Hello, hello. I was in a car accident on two separate occasions, once in 2020 during my FantasyRP arc— and very recently, hence my in-game inactivity.
Chances are (although it's entirely dependent on the circumstances), you will not be able to react when you get rear ended or T-boned by another vehicle. You won't be able to react if someone decides to tailgate you whilst the car is moving relatively fast. You just have to keep your fingers crossed and hope whoever is driving the car (unless that's you) is able to react fast enough to absorb the crash's infliction accordingly, or completely dodge the other vehicle.
In both cases, I fell unconscious (the opposing vehicle used a spiked bat on me and inflicted 3 K.O points) and only woke up in the hospital. It's not fun, really, and causes a lot of long-term or short-term damages (Whiplash, permanent lower back pain, tender tissue as a result from eerie bruises, broken bones, etc.)