A passenger in a moving vehicle is one of the few audiences left that is not being asked to do anything else. There is no feed to scroll, no skip button after five seconds, and no second tab. The screen is simply there for the length of the journey.
That is the whole idea behind in-cab advertising. A billboard waits on a wall for the city to pass it. A cab screen travels the city instead, carrying the same advert through business districts, shopping streets and residential routes in a single shift.
The part that matters commercially is what happens afterwards. Every play is recorded on the device with a timestamp and GPS position, and every QR scan is attributed to the campaign that produced it. So the question stops being “do you think anyone saw it?” and becomes a report you can read.