Why Entryway Floors Get Slippery: Wet vs Dry Friction, Mats & Slip Physics

System Context

Flooring is the entryway layer that converts upstream moisture and debris into immediate traction outcomes at the foot–surface interface. It sits downstream of environment and seating, and upstream of lighting and circulation, meaning flooring failures propagate instantly into slips and falls.

In this series, upstream risk is defined in Entryway Layout / Transition Design (Article #1: Environment layer) and load amplification is covered in Entryway Seating Engineering (Article #2: Seating layer). This article explains how friction variability, threshold geometry, and mat interfaces convert those upstream inputs into measurable slip risk.

I. Concept Reframe

Entryway flooring failure is defined as the collapse of friction during rapid environmental state changes. This occurs when the available Dynamic Coefficient of Friction (DCOF) drops below the friction required by normal gait during the transition from wet to dry zones. The result is a traction loss event that can outpace balance correction.

The engineering mistake is treating “slippery” as a material attribute. In entryways, the dominant failure mode is friction instability over time—a moving target created by moisture films, debris, thresholds, and mat interfaces.

Before using any metric, the system question is simple: does the floor deliver a consistent friction state across the first steps, or does it oscillate between high and low traction states?

II. What Is Entryway Floor Friction?

Entryway floor friction is the resistance that allows controlled walking during rapid transitions from outdoor to indoor conditions. Many homeowners ask why entryway floors are slippery when wet; the answer lies in how moisture alters surface contact and reduces the Dynamic Coefficient of Friction (DCOF) faster than balance can adapt. This is a timing problem as much as a material problem.

Friction is not a single value; it is a state that depends on surface microtexture, contamination (water, salts, oils), contact pressure during heel strike, and how quickly the wet film is displaced. In entryways, these variables change within seconds, so the same floor can behave “safe” on one step and “unsafe” on the next.

The table below answers: When does this fail? It compares how wet and dry states shift traction variables that determine DCOF and real-world slip resistance. The goal is not to label materials “good” or “bad,” but to identify when friction becomes mechanically unstable.

Dry vs Wet Entryway Flooring (Engineering Comparison)

The practical conclusion is that wet entryways are not merely “lower friction.” They are less predictable friction, which increases slip probability during normal gait timing.

Measurement clarity: DCOF is a lab-measured traction indicator, but real entryway risk is governed by the film state (water/contaminants), the shoe sole, and the timing of wet→dry transitions inside the first-step window.

In controlled gait and slip-response literature, balance correction is often discussed in the ~150–250 millisecond range (typical reported values for rapid corrective response). When friction conditions change within this window, corrective response can be mechanically constrained.

III. Geometry / Fit Variable

Mechanism summary: entryway slip risk rises when surface microtexture is temporarily neutralized by films and peak heel-strike pressure, not when materials are inherently “smooth.”

Foot–floor contact geometry is defined by the contact patch, the local surface microtexture, and the pressure distribution across the stance phase. In entryways, the highest-risk moment is typically heel strike because pressure concentrates and the wet film can persist under load. Geometry matters because friction is produced at the interface, not in the material label.

The mechanism is contact-state mismatch: microtexture that works when dry can become functionally smooth when a continuous water film forms, while contaminants can “fill” texture valleys and reduce wet-floor traction. The same shoe can experience different traction on adjacent zones (door-side wet patch vs interior dry patch) within a single step sequence.

Entryway Contaminant Taxonomy (What the “Wet Film” Really Is)

“Wet” entryways are not all the same. Different contaminants form different films, which changes how quickly microtexture can re-engage and how stable traction remains across steps.

These interface variables determine when the system crosses a safe friction threshold. Changes in texture, contamination, or pressure concentration directly alter DCOF and drive wet-floor friction loss.

The conclusion is that slip risk rises when surface microtexture is functionally neutralized by water films or debris while peak contact pressures remain high. In this condition, the floor has not become unsafe because it is smooth, but because the mechanisms that normally generate friction are temporarily disabled at the exact moment traction is required.

This failure mode concentrates near wet/dry boundaries, such as immediately inside an exterior door. Within the first steps indoors, the foot can experience multiple friction states inside a single gait cycle: planned traction → friction collapse → abrupt recovery. When this transition occurs faster than balance can respond, corrective capacity becomes mechanically constrained.

Engineering takeaway: entryway slips occur when friction transitions outpace gait adaptation—most often during wet→dry changes under heel-strike load—rather than because the floor is inherently smooth.

The mechanism unfolds as a short, repeatable sequence. First, the nervous system plans the step assuming outdoor traction. Second, heel strike lands on a thin wet or contaminated film, compressing the interface and preventing microtexture engagement. Third, the next step suddenly encounters a drier zone where friction returns, forcing rapid posture correction across mismatched traction states.

From an engineering perspective, the critical issue is therefore friction variability under peak load. Heel strike concentrates force over a small contact patch; if a continuous film is present, the available DCOF collapses precisely when shear demand is highest. Even brief lubrication at this moment can initiate a micro-slip that propagates downstream.

