One of the pivotal scenes in the Crash movie is the accident in which Ballard is injured, and in the most direct way possible, is indoctrinated into the world of Crash.  It is through the accident that he meets Vaughan, Dr. Remington, and is led into the underground culture of technological sex.

In the novel the incident takes place in Chapter 2.  Most of the elements of this chapter are included, verbatim, in the movie:  The generic models of cars, the death of Dr. Remington's husband, the blood blister of the triton signature from the radiator emblem on the dead mans hand. 

However, certain things have changed, with some elements added, and others ignored.  For example, in the movie Dr. Remington pulls at her seat belt, exposing her breast.  This most obvious sexual connection, with the knowing looks between herself and Ballard, is poignant.  However, in the novel, this does not take place.  On the other hand, the novel goes into detail regarding the arrival of helpers and emergency service personnel.  In the movie this is excluded entirely, and we next pick up the story with Ballard already in Hospital.

Aside from these differences, Chapter 2 also introduces the character of Renata, Ballard's secretary.  We know scenes of Ballard and Renata making love were filmed, and then cut from the theatrical release of the film.  In fact, Renata does not appear at all in the finished movie.  In this chapter Renata is described as "freeing herself from an unsettling affair with me [Ballard]."

This love affair, apparently into its death spirals, is key because it saves Ballard's life.  Both Ballard and Dr. Remington are shown to have been wearing seat belts, presumably her husband was not.  Ballard's wearing of the belt was to spare the blushes of Renata:  "I was still wearing the safety belt I had deliberately fastened to save her from the embarrassment of embracing me."  In effect, the ending of one love affair had allowed him the opportunity to fulfill other desires.

The descriptions of what Ballard first see's out of his cracked windshield are notably muted in the opening salvo of the novel.  Ballard has yet to fully appreciate the various elements of his new focus, the extent of the opportunities it offers.  Yet at the same time, he is not concerned about his health or injuries, instead turning his attention to the crashed vehicles, to Dr. Remington and those around him melding as new shapes to create something different and new.

For someone wanting to examine pointed differences between the original text and Cronenberg's interpretation, this is an excellent place to start.  For example, the sounds of a crash can add their own sense of drama that is palpable, and the movie excels in this.  Also, by shifting the accident into being almost a personal one-on-one interaction, as though two lovers who had yet to meet had designed this elaborate coming together simply to kill a man who would be in the way - Remington's husband - Cronenberg relies on sound, facial expressions, and drama.

JG Ballard on the other hand covers the crash itself with more completeness.  There is a tragic accident, the authorities arrive, there is an aftermath.  This is layered by the central themes of Crash, the strange longing and gradual realisation that something profound has been altered by the incident.

Some of the descriptions of what Ballard see's and hears are confusing and strange, but wouldn't they be after such a massive event?  In this instance we get two takes of the same event, and they work perfectly together, anchored as they are in their respective fields.