Passenger lift traffic calculations The elements of the round trip time (rtt) are expressed as : The time to load the passengers at the main floor and close the doors The time to accelerate, decelerate and level into the floors with 2 floor jumps to the top floor The time to open the doors, unload/load passengers and close the doors with 2 floor jumps to the top floor The time to express return to the main floor. The interval time is calculated as the round trip time divided by the number of available lifts. Therefore based upon recommendations within British standards BS5655 : Part 6: 1990 and Cibse guide D:2000, the probable quality of lift service is related to the interval time as follows : 20 seconds or less indicates an excellent system 25 seconds indicates a very good system 30 seconds indicates a good system 40 seconds indicates a poor system 50 seconds or greater indicates an unsatisfactory system