You seem intent on this idea. Logically speaking, it “might” work; however, would the general public be open to the idea???
Here is the US some states have this campaign going targeting distracted driving. Get caught talking on your phone…you get a ticket. Get caught picking up your $36 cup of coffee & taking a sip…you get a ticket. We can now be charged with DWD (Drinking While Driving) which carries the same weight as a DWI (Drinking While Intoxicated).
In my particular case, my left arm is paralyzed so I have to drive with my right hand. How do I reach the roof top holder??? I can’t reach up with my left hand & IF I reach up with the right hand, my car will be in a ditch before I can even reach the cup. IF perchance I run over anyone & they die, I’m going to be charged with reckless driving as well as vehicular homicide.
Another thought…how are you going to size the cup holder??? You said it could be recessed into the roof. Will the size be scaled to handle a 12 oz cup of coffee or a coffee mug brought from home or will it be for the 40 oz stainless steel double insulated wall with a handle???
WHY would you put your groceries on top of the cup holder in your car to begin with??? I put mine in the trunk or the back seat. My cup holder is within arms length of the driver’s seat & it would be dangerous to drive with that many groceries in a position to fall over onto the driver!!!
i know that I’m only one person, but I would NOT have a need for a rooftop cup holder. As a matter of fact, I have driven off with my cup on the top of my car & after the 2nd time, I STOPPED forgetting to put it into the inside cup holder!!!
Keep working on your design & once perfected take it to the car makers & see IF they are willing to invest in it!!! Much like Darth, I didn’t invest in (nor did I buy) the pet Rock!!!