An apparatus for use during modernization of a group of elevators to maintain optimum service to a building includes an Interim Traffic Manager (ITM) system which evenly distributes hall call routing between old and new elevator controls. During the time that an elevator to be re-equipped is out of service, the original not yet re-equipped elevators, as well as the already re-equipped elevators, respond as a single bank, one elevator car per call. The ITM system is connected between the hall call registering devices and each of old and new elevator controls. When a floor push button is pressed, the ITM system uses a routing algorithm to decide whether to send the call to the old or to the new elevator control based upon traffic loading. The ITM system is an inexpensive, reusable unit which resides on the job site temporarily and, therefore, adds no additional costs to the modernization of an elevator group. Technical knowledge required for the installation of the ITM system is restricted to connecting wiring to the elevator controls and the hall call registering devices.