Thanks largely to Chevy Chase's zaniness and comic timing this is a very funny movie. Chase, whose career has taken a few pratfalls since his Saturday Night Live days, plays an all-American boot)—a father driving his family on a cross-country road trip, all preplanned down to the minute. Their destination: the Walley World amusement park. The trip is a series of mishaps and goofs. Chase, wife Beverly D'Angelo and their kids, Dana Barron and Anthony Michael Hall, visit a cousin (Randy Quaid) who likes to barbecue Hamburger Helper without the hamburger. Then they pack up old aunt Edna, who wants to visit her son en route. Only one problem—aunt Edna (Imogene Coca) dies in the car. So Chase, worried about getting to Walley World on time, decides to strap her to the luggage rack and drop her off at her son's house with a note. Directed by SCTV veteran Harold Ramis, the movie strikes one discordant note (lifted directly but inappropriately from American Graffiti ): Every once in a while supermodel Christie Brinkley, making her movie debut, pops up driving a red Ferrari, flirting with Chase for no apparent reason. When she finally lures him into a motel pool, it seems wildly out of place—the only reason for the scene is to show her in a bathing suit, which isn't reason enough. The ending is on the wishy-washy side, too, as if to say, "Gee, aren't we all having a good time?" In this case, though, that is precisely the truth. (R)