Couldn't you just use a weight sensor to determine whether an elevator cab is over x% of its rated capacity, and make that one ineligible to respond to external

Couldn't you just use a weight sensor to determine whether an elevator cab is over x% of its rated capacity, and make that one ineligible to respond to external calls until after some people exit? That wouldn't require changes to the user interface, and the doors would only open if someone can actually get on. That, plus an assumption that most trips are between the ground/garage floors and one of the higher, occupied floors should be sufficient to reduce congestion without requiring someone to push a floor button before entering the cab. Adding a clock to the elevator controller would also help, such as by stationing empty cabs at the exit floors in the mornings, and distributing them among the occupied floors in the evenings. I can think of a lot of ways to improve elevator scheduling without changing the user interface, and even if I did change that, I could certainly provide some mechanism to select a specific floor without installing a touchscreen GUI. Wave your employee access RFID badge at the button panel to pick the floor where your cubicle is, for instance.