Need speed switch for DIY funicular/hillside elevator

[astutesolutions@xxxxxxxxxxxxx]

My dad and I built a homemade hill lift (in Europe refered to as
funiculars) in Eastern Tennessee.  You can see a working demonstration
and picture of it at:

http://www.funimag.com/photoblog/index.php/20061026/a-do-it-yourself-funicular/

The tram works fine, but I am trying to figure out some kind of simple
safety mechanism in the event that the spools break loose from the
gearbox or shaft and start rotating freely.  Since my wife and kids
ride this thing, I'd like some kind of backup solution in the event of
major catastrophe.  We are using 2 cables instead of one even though
one can handle 5 times the max load, and we have a braking motor to
stop, but both of those would be moot if the spool spins out of
control.

I am thinking of having some way of measuring the outfeed speed of the
cable (or spool) and have some emergency brake apply if a limit is
exceeded.

Any suggestions?  I thought about putting fan blades at end shaft of
the spool to have air slow it down, but that would be a little
unsightly.  I'm looking for a simple solution, one preferably without a
computer since this is outdoors and needs to be functional 100% of the
time.  I'll take any flames in stride on this point.  I realize that
this group is about computer automation, but its the only  one similar
to this kind of project I could find :-)

Thanks in advance.
Jeff