An emergency control has to stay reachable under stress, gloves, low light, and noise, not just on a calm day

What it is

Emergency controls — an emergency stop (E-stop) button, a manual lever on an emergency shutoff valve — need to remain discoverable, physically reachable, and operable under stress, low light, noise, gloves, and constrained posture. Reach has four layers: whether the control can be perceived, whether it is physically accessible, whether any authority or authorization gets in the way, and whether the action can actually be completed — being visible on screen and clickable satisfies none of this.

Why it happens

Under stress, attention narrows, fine motor control degrades, and short-term memory shrinks. A deep menu, visually similar controls, or an entry point requiring a fresh login — all reasonable under normal conditions — multiply delay under stress, because every step demands a judgment made with narrowed attention and reduced motor precision, and any hesitation compounds. A control that stays in a stable location with a distinctive shape or texture, such as the raised head of a mushroom-style E-stop, lets the operator rely on trained motor memory to locate and trigger it directly, skipping the visual-search and conscious-judgment steps — consistent with Fitts' law, where a larger target, shorter distance, and more direct path all shorten time to reach it.

Where it stops holding

Reach is not license to remove guarding, interlocks, or authorization checks; the two must be balanced together (see the companion entry on guarding). Some emergencies should be handled first by automatic protection, such as an automatic interlock trip, which loosens the time budget for the human entry point since the most dangerous seconds are already covered. Differences in physical ability and workstation geometry also break a single "average reach" standard; validation needs to cover the actual population and workstation range, not one test standing in for every scenario.

Applying it

Start from the total time budget the hazard scenario allows and split it across discovery, movement, release, and execution, setting a limit for each stage, and keep location and form consistent across stations. Testing must have participants wear the actual protective equipment used on site, work under low light and noise, and carry a cognitive load or interruption to simulate stress, measuring end-to-end time from hazard onset to completed action rather than an isolated "how long to click a button."

Related

• Same group: Y4.07.2 Balancing guard and emergency response · Y4.07.3 Consistent emergency-control placement · Y4.07.4 Unambiguous feedback after emergency action
• Nearby: Y3.05 Guarding against inadvertent operation · Y4.02 Redundancy and voting
• Search terms: emergency stop · Fitts' law · reach envelope · situational stress