And how should that be taken into account with the design of touch screen controls in the vehicle ?
To be clear, you are referring to the replacement of the typical head unit hardware user interface with a tablet touch interface. I make that assumption because it is impractical and uncommon for a touch screen to be made available anywhere else on the dashboard. Keeping that in mind, one can determine typical use-cases for the tablet touch interface:
- (very likely) control of media playback,
- (quite likely) control of air-conditioning,
- (possible) control of low-priority automobile functions,
- (possible) other functions.
...and audience for the tablet touch interface:
- front seat passenger
...while passengers in the back seat must rely on control-by-proxy through either the front seat passenger or the driver.
The above is in contrast to the odometer and other such displays and controls in the car, they are intended exclusively for the driver and as such are positioned in the safest location for the driver to glance at. Consider that the Alliance of Automobile Manufacturers devised a principle that states "Visual displays that carry information relevant to the driving task and visually-intensive information should be positioned as close as practicable to the driver’s forward line of sight". From that, one can infer that a tablet touch interface positioned in the center of the dashboard is not intended for use while driving, or at least should not be intended for such purposes.
The US National Highway Traffic Safety Administration (NHTSA) conducted a study to generate visual-manual Driver Distraction Guidelines, which concluded a 12 second total eyes-off-road time (TEORT) glance criterion. The quality of the approach to determining that value is disputed: https://journals.sagepub.com/doi/abs/10.3141/2602-01
The Alliance of Automobile Manufacturers in their Statement of Principles, Criteria and Verification Procedures on Driver Interactions with Advanced In Vehicle Information and Communication Systems, determined a principle: "The system should be located and fitted in accordance with relevant regulations, standards, and the vehicle and component manufacturers’ instructions for installing the systems in vehicles" ...they go on to offer a criterion that includes specific glance time metrics: "A visual or visual-manual task intended for use by a driver while the vehicle is in motion should be designed to the following criteria: A1. single glance durations generally should not exceed 2 seconds; and A2. task completion should require no more than 20 seconds of total glance time to task display(s) and controls." https://autoalliance.org/wp-content/uploads/2018/08/Alliance-DF-T-Guidelines-Inc-2006-Updates.pdf
0 nanoseconds. There is always a risk when your attention is diverted from the outside world when in control of a vehicle.
From the viewpoint of a consumer:
Give me physical controls for the basic tasks. I always want basic navigational buttons directly on the wheel for controlling navigation through screens, volume and seeking, etc. I want dedicated buttons for car settings such as cruise control, speed limiter that will often need to be set and reset. I want physical climate controls so I can adjust temperature and enable the heated windscreen and fan settings by feel alone especially if the screen fogs up while driving.
Give me voice controls for lesser used tasks or tasks which require more configuration. If I need to call someone or set a destination on the navigation system I don't want to be faffing around with touch screens while driving and I'd rather not have to pull over every time.
Give me options that can be navigated using a selection wheel (like those in BMW, Mercedes etc) or directional buttons on the steering wheel so I don't have to try tap a button on a screen while the car is bumping around. I find this to be a terrible experience as it requires significant concentration to be able to tap small touch targets while trying to stabilize your hand with the motion of the vehicle.
Most functions should be easily controlled with both hands on the wheel and preferably with minimal focus required to look at a display so a heads up display or a display in the instrument cluster helps a lot.
I'm of the opinion that touch input should only be used as a backup. If voice recognition fails and a user needs to manually type their input they can use an on screen keyboard. If they are stopped and want to navigate directly to something on screen or quickly scroll a list they can use touch rather than moving the cursor with the buttons. Use touch to enhance the experience, not replace other input methods.