## 2 Materials and Methods
### Materials  
_The checklist._ A predefined checklist was created which allowed the experimenter to quickly score each interaction of the driver with a wide variety of functions while driving. The checklist listed various tasks--such as adjusting the volume, changing the temperature or using the direction indicator--which were categorised per function. Tasks considered for the checklist were on the tactical level of [PERSON]'s driving task hierarchy [14]. This meant that, e.g., they excluded continuous vehicle control, such as steering or braking, and took some seconds to perform. The checklist was adapted based on a pilot study and also allowed for noting down any additional tasks. This resulted in eleven main categories of functions: Radio and media, Climate control, Phone calls, Lights, Cruise control, Danger signalling, Windshield, Car setup, Settings, Extra features and Other. Table 1 provides an overview of all tasks per function. Also noted was whether or not a task was available in the car, and whether a task was performed while driving on a motorway, a rural road, a city road or in a traffic jam, or whether it was performed during stand still. In addition, the checklist included logging the time driven per road type, the weather conditions (sunny, rainy, cloudy, dark, outside temperature), additional car characteristics, the trip duration and trip distance. It also had room for additional comments. _The route._ Participants were asked to select themselves a route that was familiar to them, which would take between fifteen and thirty minutes to drive, one way. To assess whether or not participants were sufficiently familiar with the route they had chosen, they rated how familiar they were with the selected route on a 10-point route familiarity scale [12]. On average, they scored a 9.5, with a minimum score of eight and a maximum score of ten. In accordance with [PERSON], [PERSON] and [PERSON] [15] and [PERSON], [PERSON], [PERSON] and [PERSON] [16], this score is interpreted as being very familiar with the route. All trips lasted between fifteen and thirty-two minutes, with an average trip duration of 22.4 min. Per trip, 18.2 min were driven on roads in the city, which made city roads the most frequented road type. Usually, the starting point of the route was the participant's home address. Routes were driven both with a familiar car as well as with an unfamiliar car. Most participants drove these two trips during the same appointment, to minimiseweather and context differences. Nevertheless, seven out of the thirty participants had to drive the two trips on separate days. The carsFor the familiar car, participants used their personal vehicle. This resulted in a good variety of brands and models, such as BMW, Fiat 500, Fiat Panda, Fiat Sportage, MG4, Mitsubishi Spacestar, Nissan Juke, Peugeot 208, Peugeot 308 Station, Renault Twingo, Seat Arona, Seat Tarraco, Suzuki Swift, Tesla Model Y, Tesla Model 3 and Volvo XC40. The unfamiliar car for the study was provided by the experimenter. For twenty-eight out of the thirty participants, the unfamiliar car provided was a Seat Toledo from 2014. Table 2 presents an overview of both the familiar and this unfamiliar car's characteristics. Due to circumstances, two participants drove another vehicle as their unfamiliar car. One drove a Renault Megane from 2012, the other a Renault Modus from 2005. This ensured that the familiar car was indeed considerably more familiar compared to the unfamiliar car. The participants were asked to rate their familiarity with the car--both their own car as well as the car provided by the researcher--on a 10-point scale, similar to the route familiarity scale. A score of ten means one is very familiar with the car, a score of one indicates one is very unfamiliar with the vehicle. On average, participants scored 9.5 for their own car and 4.7 for the car provided by the researcher. A paired-samples test showed that these scores were indeed significantly different from each other (\(t=8.80\), \(p<0.001\)).