the editing room that you truly create the chase sequence," Turteltaub said. "An editor with fresh eyes...can create the chase scene with a pace and level of intensity that can only be done through editing." Computer-generated effects play varying roles in chase scenes, Bruckheimer said. "It depends on the director." Though chases can be almost entirely digital creations, computers are most often used to enhance the details: adding smoke to the tires, a blood splatter here and there or to replace a windshield that was removed for filming. "The crashes are all real," Dent said. It's pricey, and personally painful to Bruckheimer, to demolish so many cars. "I'm from Detroit, so I'm a car buff," he said. Studios sometimes make deals with manufacturers to supply the vehicles starring in a chase scene. General Motors provided cars for Bad Boys, Bruckheimer said, and Mercedes was a sponsor of National Treasure. But other vehicles, including the beer truck and a few Range Rovers, were bought to be smashed. The element of one-upsmanship can also add to the cost of a chase. "Everybody wants to do something unique and special that somebody hasn't tried before," Bruckheimer said. "They always want to do something that's better than the last time." Turteltaub gave his chase a special touch by taking advantage of the rearview backup cameras popular in new cars. Cage checks the camera mid-chase to discover a small, happy-looking dog behind his car. "That's something that's never been done in a car chase," Turteltaub said. "The dog's name was Strapper, by the way, and he very much liked the peanut butter we put on the lens."