Fitts’s Law can accurately predict the time it will take a person to move their pointer, be it a glowing arrow on a screen or a finger tip attached to their hand, from its current position to the target they have chosen to hit.

Much more about Fitt's Law here from Tog. This bit was instructive:

Paul Fitts was not a computer guy. He was working on military cockpit design when he discovered his famous Law. Paul Fitts never had to deal with the the issue of stability because stuff inside aircraft cockpits is inherently stable. The few things that do move only do so because the pilot moved them, as when he or she pushes a control stick to the side or advances the throttle forward. The rest of the targets the pilot must acquire—the pressure adjustment on the altitude indicator, the Gatling gun arm switch, the frequency dial on the radio, and fuel pump kill switch—stay exactly where they were originally installed. Everything in that cockpit is a predictable target, either always in the same place or, in the case of things like the throttle, within a fixed area and exactly where you left it. Once you become familiar with the cockpit and settle into flying the same plane hour after hour after hour, you hardly look at your intended targets at all. Your motor memory carries your hand right to the target, with touch zeroing you in.

I had heard of Fitt's Law but didn't know its history, and it came to mind as I was driving my Tesla Model S recently.

In almost every respect, I really love the car. I took ownership of my Model S in December 2012 after having put down a deposit over 3.5 years earlier, and I long ago stopped thinking of it as anything other than a car, perhaps the most critical leading indicator as to whether it can cross the chasm as a technology. I've forgotten what it's like to stop and pump gas (what are gas prices these days anyway?), it's roomy enough I can throw my snowboard, road bike, and other things in the back with room to spare, and I still haven't tired of occasionally flooring it and getting compressed back into my seat like I'm being propelled by a giant rubber band that has been released after being stretched to its limit. Most of all, it's still a thrill when I fire the car up to find a new software update ready to install, almost as if the Model S were a driveable iPad.

It's the ability to update the interface via software that gives me hope that a few things in the interface might be adjusted.* In a Model S, most of the controls are accessible via a giant touch screen in the center of the console. There aren't many buttons or switches except on the steering wheel which you can use to handle some of the more common actions, like changing the thermostat, adjusting volume on the sound system, skipping ahead on a musical track, and making phone calls.

When the car first came out, one of the early complaints was the lack of physical controls. I was concerned as well. Physical controls are useful because, without looking at the road, I can run my fingers across a bunch of controls to locate the one I want without activating the wrong ones by mistake as I search. With a touch screen, there is no physical contour differentiating controls, you have to actually look at the screen to hit the appropriate control, taking your eyes off of the road.

[I also confess to some nostalgia for physical controls for their aesthetics: controls in a car give physical manifestation to the functionality of a car. The more controls a car has, the more it appeals to geeks who love functionality, and physical controls also give car designers the opportunity for showing off their skills. I find many old school car dashboards quite sexy with all their knobs and switches and levers. Touch screens tend to hide all of that which has more of a minimalist appeal that may be more modern.]

In practice, I have not missed them as much as I thought I would because a lot can be operated by physical controls on the steering wheel.

However, one task that a touch screen makes difficult, in practice, is hitting a button while in a car that's in motion. It turns out that road vibration makes it very hard to keep your arm and hand steady and to hit touch targets on a touchscreen with precision. That's why I rely so much on my steering wheel controls in the Model S to do things like adjust the volume or change the temperature. Not only are the controls accessible without having to move my hands or look at the touchscreen, but the steering wheel acts as an anchor for my hand, taking road vibration out of the equation.

Maybe there is an analogue to Fitt's Law for touch screens interfaces in cars or other places where your body is being jostled or in motion. What you'd like is maximum forgiveness in the UI in such cases because it's hard to accurately hit a specific spot on the screen.

Matthaeus Krenn recently published a proposal for touch screen car interfaces that takes this idea to the logical extreme. You can read about it and watch a video demo as well. Essentially Krenn transforms the entire touchscreen in the Tesla into one single control with maximum forgiveness for your fingers to be jostled horizontally since only the vertical movement of your hand matters. By using the entire screen and spreading the input across a larger vertical distance, you can have a much larger margin of error to get the desired change. Krenn also tracks the number of fingers on the screen to allow access to different settings.

This is an interesting proposal, but for some of the most accessed functions of the car, controls on the steering wheel are still superior. The left scrollwheel on the Model S steering wheel is more convenient for changing the volume of the stereo and toggling between play and stop (you can press the scrollwheel) than the touchscreen. The right scrollwheel is more convenient for changing the car temperature and turning the climate control on and off than the touchscreen. Both scrollwheels allow you to keep both hands on the steering wheel rather than having to take the right hand off to access the touchscreen.

Actually, the ideal solution to almost all of these problems is a combination of the steering wheel controls and another interface that already exists in the car: voice. The ideal car interface from a safety standpoint would allow you to keep your eyes on the road and both hands on the steering wheel at all times. The scrollwheel and steering wheel buttons and voice commands satisfy both conditions.

In the Model S, to issue a voice command, you press and hold the upper right button on the steering wheel and issue your voice command, after which there is a delay while the car processes your command.

Unfortunately, for now, the number of voice commands available in the Tesla Model S are quite limited:

Navigation — you can say "navigate to" or "drive to" or "where is" followed by an address or destination
Audio — you can say "play" or "listen to" and then say an artist name or song title and artist name and it will try to set up the right playlist or find the specific track using Slacker Radio (one of the bundled audio services for Model S's sold in the U.S.)
Phone — if you connect a phone via Bluetooth, you can say "call" or "dial" followed by the name of a contact in your phone contact book

I'm not sure why the command list is so limited. When I first got the car I tried saying things like "Open the sunroof" or "Turn on the air conditioning" to no avail.

Perhaps the hardware/software for voice processing in the car aren't powerful enough to handle more sophisticated commands? Perhaps, though it seems like voice commands are sent to the cloud for processing which should enable more sophisticated voice processing when you have cellular connectivity. Or perhaps the car can offload voice processing to select cell phones with more onboard computing power.

In time, I hope more and more controls are accessible by voice. I'd love to have voice controls passed through to my phone via Bluetooth, too. For example, I'd love to ask my phone to play my voicemails through the car's audio system, or read my latest text message. For safety reasons, it's better not to fiddle with any controls while driving, analog or touchscreen-based.

Perhaps this is a problem with a closing window given the possibility of self-driving cars in the future, but that is still a technology whose arrival date is uncertain. In the meantime, with more and more companies like Apple and Google moving into the car operating system space, I hope voice controls are given greater emphasis as a primary mode of interaction between driver and car.

* One other thing I'd love to see in a refresh of the software would be less 3D in the digital instrument cluster above the steering wheel. I have some usability concerns with the currently vogue flat interfaces in mobile phone UI's, but the digital instrument cluster in a car is not meant to be touched, and the strange lighting reflection and shadow effects used there in the Tesla feel oddly old-fashioned. It's one interface where flat design seems more befitting such a modern marvel.