TALKING on a cellphone is more distracting for a driver than talking to a fellow passenger. And now we may know why.

While a car is moving, the strength of signal received by a driver’s phone continually changes, and the phone often has to switch from one base station to another during a call. That causes a slight loss of sound quality, forcing the driver’s brain to work harder to work out what the person at the other end is saying, say Takashi Hamada and colleagues at the National Institute of Advanced Industrial Science and Technology in Tokyo, Japan.

Hamada’s …