Drivers only have to give UPS 30 days notice before retiring and it goes through HR and not the center, so there is no way for UPS to predict retirements. They can base it on what people say but that is unofficial. Also, someone can start retirement process and change their mind.
There will be a few retirements in the next year but not a significant number. Most drivers will hold off till Jan 2017 when the 30 year pension will jump to $3400 and the 35 year pension will pay $3900. I know one driver who will be retiring June 2016- Me
Call local 100, (513) 769-5100, and ask. They will most likely say 2016-2019, since 30 years ago was a big time for hiring, or that a lot of feeder drivers are committed for life.
If its anything like my feeder department the rumor has always been a bunch are just about to go. I've been hearing that for the last 4 years and it has yet to happen while other drivers tell me they have been hearing it for longer than that.