It is about person-to-person interactions, and though many -- most, even -- of the interactions in Crash are racially charged, race itself is not actually the focus. Haggis takes a far more narrow and specific view of the issue, according to UC Davis' Hsuan L. Hsu writing in Film Criticism. He points out that it is not actually any racial factor that leads to the stereotyped views helped by many of the characters of themselves and others, "but historically specific practices of racism" (Hsu 2006). In many ways, Crash presents a microcosm of Los Angeles and United States history of racial views; it is a condensed explanation of the creation and maintenance of racist policies at the institutional level through the widespread and generally unplanned collective racism of individuals. This is what makes the film so engaging to critics and movie goers alike: it is honest about racism and the way it still pervades society, whether we like it or not. It helps us to understand why this...

Every character is flawed -- some more than others, but all are equally and honestly human. The many different races presented in the movie with remarkably similar characteristics and emotions -- industriousness, impatience, jealousy, anger, frustration, and compassion -- reflect the fact that everyone is truly the same underneath whatever visible or audible differences might seem to exist. Crash manages to deliver this message without becoming cliche. Instead, it succeeds in its message because it succeeds in creating realistic characters in compelling situations. It is, in short, an excellent film.
References

Hsu, H. (2006). "Racial privacy, the L.A. ensemble film, and Paul Haggis's Crash." Film criticism, 13(4), pp. 12-34.

Taulbert, S. (2006). "Film Review of the Movie Crash." Pastoral psychology, 55(2), pp. 247-51.