Human factors & ergonomics engineering of an electric scooter as the 30th engineering axis: human-machine fit axis — ISO 9241 series + ISO 7250-1:2017 + ISO/TR 7250-2:2010 + ISO 11226 + ISO 11228 + ISO 14738 + ANSI/HFES 100 + ANSI/HFES 200 + DIN 33402-2 + IEC 62366-1:2015 + ISO 26262-3:2018 controllability + ISO 2631-1 WBV + ISO 7730 thermal comfort + ISO 8995 lighting + WCAG 2.2 + SAE J2944 + NHTSA Driver Distraction Guidelines

In our engineering-guide series we described the battery with BMS and thermal-runaway intro, brake system, motor and controller, suspension, tyres, lighting and visibility, frame and fork, display + HMI, SMPS CC/CV charger, connector + wiring harness, IP protection, bearings with ISO 281 L10, stem and folding mechanism, deck, handgrip + lever + throttle, wheel as an assembly, threaded-joint engineering as the joining axis, thermal management as the heat-dissipation axis, EMC/EMI as the interference-mitigation axis, cybersecurity as the interconnect-trust axis, NVH as the acoustic-vibration-emission axis, functional safety as the safety-integrity axis, battery lifecycle engineering as the sustainability axis, repairability as the repairability axis, environmental robustness as the environmental-conditioning axis, privacy and personal-data protection as the privacy-preservation axis, reliability engineering as the reliability-prediction meta-axis and software & firmware engineering as the SW-process axis. These 29 engineering axes described subsystems, joining techniques, thermal and electromagnetic phenomena, safety, sustainability, repairability, environmental conditioning, privacy, reliability engineering and the SW process — a few of them episodically touched ergonomics (handgrip diameter in the hand axis, the ≥44 px tap-target in the display axis, brake-lever force in the brake axis), but none of them described the human-factors engineering toolkit itself: how the fit between rider and scooter is systematically engineered with respect to anthropometric coverage (P5–P95), postural envelope, control reach, display glance-time, cognitive workload, situation awareness and controllability for ASIL determination.

Human factors & ergonomics engineering is the human-machine fit axis of the entire e-scooter. It provides process standards (ISO 9241-210:2019 Human-Centred Design + ISO 9241-220:2019 HCD process + IEC 62366-1:2015 Usability Engineering Process — a methodology that, despite its medical-device origin, transfers cleanly to any safety-relevant human-machine system), usability definitions (ISO 9241-11:2018 — effectiveness/efficiency/satisfaction in a specified context of use), anthropometric databases (ISO 7250-1:2017 with 60+ body measurements + ISO/TR 7250-2:2010 statistical summaries for 20+ national populations + DIN 33402-2 + ANSUR II + CAESAR), postural norms (ISO 11226 static + ISO 11228 manual handling 4-part), workstation design (ISO 14738 anthropometric workstation), ergonomic-analysis methods (RULA + REBA + OWAS + NIOSH Lifting Equation + Snook–Ciriello tables + Strain Index + ACGIH TLV-HAL), display ergonomics (ISO 9241-300 series + ISO 9241-303:2011 visual ergonomics for LCDs), input-device ergonomics (ISO 9241-400 series + ANSI/HFES 100-2007 + ANSI/HFES 200-2008), vibration exposure (ISO 2631-1:1997 + ISO 2631-4 vibration in vehicles), thermal comfort (ISO 7730 PMV/PPD), lighting (ISO 8995), accessibility minima (WCAG 2.2 target size + contrast — the same W3C standard that feeds the dashboard as the touch-target oracle), driver-distraction operationalisations (SAE J2944:2015 lexicon + NHTSA Driver Distraction Guidelines DOT HS 811 547 with the 2-second and 12-second glance limits) and controllability classification for ASIL determination (ISO 26262-3:2018 Annex B: C0 simply controllable / C1 normally controllable / C2 difficult to control / C3 uncontrollable).

This is the thirtieth engineering-axis deep-dive in the guide series — and the thirteenth cross-cutting infrastructure axis (parallel to joining DT + heat-dissipation DV + interference-mitigation DX + interconnect-trust DZ + acoustic-vibration-emission EB + safety-integrity ED + sustainability EF + repairability EH + environmental-conditioning EJ + privacy-preservation EL + reliability-prediction EN + SW-process EP, and now human-machine-fit ER). Like the reliability and SW axes, the ergonomics axis has no separate “hardware” implementation — it is a methodology that defines which specific component from each of the 29 prior axes sits in front of you, how high the handlebar is, how far you must reach to the brake lever, how fast you can read the dashboard, and how much attention you have left over for the world around you.

1. Ergonomics ≠ UX ≠ HMI ≠ accessibility: a distinct axis

Ergonomics, UX, HMI and accessibility are often conflated but solve different problems:

Dimension	Ergonomics (ER)	UX	HMI / display (previously under display axis)	Accessibility (WCAG)
Question	Do human and machine fit, statically and dynamically?	Is it pleasant to use?	What does the machine show?	Can users with limited abilities use it?
Discipline	Anthropometry + biomechanics + cognitive psychology	Design + behavioural research	Embedded SW + display engineering	Accessibility standards
Foundation standard	ISO 9241 series + ISO 7250 + ISO 11226/11228	Nielsen heuristics + Norman DOET	ISO 9241-300/400 + automotive HMI guidelines	WCAG 2.2 + EN 301 549
Metric	Reach percentage, MVC, RPE, RULA score	SUS score, NPS, task completion	Glance time, character size, contrast	Pass/fail on 87 WCAG SC
Validation cycle	RULA/REBA/OWAS + NIOSH + ALT with humans	Usability test with users	HIL + glance-time study	Automated + manual SC test
Trigger	“Can a P5 female reach the brake?”	“Do users like it?”	“What’s on the dashboard?”	“Can a dim-vision user read it?”

