Clearly, the film makers have constructed images which contravene the physical laws of the world we actually live in. They have done so for comedic effect. They have done so in service of the narrative which asserts a taboo of not wearing seat belts. The seat belt rule set up in the beginning of the film comes back later and again, we see a body flying through the windshield:
In this instance, it makes a little more "real world sense" that the zombie in the back seat flies forward through the windshield, but again: comedic effect, and the comedic effect of repetition. In both cases, the visual effect is affective when audiences suspend their disbelief. We're already not questioning a world where zombies are real, so "how" she flew out the window like that is simply the magic of movies, no?
The audience is sympathetic to the woman because obviously she was in shock. While she is momentarily distracted by her dashboard bobbleheads (a symbolic reminder of a safe, suburban lifestyle that has now been shattered) the driver was likely not paying attention to how heavy her foot was on the accelerator. In her distraction, she doesn't notice the cross traffic and by the time of the impact, her rule breaking negligence of seat belt use renders her a car crash projectile. Truly a tragi-comic demise, but such is cinematic hyperbole.
Tho U.S. cars have required air bags since 1998, there also appears to have been no airbag to prevent her from launching through the windshield. Who knows - it may have been an older model, but it looks of the movie's era when Zombieland came out in 2009. The van in the movie has had make and model identifications removed, however, frontal collision tests with similar models from 2009 clearly show the likelihood of the driver's body slamming down upon the steering wheel:
("Naturally", I presume you'll appreciate the clip is regarding the Australasian New Car Assessment Program).
This video also shows what the driver's body is likely to do in a car crash when the driver is without a seatbelt:
Culturally, the U.S. has a long history of "shocking" movie audiences with films like "Blood on the Windscreen" and "Mechanized Death" to drive home the point of safe driving. These films are still shown to this day in Driver's Education classes (often assigned by the courts to those who have violated traffic laws). Perhaps the joke simply plays better to an audience predisposed to imagining all sorts of terrible consequences to unsafe driving practices?
All that said, in real life and at high speeds, car crashes are very unpredictable, so do always wear your seatbelt. In case you are tempted to drive without seatbelt or airbags, here's a crash test dummy example of what is likely to happen in a head on collision. Note that it would be awful, but without any other directional momentum, you are unlikely to fly through the windshield:
