Here's some wiki on the subject:
"Gone in 60 Seconds
Further information: Gone in 60 Seconds (1974 film)
For his venture into film, Halicki decided to make the film that became Gone in 60 Seconds in 1974. He wrote, directed, produced and starred in the film; and with his business sense, he copyrighted the terms "Gone In 60 Seconds" and "Eleanor."
There was no official script for the movie, apart from several pages outlining main dialog sequences. Halicki supplied most of the cars, and used repeated footage of the same vehicles and shots of public incidents to increase the footage. The scene in which a train derailment is observed was not part of the original shooting script, but it is in fact a real train that derailed and when the director heard about this he wanted to incorporate it into the film.
According to people on the set, after the mishap when a driver missed a mark and caused "Eleanor" to hit a real light post at 100 mph, the first thing that Halicki said when he regained consciousness was "Did we get coverage?" To achieve the effect of cars sliding into each other when hit by the patrol car at Moran Cadillac, the filmmakers put oil under the tires of the first few cars to help them slide. When it came time to do the stunt, it worked too well and many of the agency's own Cadillacs that were for sale were badly damaged, resulting in Halicki having to purchase all of them. Halicki compacted 10 vertebrae performing the film’s 128-foot-long (39 m) jump finale, that he walked with a limp afterwards.[2]
Much of the action/dialog was improvised, which caused many problems for the editor, Warner E. Leighton, who never knew what footage was being dumped on him or where in the movie it belonged. In the DVD audio commentary, he described the script for the construction site portion of the main pursuit as a piece of cardboard with a circle on it. Halicki pointed at it and said, "That's the dust bowl. We went around it twice. There's your script.""
From here:
Wiki on Halicki
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