A canonical worked example: the brake lever on the e-scooter handlebar. The HMI axis (display-engineering article) said: “The brake lever must emit tactile feedback at the click point.” UX said: “Users prefer a sporty look.” Accessibility said: “Target ≥44 × 44 CSS-px” (WCAG 2.2 — for touch controls). Ergonomics said something completely different and measurable: “the reach distance from the centre of the grip to the tip of the pulled lever shall lie within the P5-female index-finger length (~64 mm, ISO 7250-1 + ANSUR II) minus a safety margin for wet-glove operation = target reach ≤ 55 mm, and the lever pull force (DIN 33411-5 + ISO 9241-410) shall be ≤ 45 N for full-stop modulation, which sits at the P5 female grip strength (~150 N) × 30 % threshold per the Borg CR10 RPE 3 ‘moderate exertion’ level.”

Ergonomics does not intersect with UX (the “pleasant?” question), does not duplicate HMI (the “what to show?” question), and does not repeat accessibility (the “can I use it at all?” question). Its question is narrow and measurable: do human and machine fit, statically and dynamically, across the P5–P95 range?

2. ISO 9241 series — the foundation of ergonomics standards

ISO 9241 “Ergonomics of human-system interaction” is a multi-part series (~40 active parts) that has undergone two conceptual recalibrations: the original (1992–2008) focused on office workplaces with desktop computers; the 2018+ redesign extended scope to “interactive systems, including built environments, products and services”. The e-scooter enters scope as an “interactive system” (HMI + control loop + physical user).

Sub-series	Topic	Key parts
9241-1xx	Software ergonomics	9241-11:2018 Usability: Definitions and concepts; 9241-110:2020 Interaction principles; 9241-112:2017 Information presentation; 9241-125:2017 Visual presentation; 9241-129:2010 Individualization; 9241-143:2012 Forms
9241-2xx	HCD process	9241-210:2019 Human-centred design for interactive systems; 9241-220:2019 Processes for enabling, executing and assessing HCD within organisations; 9241-231:2017 Recommendations for tactile / haptic interactions
9241-3xx	Displays	9241-300:2008 Introduction; 9241-303:2011 Requirements for electronic visual displays; 9241-305:2008 Optical lab test methods; 9241-307:2008 Analysis and compliance test methods for electronic visual displays; 9241-310:2010 Visibility, aesthetics and ergonomics of pixel defects
9241-4xx	Physical input devices	9241-400:2007 Principles and requirements; 9241-410:2008 Design criteria for products; 9241-411:2012 Evaluation methods; 9241-420:2011 Selection procedures; 9241-460:2018 Tactile and haptic interactions
9241-5xx	Workplace ergonomics	9241-500:2018 Ergonomic principles for the design of workplaces
9241-9xx	Telework / mobile	9241-960:2017 Framework and guidance for gestures; 9241-810:2020 Robotic, intelligent, autonomous systems

Key definition (ISO 9241-11:2018 § 3.1.1):

Usability — extent to which a system, product or service can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.

The three dimensions of usability (operationalised in ISO 9241-11:2018 § 7):

• Effectiveness — accuracy and completeness with which users achieve specified goals; e-scooter operationalisation: percentage of successful emergency-stop attempts in a dry test from 3 m approach (target ≥ 95 % per ISO 26262-3 controllability C0).
• Efficiency — resources expended relative to results achieved; e-scooter operationalisation: average glance time at the dashboard per kilometre, per the NHTSA Visual-Manual Guidelines target ≤ 2 s single + ≤ 12 s aggregate.
• Satisfaction — the extent to which the user’s physical, cognitive and emotional responses meet the user’s needs and expectations; operationalisation: SUS score (System Usability Scale) ≥ 68 = “above average”, ≥ 80 = “excellent”.

Usability is always declared in a concrete context of use (ISO 9241-11:2018 § 3.1.6 — users + goals + tasks + resources + environment). The same e-scooter may have high usability in the context “dry urban commute by P50 male aged 25–45” and low usability in the context “wet-road emergency stop by P5 female with gloved hands”.

3. ISO 9241-210:2019 — six principles of human-centred design

ISO 9241-210:2019 “Human-centred design for interactive systems” replaced the outdated ISO 13407:1999. It is a process standard — it does not prescribe specific methods but lays out the six principles any product’s HCD process must satisfy:

1. Design is based on an explicit understanding of users, tasks and environment.
2. Users are involved at all stages of design and development.
3. Design is driven and refined by user-centred evaluation.
4. The process is iterative.
5. Design addresses the whole user experience (not just a single interaction).
6. The design team includes multidisciplinary skills and perspectives (ergonomics + UX + engineering + domain experts).

HCD process (ISO 9241-210:2019 § 5) — a 4-activity loop:

       Plan HCD process
              ↓
   ┌→ Understand context of use ─┐
   │           ↓                  │
   │   Specify user requirements  │
   │           ↓                  │
   │   Produce design solutions   │
   │           ↓                  │
   └── Evaluate against requirements
              ↓
       Solution meets requirements?
              ├── No → loop back
              └── Yes → deploy

Each iteration may reuse evidence from previous ones (an e-scooter design does not start from a blank slate — the ANSUR II + CAESAR + ISO/TR 7250-2 anthropometric databases already exist and reusing them is normative practice).

