Maybe this is a "Fred and Barney" thing since I was on the street before airbags and some cars did not have shock absorbing steering columns.
On city streets (speeds under sixty) I went on numerous one and two car collisions. I sensed a division between rollovers and spin-arounds (including one 1973 VW Bug that somersaulted): Occupants of cars which rolled (level streets) tended not to be as severely injured or killed (severely killed?) as occupants of cars which kept their shiny side up but spun, abruptly halted or slid a short distance to a halt.
Mr Chan says to avoid injury from a fall or throw, roll out of it, "always keep it moving". Maybe the same applies to MVA's under a certain speed? Any other observations out there?
From the physics point of view, you must consider the forces on the occupant, not the car.
Remember from the start of this, the old saying, it's not the fall that kills you, but the sudden decelleration. Essentially, large forces are bad for you. However, you can survive large forces for very short amounts of time (which means little energy is actually put in). Jackie Chan's point is that if you keep rolling, you minimize the deceleration forces AND you spread them out so that no one body part receives large forces for a long (relative) time and perhaps you carry your momentum back to a standing and fighting position meaning you didn't completely decelerate.
OK. That is good for a bare human at human speeds. Look at it from a human in a car at car speeds:
The car is made to collapse increasing deceleration times for items (people) INSIDE thereby reducing peak forces and displacement (intrusion into the occupants area). That's good because the seat belted occupant cannot roll inside the vehicle. The unrestrained occupant bouncing inside the vehicle will find small surfaces to impact which does not dissapate, spread out, or otherwise divide and reduce decelleration forces over area and time.
Similarly, vehicles are not built strongly to prevent intrusion on the top. There simply isn't material or space to collapse and absorb.If you get a rollover that doesn't intrude the roof etc, then you have to have a restraint system that keeps the passenger from hitting the roof. If you have these things at low speeds, then yes you can see low injury rates. I've seen it tons of times with cars rolling slowly in snow. Snow collapses for the car. If you have hard ground, it's not as pretty. If you have intrusion and not a lot of head clearance, not pretty. I'm sure you've seen that. If there's a lot of junk in the car to get thrown around in the meat grinder, definitely not pretty. Think: how well would the human survive tucking and rolling at 50mph without a metal and plastic cage around them?
OK that's my stab at explaining the kinematics of why rolling cars don't offer lesser injury rates in most situations.
