Answer
See work.
Work Step by Step
Newton's First Law (inertia) applies here. If the vehicle suddenly slows due to a head-on collision, the passenger keeps moving at the same velocity at which she was moving (faster than the vehicle's current velocity), and so will lurch forward relative to the car, through the windshield.
If the passenger is restrained by a seatbelt, when the car slows, the belt “locks” and exerts a backward force on the passenger, slowing her down, and keeping her in the car.
