HMI Development and Human Reaction Time

Whether an alarm is triggered, a device status changes or a room function needs to be adjusted: how quickly and reliably users respond depends not only on their personal reaction times. Equally important are the visibility of the signal, the clarity of the operating logic, the layout of the controls and the system’s feedback.

Particularly in medical technology and building automation, control elements must therefore be designed as part of a complete response chain. A technically fast control system is not sufficient if users overlook a display, confuse one function with another, or do not receive clear confirmation after entering a command.

Immediate response: In the case of a simple reaction to an expected visual stimulus, the human reaction time is often between approximately 200 and 250 milliseconds. A large, technically calibrated study found an average of 213 milliseconds among adults after correcting for measurement delay. However, if a person first has to assess the displayed state and choose between several actions, the reaction takes significantly longer. When designing control elements, it is therefore not just the average value that is decisive, but the entire sequence comprising perception, decision-making, action and system feedback. Study on factors influencing simple reaction time
 

How does a response occur at the control element?

A user action can be broken down into several steps:
• A visual, auditory or tactile stimulus reaches the user.
• The user recognises and interprets the information.
• They decide what action is required.
• They locate and operate the appropriate control element.
• The system recognises the input.

Feedback confirms the input or indicates the new state.

Each of these steps takes time and can be influenced by the design of the control system. A display with low contrast delays perception. Similar symbols or unclear labelling make decision-making more difficult. Small touch areas, closely spaced buttons or a lack of haptic feedback prolong the execution of the action. If no feedback is provided, the user cannot be certain whether their input has been registered.
In simple reaction tests, it is clear in advance which stimulus will appear and which button must be pressed. In real-world applications, the situation is more complex. Medical staff may need to prioritise an alarm, check a patient’s vital signs and then select a safe course of action. In building automation, users can switch between lighting, temperature, shading, ventilation and scene control.
 

The greater the number of possible displays and actions, the more important it is to have a clear hierarchy of information and predictable operating procedures.

What factors influence reaction time?

• Type and clarity of the signal

For simple tasks, auditory signals are often perceived more quickly than purely visual cues. In one study, the reaction time for a simple auditory signal was around 161 milliseconds, whilst that for a visual signal was around 207 milliseconds. However, as soon as attention was divided across multiple sensory channels, reaction times slowed down. Study on divided attention
In practice, this means that more signals do not automatically lead to better perception. Light, sound and haptic feedback should each fulfil a defined function. If too many states flash or are signalled acoustically at the same time, the cues compete for attention.

• Complexity of the decision

A clearly labelled button usually enables a faster action than a function that has to be searched for in a multi-level menu. For frequently used or time-critical functions, the operating paths should therefore be as short as possible.
However, the aim is not the shortest reaction time at any cost. A quick but incorrect input can have significant consequences, particularly with medical devices. Speed and operational safety must therefore be considered together.
Attention and the environment
 

Noise, time pressure, poor lighting, multitasking or wearing gloves can increase reaction times. The location also influences which signals are reliably perceived.
When it comes to the development of medical user interfaces, the FDA cites, amongst other things, changing lighting conditions, high noise levels, distractions and movement as relevant environmental factors. These can make it more difficult to recognise displays, alarms and control elements.
 

FDA: Human Factors Considerations
In building automation, other situations must be taken into account: a control panel may be used in a dark hotel room, a brightly lit entrance hall, a plant room or a public area. Furthermore, the user group often ranges from trained building staff to people operating the system for the first time.
 

Technical response time

In addition to human response time, there is the technical latency of the system. This can be caused by, amongst other things:

• Input from the touch sensor or push button
• Processing by the electronics and software
• Communication with the building or device control system
• Updating the display
• Switching a connected actuator
• Feedback on the status achieved

A user interface can be clearly laid out and yet still feel unreliable if input is confirmed with a delay. Human reaction time and technical system latency should therefore be measured separately, but assessed as a single chain of operation.
Designing user interfaces for medical devices

With medical devices, the focus is not only on efficiency but, above all, on avoiding risks associated with use. To this end, IEC 62366-1 describes a usability engineering process that enables manufacturers to analyse, develop and evaluate usability in relation to safety. This also includes consideration of foreseeable user errors. IEC 62366-1:2015
 

This gives rise to several requirements for the design of the control elements.