4. ISO 9241-110:2020 — seven interaction principles

ISO 9241-110:2020 replaced ISO 9241-10:1996 and ISO 9241-110:2006. It lists 7 interaction principles for any interactive system (operating an e-scooter unambiguously falls within this scope):

1. Suitability for the user’s tasks — the system supports the task effectively and efficiently; e-scooter operationalisation: the dashboard shows remaining range, switching cruise control is a one-press operation.
2. Self-descriptiveness — each step is understandable without external help; e-scooter: dashboard icons are stand-alone interpretable per ISO 7000:2019 / ISO 7001:2007.
3. Conformity with user expectations — the system behaves as the user expects (population stereotypes); e-scooter: the throttle rotates in the direction of the traffic stereotype, the brake lever is on the left/right per regional motorcycle convention.
4. Learnability — the system helps the user learn it; e-scooter: tutorial mode + speed-limited learning mode for the first 50 km.
5. Controllability — the user controls the pace and direction of interaction (≠ ISO 26262 controllability — here we mean UI control); e-scooter: cruise-control toggle, ride-mode selection, fallback when power assist is disabled.
6. Use-error robustness — the system makes use errors detectable and recoverable; e-scooter: throttle release is detected within 100 ms, brake lever recoverable after a soft lock.
7. User engagement — the system motivates safe and effective use; e-scooter: positive feedback when adhering to eco-mode or successful hazard avoidance.

Note principles 3, 5 and 6 in particular — they interface directly with functional safety (the ED axis), because population stereotypes and robust error recovery determine whether the user can recover from their own slip (an active error per Reason’s taxonomy) without causing a hazard.

5. ISO 7250-1:2017 + ISO/TR 7250-2:2010 — the anthropometric foundation

ISO 7250-1:2017 “Basic human body measurements for technological design” defines 60+ standard body measurements with anatomical landmarks. Confirmed in 2023 — still the active edition (corrected version 2025-04 with European Norm endorsement). The most important for the e-scooter:

Measurement	Definition	P5 female	P50 mixed	P95 male	Use case
Stature (height)	Perpendicular distance from floor to vertex top of head, standing	1 510 mm	1 720 mm	1 880 mm	Handlebar height range
Eye height	Vertex-to-eye − 30 mm; floor to outer canthus	1 405 mm	1 605 mm	1 765 mm	Forward sight-line, mirror placement
Shoulder (acromial) height	Floor to lateral acromion	1 240 mm	1 425 mm	1 565 mm	Handlebar grip zone
Elbow height	Floor to radiale (lateral elbow)	925 mm	1 075 mm	1 200 mm	Comfortable hand position
Knuckle height	Floor to distal head of metacarpal 3	685 mm	780 mm	870 mm	Brake-lever lowest reach
Hand length	Distal wrist crease to middle-fingertip	162 mm	185 mm	210 mm	Grip span design
Hand breadth (at metacarpals)	Across metacarpals 2–5	73 mm	84 mm	95 mm	Grip diameter
Grip diameter (inside)	Inner diameter of cylinder closed by index + thumb tip	38 mm	47 mm	58 mm	Handgrip outer-diameter target
Hip breadth (sitting)	Across the widest part of the hips	320 mm	365 mm	430 mm	Deck-width minimum
Foot length	Heel-most-posterior to longest toe	230 mm	260 mm	290 mm	Deck length min/max
Foot breadth	Across the metatarsals	86 mm	99 mm	113 mm	Deck width for stance

Sources (population pooled — ISO/TR 7250-2:2010 + ANSUR II 2012 US Army + CAESAR civilian North America/EU + DIN 33402-2:2020 German civilian):

• ANSUR II: 4 082 male soldiers + 1 986 female soldiers (US Army, 2012). Public-domain CSV via DTIC.mil.
• CAESAR: 4 400 subjects (US 2 400 + Italy 800 + Netherlands 1 200), 1998–2000. 3D scans plus manual measurements.
• DIN 33402-2:2020: ~3 000 German civilians aged 18–65.
• ISO/TR 7250-2:2010: statistical summaries for 14 national populations (US, UK, Germany, France, Netherlands, Japan, Korea, China, India, Mexico, Brazil, Italy, Spain, Australia).

Design rule of thumb (ISO 14738:2002 + ISO 9241-110:2020):

• Reach-critical parameter (brake-lever pull distance) → P5 female (smallest 5 %).
• Clearance-critical parameter (deck width to accommodate feet) → P95 male (largest 5 %).
• Adjustment-critical parameter (handlebar height) → adjustable from P5 female to P95 male (~580 mm adjustment range for handlebar from 880 mm to 1 460 mm above the deck — ergonomically valid for the standing rider; the sitting e-scooter is a separate axis).

P5–P95 covers 90 % of the adult population. Coverage of P1–P99 (98 %) requires a larger margin in the design and is mandatory for public shared scooters (Lime / Bird sharing class).

6. Standing-rider postural envelope — ISO 11226 + ISO 14738

The classical sitting workstation in ISO 14738 does not cover the standing rider of an e-scooter. Instead ISO 11226:2000 “Ergonomic evaluation of static working postures” is used — it defines unacceptable (red) / questionable (yellow) / acceptable (green) static posture ranges across the main joints.

