You press a button and wait for your elevator. How long before you get impatient and agitated? Theresa Christy says 20 seconds.

As a mathematician steeped in the theories of vertical transportation at Otis Elevator Co., Ms. Christy, 55, has spent a quarter-century developing systems that make elevators run as perfectly as possible—which means getting most riders into a car in less than 20 seconds. "Traditionally, the wait time is the most important factor," she says. "The thing...