Well, this is interesting. Apparently UPS drivers don't turn left. Like, ever.
The goal is to save gas.
Well, it's not really the "left" part that matters, but turning across traffic, which you have to do when you turn left unless you're turning from a one-way street. Driving in Dallas, I avoid left turns and left turn lanes as much as I can, although I do so not so much to save time or gas but to save what's left of my sanity — and to minimize the risk of accidents.
Circ managers using CMAssistant: If you aren't doing this already, consider revising a couple of your routes to avoid left turns, and after a month or so, find out what your drivers think of the change.