When interviewed by Julia Belluz for Vox about the surprising results of the study he lead, Daniel Kaffine said, “We were expecting to find maybe a five to ten percent reduction in accidents. We had read the studies that talking on your phone is as dangerous as drinking and driving.” Further, “Only after spending a ton of time looking at the data, slicing it in different ways, we eventually came to the conclusion that there was no evidence of a decline in accidents. It took a while for us to convince ourselves that there wasn’t something there.”
In the published study found in Transportation Research Part A: Policy and Practice Volume 66, August 2014, Pages 162–172, the author states that the expectation for accident reduction was predicted to be somewhere between 2% to 21%. The actual effect of the ban resulted in estimates that ranged from -3% to 2%, effectively not a factor at all.
There are numerous speculations on why the ban had no effect on accidents. Either people simply ignored the law, or people switched to other behaviors such as holding the cell phone below the window while on speakerphone to avoid being caught.