I maintain that cell phones are one of the biggest officer safety threat faced today.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3584251/
The science on cell phones and distracted driving (and being distracted from life) is pretty solid. If you're on your phone, you can't really pay attention to the world around you. If you're thinking about your phone, you can't really pay attention to the world around you. People think they can multitask with a phone and pay attention to something other than that phone, they can't. The whole hands free stuff is based off of research that people just holding their phones think about their what they are going to do with the phone and stop paying attention I'm not trying to a make a that you can't walk and chew bubble gum, it's entirely possible to simply carry a phone and not be distracted, but if you're not planning on using it, just put it up.