• Clearly distinguish critical functions: Functions that may affect treatment, diagnosis or patient safety must not be easily confused with everyday settings. Spacing, position, shape, colour and labelling can help to distinguish them.

• Link alarms to action: An alarm should not merely attract attention. The user must understand what condition is present, how urgent it is and what action is required. Audible alerts therefore need to be clearly supplemented by visual cues.

• Prevent accidental activation: Critical changes may require deliberate confirmation, prolonged activation or a clearly defined physical button. However, in an emergency, this safeguard must not hinder the operator through unnecessarily lengthy procedures.

• Take glove operation into account: Touch sensors, spacing and activation areas must be suitable for use with medical gloves. Particularly small touch targets increase the risk of triggering an adjacent function.

• Allow for cleaning and disinfection: Sealed glass surfaces can be designed to meet hygiene requirements. At the same time, it must be checked whether cleaning movements could cause unintended inputs. A cleaning mode or a temporary operation lock may be useful here.

• Confirm inputs and states separately: After a button is pressed, the system should first indicate that the input has been recognised. It must then be clear whether the desired device state has actually been achieved. A pressed button does not automatically mean that a function has been successfully executed.

The FDA also emphasises that medical devices must capture user inputs and provide feedback to users on their effects. The aim of human factors engineering is to minimise usage-related risks and to enable safe, effective operation by the intended users. FDA: Human Factors and Medical Devices

Designing control elements for building automation

In building automation, the focus is often on comfort, energy efficiency and the simple control of multiple systems. Control elements must be intuitive even without training, whilst at the same time conveying complex technical relationships.
Make frequently used functions directly accessible: lighting, temperature, shading and, where applicable, ventilation should not be hidden behind an unnecessarily large number of menu levels. For functions used regularly, dedicated buttons, rotary controls or permanently visible touch areas can offer advantages.

• Clearly indicate room reference: users must be able to recognise which room, zone or group of lights they are currently controlling. Particularly with central control panels, a lack of room labels can lead to incorrect operation.
• Display the system status rather than just the command: After pressing a button, it should not just be the selected option that lights up. The panel should display the actual status, such as ‘Blind moving’, ‘Temperature setpoint 22 °C’ or ‘Light off’.
• Design response times to meet expectations: A light switch is associated with an almost immediate effect. For slower processes such as adjusting blinds or changing the temperature, the system should immediately confirm that the process has started, even if the physical result only occurs later.
• Enabling ease of use in changing light conditions: Contrast, brightness and backlighting must function effectively in both dark and very bright rooms. A display that is consistently too bright can be just as disruptive in bedrooms or patients’ rooms as labelling that is illegible in daylight.
• Take different users into account: Building services are operated by residents, guests, cleaning staff, facility management and service technicians. The interface should remain simple for everyday functions and clearly separate administrative settings from normal use.

The general interaction principles of ISO 9241-110 support this design approach. These include task suitability, self-descriptiveness, conformity to expectations, controllability and robustness against user errors. Their relative importance depends on the users, the task, the environment and the consequences of potential errors. ISO 9241-110

EVYTRA develops bespoke HMI systems for demanding applications. Touch surfaces, buttons, displays, lighting, haptic feedback and acoustics are tailored to the specific usage situation. The spectrum ranges from custom-printed glass to complete OEM control systems. EVYTRA HMI and control systems

Are you planning a medical device or a control solution for building automation? EVYTRA helps you to combine user requirements, technical functions and clear operational feedback into a suitable HMI concept.

Conclusion: Good control elements support the entire response process

The average human reaction time of around 200 to 250 milliseconds is a useful benchmark. However, it is not sufficient when designing a control system. The decisive factor is the entire chain of perception, interpretation, decision-making, action and technical feedback.
In medical technology, the focus is on protection against operating errors, reliable alarms, hygiene and validation with representative users. In building automation, direct operating paths, clear status indicators and intuitive use by varying groups of people are crucial.
Work with EVYTRA to develop a bespoke control system for medical technology or building automation: from the usability concept through to a functional prototype and a production-ready OEM product.