Standing-rider neutral posture (rider stance, head up, hands on the handlebar, knees soft):

Joint	Acceptable (green)	Questionable (yellow)	Unacceptable (red)	E-scooter target
Neck flexion	0–20°	20–25°	> 25° sustained	5–10° (head-up gaze 5 m ahead)
Trunk flexion (forward)	0–20°	20–60°	> 60°	5–15° (slight forward stance)
Trunk lateral bend	0° (no bend)	0–10°	> 10° sustained	0° (symmetrical loading)
Shoulder flexion	0–20°	20–60°	> 60° sustained, no support	30–45° (relaxed, hands at hip-shoulder level)
Shoulder abduction	0–20°	20–60°	> 60° sustained	5–15° (handlebar width matches biacromial)
Elbow flexion	60–100°	100–135°	< 60° with force	100–135° (loose grip)
Wrist extension	0–30°	30–45°	> 45°	5–15° (handgrip slightly above wrist line)
Wrist ulnar deviation	0–10°	10–15°	> 15°	0–5° (handlebar 22–25 mm diameter, grip aligned with forearm)
Knee flexion	5–10° (soft)	0° (locked) or > 30°	locked-out or deep squat	5–10° (soft knee, dynamic shock absorption)
Ankle dorsiflexion	0–10°	10–20°	> 20° sustained	0–5° (flat foot on deck)

Stability cone (per CAREN gait studies + Pheasant 1996 Bodyspace):

• AP stability (anterior–posterior): foot length × 0.8 = ~200 mm — target deck length ≥ 220 mm for a P50 user without heel-toe lockup.
• ML stability (medio-lateral): hip breadth × 0.4 = ~145 mm — target deck width ≥ 160 mm for a P50 user without heel-toe stagger.
• Combined stability cone: a P5 female may lose stability at > 4° lateral tilt + cornering G-load > 0.4 g; a P95 male at > 6° + 0.6 g (greater body mass = larger inertial restoring torque).

How wear-out correlates with the EN reliability axis: a standing posture with knees locked > 30 minutes triggers venous pooling and peripheral fatigue (β > 1 in a Weibull analysis of attention loss); a soft-knee dynamic stance triggers reactive small-muscle co-contraction that maintains attention engagement (constant-failure-rate regime — β ≈ 1).

7. Control reach and lever force — handgrip + brake lever + throttle

The e-scooter has three main control surfaces on the handlebar:

A. Handgrip (covered in the hand axis already — here, the ergonomic-fit aspect). ISO 9241-410:2008 + DIN 33411-5:1999 grip-strength data:

• Outer diameter: 30–35 mm (sweet spot for hand breadth 73–95 mm; finger-thumb opposition ≈ 60 % of hand breadth ≈ 50 mm).
• Effective length (grip-engagement zone): ≥ hand breadth + 20 mm = ≥ 115 mm (P95 male).
• Surface friction: dry COF ≥ 0.6 on rubber compound; wet COF ≥ 0.4 (per DIN 53516 abrasion + Schallamach friction tests). Less than 0.3 → grip-slip risk; greater than 0.8 → blister formation in sustained riding.

B. Brake lever. ISO 9241-411:2012 + DIN 33411-5 + CIE 17.4 “luminaires control” pull-force range:

• Reach distance (from grip centre to the tip of the pulled lever in the ready position): ≤ 55 mm (P5-female index-finger length 64 mm − 15 % glove margin).
• Full-stop pull distance (lever travel): 35–55 mm (ergonomic range, non-fatiguing even with 30 emergency stops per year).
• Initial activation force: 5–15 N (light initial bite — minimum tactile detection per Weber’s law thresholds).
• Full-stop pull force: 30–60 N (P5-female grip strength of 150 N × 0.3 = 45 N median target). Older scooters with friction brakes often required 80–120 N — questionable for a P5 female with gloves.
• Modulation gradient: roughly linear, total pull travel ≥ 25 mm for precise control (Fitts’s law: as target width increases, movement time decreases).

C. Throttle (thumb throttle, lever throttle, twist throttle). ISO 9241-410:2008:

• Thumb throttle: peak thumb-tip force 25–60 N (P5 female 25 N per DIN 33411-5); throttle spring return force ≤ 8 N (avoid sustained MVC > 15 % during cruise — fatigue threshold per Borg CR10).
• Twist throttle: rotation 20°–35° from idle to full throttle; torque ≤ 0.25 N·m peak (light forearm rotation, ulnar deviation ≤ 10° per ISO 11226 green zone).
• Lever throttle (squeeze): pull distance 15–35 mm; force 10–25 N — similar to brake lever but with a lower MVC because cruise applies sustained engagement.

Adherence to these ranges is verified by HALT with humans (Hobbs method, EN axis step 13): 6 cycles × 8 hours × mixed-percentile users (2 P5 female + 2 P50 mixed + 2 P95 male) with RULA scoring after each cycle. RULA score 1–2 = acceptable; 3–4 = further investigation; 5–6 = change soon; 7 = immediate change required.

8. Display ergonomics — glance time + character size + viewing distance

The e-scooter dashboard is an automotive-like glance-time problem. The driver-distraction literature (NHTSA + SAE J2944) establishes operational glance limits:

• Single glance ≤ 2.0 s (NHTSA Visual-Manual Guidelines 2013) — otherwise cognitive tunnelling blocks peripheral processing.
• Total glance ≤ 12.0 s for a single task (NHTSA cap).
• Glance accumulation ≤ 50 % of road time (eyes-off-road percentage).

Achieving these limits requires:

Angular character size (ISO 9241-303:2011 § 5.3.2):

• Minimum legible = 20 arc-min in height (i.e. 1/3 of a degree of visual arc) at 100 % legibility.
• Comfortable reading = 24–30 arc-min.
• Computation: character_height_mm = (viewing_distance_mm × tan(arc_min × π / (60 × 180))).
• For a viewing distance of 600–800 mm (handlebar-mounted dash to eye, standing rider): minimum character height 3.5–4.7 mm, 4.2–5.6 mm for comfortable reading.

Standard dashboard glyphs (e-scooter):

• Speed digits: 12–18 mm (comfortable reading at 700 mm).
• Battery percentage: 6–9 mm.
• Mode indicator: 6–9 mm + icon ≥ 8 × 8 mm.
• Warning icons: 10 × 10 mm + ISO 7000 / ISO 7001 standardised pictograms.

