Whilst trying to practice my landings in helicopters, I often have to get out and listen for some time as to whether my character sucks his teeth and moans or not before realising that I've been injured. Likewise, when my helicopter or car crashes/explodes, I find that the only indication I have of my demise is my screen looking down at my groin and the car interior gains a damage decal over the normal texture. As someone who uses vehicles quite regularly, I can't help but think that this could be improved somewhat.
A violent jerk forward, perhaps, or a sudden red flash akin to those you get when hit on foot. Preferably, both combined and some reaction from the vehicle itself. If that landing hurt me, tell me in some way rather than having to wait for the annoying injured sound effect to begin looping. Equally, if I die in a catastrophic explosion, make me regret it, rather than a half-hearted slump in my seat, despite the vehicle exploding around me.