The Evolution of HMI: Shape Coding and the Art of Switch Safety Design

I’m joined by the fantastic Tim Cherny of Arrival fame as a collaborator on this subject. 

The history of Human-Machine Interaction (HMI) is rooted in the friction between the machine's ability, and its user goal. Machines, in their purest mechanical state, could do nothing without human involvement. They lacked intelligence, autonomy, and contextual awareness. They were mere mechanisms, waiting for human input to determine what should be done, when, and how.

Whether it was an industrial press, a bi-plane cockpit, or an early automobile, every action required human decision-making. The burden of responsibility fell entirely on the operator, who had to understand, react, and execute commands in real time. These systems did not account for errors, stress, or the cognitive load on their users. They simply functioned according to their design, leaving humans to fight against the limitations of both machine and mind.

This challenge became even more apparent in high-stakes environments like aviation, military operations, and industrial control. Operators were forced to make split-second decisions under extreme pressure, where a single misstep could lead to catastrophic consequences. The design of controls and interfaces had to evolve—not just to be more efficient, but to accommodate human psychology, motor skills, and cognitive limitations.

Shape Coding and the Art of Switch Safety Design

The field of shape coding emerged as a response to this challenge. Pioneered by Alphonse Chapanis and Paul Fitts during the Second World War, shape coding aimed to make control interfaces more intuitive by leveraging human touch and muscle memory. Today, these principles are embedded in cockpit designs, industrial machinery, and a variety of critical control systems.

A perfect example of shape coding in action is the evolution of the flight stick in fighter jets like the F-16. The early F-16A, which entered service in 1976, had three primary controls under the pilot’s thumb: Trim, Target Management, and Store (Missile/Bomb) Release. The latter was naturally coloured red for immediate recognition. As aircraft technology advanced, so did its control systems, leading to the modern F-16V, where six distinct controls are shaped to provide a unique tactile feel, enabling pilots to operate them without looking.

Throttle design followed a similar path. The F-16 throttle features an array of controls designed for efficiency and precision, including the radar cursor control and the crucial ‘Dogfight’ switch. This particular switch, while sharing shape coding with the speed brake, requires a deliberate movement to activate, reducing the risk of accidental engagement during critical moments.

Reducing Cognitive Load

HMI design must account for the way human brains function under stress. In an emergency, people may be unable to process all available information, leading to critical oversights. The challenge is to design interfaces that gently shift the user's focus when necessary—through shape, size, position, and grouping of buttons and controls.

Over time, improvements in control layouts, haptic feedback, and intuitive placement have helped reduce the likelihood of fatal errors. In 1989 John Barnard removed the traditional gear stick and introduced shift paddles behind the steering wheel of the Ferrari 640 formula one car. This dramatically reduced gear change time and decreased cognitive load during high speed turns. Today modern high speed scooters such as iScooter iMax 6 feature shape coded inputs to aid in keeping the rider’s eyes on the road.

Shape Coding Beyond Vehicles

The principles of aviation switch design have influenced other industries as well. For instance, Doyon Drilling Rigs adopted a split-stick control system where shape-coded rollers allow operators to manipulate the drill string axis intuitively.

Similarly, crane manufacturer Liebherr implemented dimpled concave hat caps in their INTUSI crane cockpit, providing natural hand positioning for improved precision.

The Role of Switch Safety in High-Risk Environments

Shape coding not only enhances usability but also significantly improves safety. Critical switches in high-risk environments are often protected by guards, recessed panels, or tactile differentiation to prevent accidental activation. A striking historical example is the Apollo 11 control panel, where every switch featured rail guards to avoid unintended engagement in zero gravity. Some of the most mission-critical switches—such as those controlling abort functions and module separation circled below—were further protected by physical barriers and chevrons to ensure they were only used deliberately.

In some instances switch guards are there to protect the machine, not the crew. The F-15 Strike Eagle features a guarded switch labelled V-Max which when opened and activated boosts fuel flow to both engines providing an additional 4% thrust during combat. The downside is both engines need to be rebuilt when the plane returns. The Ford GT, Gordon Murray T.50 and Hennessey Venom F5 all have driving modes named after this switch, and Czinger named a car after it.

Intelligent switch design can be deployed to protect the machine or protect the human, and in some instances protect the recipient of a machine's function.    

When defibrillator manufacturer Zoll were upgrading their R Series systems, they opted to recess and hide the Power Output and Rate behind a flap door. While far less likely to injure the patient than say poorly placed shock pads, too much power administered to a child can be lethal. With this updated recess the unit can be pre-configured at a lower output before being installed in a paediatric ward with less risk of control interference.

A Closing Lesson of Poor Switch Design

Despite the advancements in HMI design, history has shown how crucial intuitive controls are, particularly in high-stakes scenarios. A final example occurred during the 2021 Azerbaijan Grand Prix when Lewis Hamilton accidentally engaged a brake-bias switch, causing him to lock up at the first turn during a race restart and lose a potential victory.

Mercedes added a switch guard for the next race but the damage was done, and an easy 25 points was lost. It was a minor human error that went on to have large consequences. Max Verstappen went on to win the championship that year by only 8 points.