Luminance contrast (ISO 9241-303:2011 § 5.5):

• Daytime (photopic, ≥ 100 cd/m² ambient): display ≥ 500 cd/m² for legibility; ≥ 800 cd/m² for direct-sunlight readability.
• Night (mesopic, 0.01–3 cd/m² ambient): display ≤ 30 cd/m² (avoid pupil constriction and dark-adaptation loss); automatic dimming mandatory.
• Contrast ratio (foreground-to-background luminance): ≥ 5:1 character-to-background for AA legibility per WCAG 2.2 SC 1.4.3; ≥ 7:1 for AAA SC 1.4.6.
• Veiling glare (specular reflection from the sun): mitigated by an anti-reflective coating (≤ 2 % reflectance per ASTM E430) + matte finish + optionally a polarised filter.

Viewing geometry:

• Vertical viewing angle: 0°–25° below horizontal (eye line); the e-scooter dashboard on the stem top typically sits 30°–40° below horizontal — borderline acceptable per ISO 9241-303. Better — stem-mounted dashboard angle adjustment of ±10°.
• Horizontal viewing angle: ±15° from forward line.
• Reading distance: 500–800 mm comfortable; below 400 mm — head-down posture issues (neck flexion > 25° red zone).

9. Cognitive ergonomics — workload + situation awareness + attention

Cognitive-ergonomics standards are less formal than anthropometric ones — mostly frameworks and assessment tools rather than requirements specs:

Workload assessment. NASA-TLX (Task Load Index, Hart & Staveland 1988) — a 6-dimension subjective rating (mental demand, physical demand, temporal demand, performance, effort, frustration) on a 21-point bipolar scale. NASA-TLX is the default workload measure in safety-critical interaction research (aviation, automotive, medical). E-scooter scenarios: low workload (eco-mode cruise on a quiet bikeway) vs high workload (rush-hour urban junction). High workload reduces situation awareness — the interdependence is measured by crossing NASA-TLX with SAGAT.

Situation awareness (Endsley 1995 model, ISO 11064-1 reference):

• Level 1 SA — perception of elements in the environment (other vehicles, pedestrians, road surface, dashboard alerts).
• Level 2 SA — comprehension of the current situation (combining perceptions into a pattern — “the cyclist ahead is braking”).
• Level 3 SA — projection of future status (anticipating “what will happen in 3 seconds”).

E-scooter operation requires high Level 3 SA — recovery time from detected hazard to physical manoeuvre = perception (200 ms) + decision (300–500 ms) + action (200–400 ms) ≈ 1 s. Brake distance at 25 km/h cruise on dry asphalt ≈ 7 m. SA failure → late detection → critical reduction of usable brake distance.

Attention failures (Wickens + Hollands 2000):

• Attentional capture — a salient stimulus pulls attention (a loud alert → eyes on dash → eyes off road).
• Attention tunnelling — sustained focus on a single source (cruise-control comfort → reduced scanning of the road environment).
• Inattentional blindness — failure to notice an unexpected stimulus in the full attentional field.
• Change blindness — failure to notice a change between two scenes.

Mitigation — interaction-design principles:

• Mandatory acknowledgement only for life-safety alerts (low cry-wolf rate).
• Multimodal cueing — visual + auditory + haptic — for critical alerts (per ISO 9241-460:2018 tactile/haptic).
• Pre-emptive cueing — early-warning audio precedes the visual icon (advance attention shift).
• Workload-adaptive interfaces — suppress non-critical info during high-workload periods (eco-mode tip → suppressed at high speed + cornering G-load > 0.3 g).

10. ISO 26262-3 controllability (C0/C1/C2/C3) — interface to functional safety

ISO 26262-3:2018 “Concept phase” Annex B defines controllability as one of the three dimensions of the HARA (Hazard Analysis and Risk Assessment) alongside exposure (E0–E4) and severity (S0–S3). Together the three dimensions multiplicatively determine the ASIL (Automotive Safety Integrity Level QM/A/B/C/D).

Class	Definition (ISO 26262-3 Annex B)	E-scooter example
C0	Controllable in general — > 99 % of drivers can avoid harm during the specific operating situation	Normal acceleration delay after throttle release — the driver waits for the response and corrects
C1	Simply controllable — 99 % of drivers can avoid harm	Sudden cruise-control engagement at low speed (driver applies brake)
C2	Normally controllable — 90 % of drivers can avoid harm	Loss of regenerative braking during descent (driver shifts to mechanical brake within 1 s)
C3	Difficult to control or uncontrollable — fewer than 90 % of drivers can avoid harm	Unintended full throttle in a corner-leaning posture (controllability < 90 % — single-axis loss of stability)

Determining the C-class requires expert evaluation — typically 10+ experienced engineers / test riders score the scenario, with conservative aggregation (median + 1σ shift toward less controllable). A common bias is overestimation of one’s own population’s controllability vs inexperienced / elderly / wet-condition populations.

Controllability is correctly measured only after the ergonomic-fit check of the previous sections — otherwise the C rating reflects a mismatch (e.g. a P5 female cannot reach the brake lever) rather than the inherent controllability of the system. That is why ergonomics is a prerequisite for the ISO 26262 HARA.

ASIL determination example: “Loss of mechanical brake while descending at 25 km/h on a 5 % grade”.

• S (severity): S2 — severe injuries possible (AIS 3–5 scale).
• E (exposure): E3 — medium probability (descent ride > 5 % of total ride time).
• C (controllability): C2 — normally controllable through transition to regenerative braking + dynamic foot-down + steering deceleration; ~ 90 % of riders manage without impact.
• ASIL = S2 × E3 × C2 = ASIL B (per ISO 26262-3 Table 4) — a mid-level safety-integrity target for brake-system redundancy.

