Surely escalators could be fitted with sensors so that as someone approaches, it turns on and it is timed to run for just longer than it takes to travel a full length.
You would also err on the side of caution and make sure it is enabled earlier than required and disabled much later.
I suppose all the extra on-off would cause wear on the components/motors faster than continuous running so it depends if the off time saves significant money.
In the UK it seems less common place but having been to Germany on a couple of occasions, it seems the norm there that escalators stop completely when not in use.
What is becoming common in this country, however, is them slowing down when out of use (I guess it means they don't look broken as stopped ones often can) - the "new" (they have been there for about 4 years or so now) ones at Birmingham International do this. I think the have sensors at both ends (naturally, they are reversible) so getting off the top seems to reset the timer, in the same way going to the wrong end of an escaltor will also start it moving at normal pace.
From what I know though, escalators when out of use should be fairly efficient as it should be a harmonic (is this the right word?) system - in other words, the weight is in balance and thus when there is no-one on it, the overall weight is zero and therefore it takes very little energy to keep it moving
