"David Cartwright" wrote:
I've not had a take-off clearance when they guy in front still had his
wheels on the runway, but I _have_ had plenty when he was only a hundred or
two feet in the air.

What they do at HPN sometimes is get two spam cans in position and hold at
the same time. If there's two guys ready to go, and the gap in traffic is
just right, they'll have the first guy "taxi 500 feet up the runway and
hold" as soon as landing traffic has crossed the threshold. While he's
doing that, they'll have the guy behind him taxi into position on the
numbers. When the landing traffic turns off the runway, they'll clear the
first spam can for takeoff, and as soon as he's in the air, clear the
second guy with, "maintain visual separation".

It takes a tower controller who's on the ball, and pilots who follow
instructions, but it can mean getting two guys out in a gap that would
otherwise only allow a single departure.