I'd say that's a fair assumption for the vast majority of people who drive and have mobile phones, given the ubiquity of mobile phones and the likelihood that m

I'd say that's a fair assumption for the vast majority of people who drive and have mobile phones, given the ubiquity of mobile phones and the likelihood that most people will carry their phones with them at all times (myself included). Thus, it would be your behavior which would be considered anomalous. So yes, definitely an assumption, although not an unwarranted one.