Good point - the system would have to monitor the aircraft after it was
in position and during the start of the takeoff roll to determine which
intersecting runway was being used.

.... and then what? The aircraft is rolling on the runway, it takes
maybe thirty seconds to get airborne. It would take five to ten seconds
to ascertain that the wrong (intersectiong) runway was being used.
(remember, aircraft are also cleared to taxi on the "wrong" runway to
get to where they are going; the system needs to know this is an
attempted takeoff). With twenty seconds to go, the system has to notify
the pilot (in a way that doesn't make other pilots who may be doing a
correct concurrent takeoff think the message is for them) and then the
pilot has to hear, evaluate, react, and be able to take corrective action.

It's not as easy as it sounds, and its usefulness is still questionable.

Another good point - The alarm would have to be in both tower cab and
the cockpit and have to be audible to be effective.

That's not the point. The point is that if the system itself is down,
there will be no alarm anywhere. Then somebody will propose some other
system that will address the same issue another way, and more money will
be spent on research, development, installation, and supervision, and
the end result will be that the money comes from somewhere - perhaps
draining =other= safety programs.

...there may be no way of keeping humans from making a
mistake and attempting to depart on the wrong runway.

That's for sure.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.