DB>You may think you have the right to abscond with the property of a
  >private citizen, for whatever reason... you don't.
CH> You are not even close to reality on this, Don.
Welllll. . . he is close but not quite dead on.  If I
were in a major city and someone in a uniform ran over
to my car and demanded that I get out and give it to
him the odds are he wouldn't get it until he proved to
my satisfaction that he was truly a LEO.  Anyone can go
out and get a cop uniform and stick a toy gun in a
holster and claim to be a LEO.
I don't know how the laws read when an immediate threat
is involved but in other emergencies situations the
governmental agency and agent have to provide a receipt
for any item they take.  I think if the driver demanded
a receipt and the officer refused there might be legal
standing to refuse to turn the car over to him.
If a LEO took a car and the courts ruled that his
emergency wasn't then the cop, his department and the
city could be in for a rather nasty law suit.  There
would be several Constitutional issues involved.
Also the owner could have things to sue for and might
win.  In todays system the "emotional trauma" of having
an armed man demand your car could very easily win in a
court room.
Other things I haven't seen covered are the cost of use
of the car and any hidden damage.
If the car was used at high speeds there could be
damage that would not show up for a while.  A damaged
wheel bearing or spindle and brake drum/rotor damage
pop into mind.  You wouldn't know that there was a
couple hundred dollars of damage done for a couple of
months.
But as almost everyone has stated the odds a LEO ever
needing to take a car are incredible small and the odds
of it being mine or anyone else's on the echo are even
smaller.
                      Remember:  Freedom isn't Free!
--- timEd-B11
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