11. ISO 2631-1 — whole-body vibration exposure limits

ISO 2631-1:1997 + ISO 2631-4:2001 regulate whole-body vibration (WBV) exposure. E-scooter cruise emits WBV in three axes (X — fore-aft, Y — lateral, Z — vertical).

Weighting: the kerb-shock + cobble vibration spectrum (4–80 Hz dominant) is filtered through the W_d (horizontal) and W_k (vertical) frequency-weighting filters per ISO 2631-1 Annex.

Daily exposure metric:

A(8) = a_w × √(t / 8 hours)

where a_w = weighted RMS acceleration (m/s²) and t = daily exposure time.

Action / limit values (EU Directive 2002/44/EC):

• EAV (Exposure Action Value): A(8) = 0.5 m/s² — monitoring + action required.
• ELV (Exposure Limit Value): A(8) = 1.15 m/s² — must not be exceeded.

E-scooter measurement (typical asphalt, suspension OFF, foot on deck): a_w ≈ 0.6–1.2 m/s² on a rough surface, 0.2–0.4 m/s² on a smooth one. An 8-hour daily cruise → close to the ELV → suspension becomes an occupational ergonomic mitigation, not just a comfort feature. Cumulative WBV exposure is linked to lower-back disorders (LBD), carpal tunnel syndrome (hand-arm vibration), digital ischaemia (“white finger” — separate ISO 5349-1:2001 hand-arm scope).

12. WCAG 2.2 + accessibility as the interface to ergonomics

WCAG 2.2 (October 2023) added 9 new success criteria; the ones relevant to the e-scooter HMI / companion app:

• SC 2.5.5 Target Size (Enhanced) AAA: ≥ 44 × 44 CSS-px for pointer targets. In the e-scooter dashboard context — a physical touch screen or hard button — target ≥ 12 × 12 mm (no scale factor; for glove-wearing — 15 × 15 mm).
• SC 2.5.8 Target Size (Minimum) AA: ≥ 24 × 24 CSS-px — fallback minimum.
• SC 1.4.3 Contrast (Minimum) AA: 4.5:1 for text < 18 pt / < 14 pt bold; 3:1 for larger text.
• SC 1.4.6 Contrast (Enhanced) AAA: 7:1 / 4.5:1.
• SC 1.4.11 Non-text Contrast AA: 3:1 for UI components + graphical objects.

WCAG operationalises accessibility as a superset of usability for non-typical populations — older adults (presbyopia + arthritis), motor-impaired, vision-impaired, hearing-impaired. WCAG compliance partially covers anthropometric outliers already (advanced age = different anthropometric percentile dynamics).

13. Cross-axis matrix — ergonomics relevance to the 29 prior axes

Engineering axis (prior)	Ergonomic concept (this axis additionally constrains)
DT Joining (fastener torque)	Owner-serviceable joint torque ≤ 30 N·m for P5 female with 200 mm wrench arm
DV Heat dissipation (thermal)	Heat-emission zone of handlebar ≤ 40 °C contact temperature per ISO 13732-1
DX EMC/EMI	High-pitched audio alerts ≥ 65 dB in the dominant 2–4 kHz hearing-sensitivity band
DZ Cybersecurity	User-facing security UI with reading time ≤ 30 s, plain-language ≥ 9th-grade
EB NVH	Tyre/motor noise contributes 50–80 dBA at rider ear; subjective annoyance modulated by harmonic content
ED Functional safety	C rating impossible without an ergonomic-fit prerequisite (this is section 10)
EF Sustainability	Repair-friendly tool types reduce cognitive load on the DIY owner
EH Repairability	Cover-screw access targets reach + tool-engagement per ISO 14738
EJ Environmental conditioning	Glove-wearing changes hand breadth + grip friction — design margin
EL Privacy	Onboard consent dialog readable within 2 s glance limits
EN Reliability	RPN-FMEA for use error adds a controllability dimension
EP SW process	HMI software ASIL B (typical) → MISRA C compliance + ASIL-B partition
Battery / BMS	Charging-port reach ≤ 600 mm from the ground (deck-storage case)
Brake system	Lever force (this article) determines pad/disc sizing back-stop
Motor + controller	Throttle response curve has a perception-action coupling threshold (200 ms)
Suspension	WBV mitigation per ISO 2631 (section 11) — suspension is an ergonomic intervention
Tyre	Rolling resistance affects expected pedalling effort; grip loss is a C2/C3 controllability scenario
Lighting	ISO 8995 luminance + WCAG contrast — joint optic-ergonomic
Frame + fork	Stem reach + handlebar offset determine shoulder + elbow joint posture
HMI / display	Glance time + character size (sections 8 + 12 — joint ownership)
Charger	Charging-port plug force ≤ 30 N for P5 female
Connector + harness	Connector mating direction follows the population-stereotyped twist (clockwise tight)
IP protection	Cover-removal procedure tool-free for IP-rated user-serviceable parts
Bearing	Re-grease intervals communicated in calendar time + cognitive-easy units
Stem + folding	Folding-mechanism activation force ≤ 60 N per ANSI/HFES
Deck	Width + length ≥ section 6 anthropometric stability cone
Handgrip + lever + throttle	Sections 5 + 7 own directly
Wheel + rim	Carry handle when wheels off floor — grip-force range section 5
Fastener (joint)	Same as DT — owner-serviceable joint torque

Each prior axis acquires an ergonomic constraint as a post-condition of its own decision (e.g. the brake-system designs pad/disc geometry to deliver target friction, BUT ergonomics constrains the brake-lever force-pull characteristic curve, which in turn feeds back into the required pad/disc μ × area capacity).

