phonetard

Noun: A person behaving abnormally, usually delayed or causing delay, due to cell phone use.

Traffic was moving along great until I found myself behind this total phonetard doing ten miles per hour under the speed limit.

We were leaving the movie but were stuck behind this huge, fat phonetard who was oblivious to the fact he was blocking the hallway.

Also phonetardedly (adverb, as in, to drive phonetardedly)