14. Owner-level ergonomic-fit practices

8-step DIY ergonomic-fit checklist:

1. Stand on a level surface, hands at your sides, soft knees. Measure your stature (barefoot).
2. Adjust the handlebar height to elbow height ± 50 mm (e.g. for a P50 male elbow of 1 075 mm — handlebar 1 025–1 125 mm above the floor).
3. Test brake-lever reach — the brake lever should be reachable from the resting position with index/middle finger without full extension. If extension is required — adjust the lever clamp position.
4. Test brake-lever pull force — full stop should be achievable with 3 of 4 fingers without a pinned-out grip; if an ulnar-deviated grip is needed — reposition the lever clamp.
5. Wear the gloves you actually ride in — wet-grip / cold-grip operations may change hand clearance.
6. Posture check at 25 km/h on flat ground — knees soft, trunk slightly forward (5–15°), shoulders relaxed not shrugged, gaze 5 m ahead.
7. Vibration sanity check — after 10 km on rough urban surface: do you feel numbness in your fingers or feet? If yes — suspension service / handgrip change.
8. Glance discipline — adopt the 2-second glance rule at the dashboard. If you consistently need > 2 s — rearrange the visible information or change the dashboard configuration.

A word for older users (P95 in age, not lower body percentile): expect slower reaction time (RT increases ~ 1 ms / year after age 25); decreased grip strength (loss ~ 1 % / year after age 50); presbyopia after ~ 45 years (near-vision degradation). E-scooter ergonomic-fit for a 65+ adult requires larger dashboard glyphs, lower top-speed gating, enhanced contrast and ride-time limits for cumulative fatigue.

15. Future axes — where the axis series will extend

Like reliability (EN) and SW process (EP), ergonomics (ER) is a process axis with a methodology overlay on every prior engineering axis. Other candidate future axes:

• Manufacturing quality (IATF 16949 + APQP + PPAP + SPC + MSA + 8D) — the production-process axis. How a concrete exemplar of those scooter parts that passed all previous axes is actually produced.
• Risk management (ISO 31000:2018 + ISO/IEC 31010:2019 + Bowtie + ALARP + LOPA) — a risk meta-axis above HARA + TARA + reliability FMEA.
• V&V engineering as a standalone axis — currently split between functional safety (ED) and SW process (EP); IEEE 1012 is a separate standard.
• Production logistics & supply chain (ISO 28000 + C-TPAT + AEO + UFLPA compliance) — a flow axis.

None of these is a prerequisite for the ergonomics axis — publication order is left to author judgement, with the main criterion “what right now is most valuable to the e-scooter power user”.

16. Reuse — the ergonomic concept as a pattern

Cross-cutting infrastructure axis pattern v13 — a thirteen-instance set (joining DT + heat-dissipation DV + interference-mitigation DX + interconnect-trust DZ + acoustic-vibration-emission EB + safety-integrity ED + sustainability EF + repairability EH + environmental-conditioning EJ + privacy-preservation EL + reliability-prediction EN + SW-process EP + human-machine-fit ER).

Ergonomics, like SW process and reliability, is a methodology layered over all others rather than a separate subsystem:

• Reliability (EN) described the formal apparatus to predict and validate the reliability of every prior axis.
• SW process (EP) described the formal apparatus to build and deliver the firmware that implements decisions from each of the 28 axes.
• Ergonomics (ER) describes the formal apparatus to fit the human to each of the 29 prior axes statically and dynamically — without it, controllability ratings (ISO 26262), accessibility compliance (WCAG) and usability scores (ISO 9241-11) remain qualitative claims without operationalisable evidence.

Recap — 10 points:

1. Ergonomics ≠ UX ≠ HMI ≠ accessibility — its own scope, its own metrics, its own standards.
2. ISO 9241 series — the foundation, 6 sub-series (1xx software, 2xx HCD process, 3xx displays, 4xx physical input, 5xx workplace, 9xx mobile/intelligent).
3. ISO 9241-11:2018 — usability = effectiveness × efficiency × satisfaction in a specified context of use.
4. ISO 9241-210:2019 — HCD process with a 4-activity iterative loop.
5. ISO 7250-1:2017 — 60+ standard body measurements; rule of thumb: reach-critical to P5 female, clearance-critical to P95 male.
6. ISO 11226 + ISO 14738 — postural envelope for the standing-rider e-scooter — a 10-joint matrix of acceptable/questionable/unacceptable ranges.
7. ISO 9241-303:2011 — 20 arc-min minimum legible character size = 3.5–4.7 mm at a 600–800 mm viewing distance.
8. NHTSA Visual-Manual Guidelines: 2 s single + 12 s aggregate glance limits — the operational definition of “the driver did not become distracted”.
9. ISO 26262-3 controllability C0/C1/C2/C3 — ergonomics is a prerequisite for ASIL determination.
10. WCAG 2.2 — a superset of usability for non-typical populations; the 44 × 44 px tap-target + 4.5:1 contrast minima already partially cover anthropometric outliers.

---

ENG-first sources (0 Russian, 25+ official):

• ISO 9241-11:2018 Ergonomics of human-system interaction — Part 11: Usability: Definitions and concepts — iso.org/standard/63500.html
• ISO 9241-110:2020 Ergonomics of human-system interaction — Part 110: Interaction principles — iso.org/standard/75258.html
• ISO 9241-210:2019 Ergonomics of human-system interaction — Part 210: Human-centred design for interactive systems — iso.org/standard/77520.html
• ISO 9241-220:2019 Ergonomics of human-system interaction — Part 220: Processes for enabling, executing and assessing human-centred design within organizations — iso.org/standard/63462.html
• ISO 9241-303:2011 Ergonomics of human-system interaction — Part 303: Requirements for electronic visual displays — iso.org/standard/57992.html
• ISO 9241-410:2008 Ergonomics of human-system interaction — Part 410: Design criteria for physical input devices — iso.org/standard/38899.html
• ISO 9241-411:2012 Ergonomics of human-system interaction — Part 411: Evaluation methods for the design of physical input devices — iso.org/standard/54106.html
• ISO 9241-460:2018 Ergonomics of human-system interaction — Part 460: Guidelines on the ergonomics of touch screens and tactile displays — iso.org/standard/74333.html
• ISO 7250-1:2017 Basic human body measurements for technological design — Part 1: Body measurement definitions and landmarks (corrected 2025-04) — iso.org/standard/65246.html
• ISO/TR 7250-2:2010 Basic human body measurements for technological design — Part 2: Statistical summaries of body measurements from national populations — iso.org/standard/41249.html
• ISO 11226:2000 / Amd 1:2006 Ergonomics — Evaluation of static working postures — iso.org/standard/25573.html
• ISO 11228-1:2021 Ergonomics — Manual handling — Part 1: Lifting, lowering and carrying — iso.org/standard/76820.html
• ISO 11228-2:2007 Ergonomics — Manual handling — Part 2: Pushing and pulling — iso.org/standard/26521.html
• ISO 11228-3:2007 Ergonomics — Manual handling — Part 3: Handling of low loads at high frequency — iso.org/standard/26522.html
• ISO 14738:2002 Safety of machinery — Anthropometric requirements for the design of workstations at machinery — iso.org/standard/27556.html
• ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations (Human Factors and Ergonomics Society) — hfes.org/Publications/Technical-Standards
• ANSI/HFES 200-2008 Human Factors Engineering of Software User Interfaces — hfes.org/Publications/Technical-Standards
• DIN 33402-2:2020 Ergonomics — Body dimensions of people — Part 2: Values — din.de
• IEC 62366-1:2015 + Amd 1:2020 Medical devices — Part 1: Application of usability engineering to medical devices — iec.ch/publications
• IEC/TR 62366-2:2016 Medical devices — Part 2: Guidance on the application of usability engineering to medical devices — iec.ch/publications
• ISO 26262-3:2018 Road vehicles — Functional safety — Part 3: Concept phase (controllability Annex B) — iso.org/standard/68385.html
• ISO 2631-1:1997 + Amd 1:2010 Mechanical vibration and shock — Evaluation of human exposure to whole-body vibration — Part 1: General requirements — iso.org/standard/7612.html
• ISO 2631-4:2001 Whole-body vibration — Part 4: Vibration in fixed-guideway transport systems — iso.org/standard/32178.html
• ISO 7730:2005 Ergonomics of the thermal environment — Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices — iso.org/standard/39155.html
• ISO 8995-1:2002 (CIE S 008/E:2001) Lighting of work places — Part 1: Indoor — iso.org/standard/28857.html
• ISO 13732-1:2006 Ergonomics of the thermal environment — Methods for the assessment of human responses to contact with surfaces — Part 1: Hot surfaces — iso.org/standard/43558.html
• W3C Web Content Accessibility Guidelines (WCAG) 2.2 (W3C Recommendation 5 October 2023) — w3.org/TR/WCAG22/
• SAE J2944:2015 Operational Definitions of Driving Performance Measures and Statistics — sae.org/standards/content/j2944_201506
• NHTSA Visual-Manual NHTSA Driver Distraction Guidelines for In-Vehicle Electronic Devices (DOT HS 811 547, April 2013) — nhtsa.gov/sites/nhtsa.gov/files/811547.pdf
• US Army ANSUR II — Anthropometric Survey of U.S. Army Personnel: Methods and Summary Statistics (2012) — dtic.mil/citations/AD1473587
• CAESAR Civilian American and European Surface Anthropometry Resource Project — humanics-es.com/CAESARvol1.pdf
• N. A. Stanton et al. Human Factors Methods: A Practical Guide for Engineering and Design, 3rd ed., CRC Press, 2017.
• C. D. Wickens, J. G. Hollands Engineering Psychology and Human Performance, 4th ed., Pearson, 2012.
• M. R. Endsley “Toward a Theory of Situation Awareness in Dynamic Systems”, Human Factors 37(1), 1995, pp. 32–64. DOI 10.1518/001872095779049543.
• S. G. Hart, L. E. Staveland “Development of NASA-TLX: Results of empirical and theoretical research” in P. A. Hancock & N. Meshkati (Eds.), Human Mental Workload, North-Holland, 1988.
• J. Reason Human Error, Cambridge University Press, 1990.
• J. Rasmussen “Skills, Rules, and Knowledge: Signals, Signs, and Symbols, and Other Distinctions in Human Performance Models”, IEEE Trans. SMC 13(3), 1983.
• J. Nielsen Usability Engineering, Academic Press, 1993; 10 Usability Heuristics for User Interface Design (1994 revised 2024) — nngroup.com/articles/ten-usability-heuristics.
• D. A. Norman The Design of Everyday Things, revised ed., Basic Books, 2013.
• S. Pheasant, C. M. Haslegrave Bodyspace: Anthropometry, Ergonomics and the Design of Work, 3rd ed., CRC Press, 2005.
• L. McAtamney, E. N. Corlett “RULA: A survey method for the investigation of work-related upper limb disorders”, Applied Ergonomics 24(2), 1993, pp. 91–99.
• S. Hignett, L. McAtamney “Rapid Entire Body Assessment (REBA)”, Applied Ergonomics 31(2), 2000, pp. 201–205.
• T. R. Waters, V. Putz-Anderson, A. Garg “Revised NIOSH equation for the design and evaluation of manual lifting tasks”, Ergonomics 36(7), 1993, pp. 749–776.
• EU Directive 2002/44/EC On the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (vibration) — eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32002L0044.