In practical terms, this explains why a floor that feels stable when fully dry can still produce slips during entry. A narrow wet strip just inside the door may be visually subtle, yet it creates a localized lubrication zone exactly where the first indoor step lands. The following step may regain friction abruptly, forcing correction while the body is still transitioning tasks.

The highest-risk situations occur when this friction variability overlaps with increased task demand. Carrying bags, turning to close the door, or stepping over a threshold raises the friction required for stability. If heel strike occurs on a partially wet surface during these actions, pressure spikes compress the film, microtexture fails to engage, and the body must manage clearance, direction change, and balance recovery simultaneously.

Chicago winter amplification: cold-weather entryways intensify this mechanism through salt and slush contamination. Deicing salts increase water viscosity and inhibit clean film breakup, while slush introduces semi-solid particles that intermittently block microtexture. The result is a surface that alternates between lubrication and sudden grip across consecutive steps, deepening friction collapse and reducing recovery predictability.

In system terms, effective entryway safety depends on maintaining predictable friction across the first steps, not maximizing surface roughness in isolated zones. When traction changes abruptly under load—especially in winter conditions— even small disturbances can escalate into slips. The risk is created by the sequence of events, not by any single material choice.

IV. Stability / Reserve Variable

In engineering terms: entryways fail when required friction rises (turning, braking, carrying) at the same moment available wet friction becomes unstable.

Friction margin is defined as the difference between available friction and the friction required by the current gait task. Slip events occur when available wet DCOF drops below the required friction during walking, turning, or carrying loads in the entryway. Stability reserve shrinks as friction margin approaches zero.

The mechanism is not “low traction” in the abstract; it is a shortfall relative to the task: stepping over a threshold, pivoting to hang a coat, or entering while holding bags increases required friction and reduces recovery time. The floor can be acceptable for straight walking but fail during a turn or transition event.

Required Friction Drivers (Why REQ Spikes in Entryways)

In AV/REQ terms, entryways are dangerous because REQ rises quickly: the tasks you do at the door (turning, braking, stepping up/down, carrying) increase shear demand exactly when available friction is least stable.

The table shows why this failure appears in entryways but not elsewhere. Standards-based wet DCOF targets assume steady, level walking, while real entryway tasks increase required friction through turns, thresholds, and load carrying. As these demands rise, the available safety margin narrows, reframing flooring performance as an engineering safety-margin problem, not a material or style choice.

Treat ANSI A326.3 values as a baseline for comparison: field safety still depends on the shoe, the contaminant film, and whether traction changes abruptly inside the first-step window.

Standards-based wet DCOF targets function as a baseline for steady, level walking, but entryway conditions routinely impose higher friction demands. Turns, thresholds, and rapid direction changes increase shear forces at the foot–floor interface, reducing the available safety margin. In these moments, a surface that meets minimum standards can still experience traction shortfalls during normal entry tasks, especially within the first steps indoors.

V. Transition Event

Threshold transition failure is defined as a gait re-planning event occurring during friction instability. When vertical change or bevel geometry forces foot clearance adjustments while traction is already changing, slip and trip risks couple together. This is why thresholds become “risk multipliers” in wet entryways.

The mechanism is dual-load: the body spends balance capacity on clearance and timing while the friction state is uncertain. A small height change can be benign on a dry, consistent surface, but become hazardous when the contact state is wet and the required friction is elevated during the same step sequence.

The table shows which geometric variables push the entryway system past a safe threshold. Common height and bevel limits are used to identify when a threshold becomes a balance disruption rather than a neutral transition, independent of floor material. These geometric breakpoints translate directly into measurable transition risk that later informs SRC.

Thresholds increase risk not solely through their vertical height, but through how that height interacts with friction instability within the same step window. When a vertical change forces clearance adjustment at the exact moment traction is uncertain, the body must divide balance capacity between lifting the foot and controlling shear forces at contact.

In practical terms, this explains why a threshold that feels benign under dry, steady walking can become hazardous during entry. If a step lands on a partially wet surface immediately before or after the height change, the required friction rises while available friction fluctuates. Even small thresholds can then disrupt timing, increasing the likelihood that a brief slip escalates into a loss of balance during the first steps indoors.

VI. Asymmetry & Real-World Distortions

Mat-interface failure is defined as an asymmetry where local friction is increased while the boundary geometry becomes trip-prone or the mat migrates under shear. Entry mats change the floor plane into multiple traction zones with a physical edge, creating a slip–trip tradeoff. Real-world distortions (curling edges, wet backing, debris) increase variance.

The mechanism has two parts: (1) grip concentration inside the mat zone and (2) edge/transition hazards at the mat boundary. If the mat backing slips, the “safe” traction zone becomes a moving surface; if the mat edge lifts or is tall, toe clearance failure becomes likely during normal gait.

The table shows how entry mats trade one form of risk for another. Characteristics that improve wet-floor traction can simultaneously increase trip onset or mat migration at the interface. This slip–trip exchange defines the core boundary condition that SRC must synthesize rather than treat in isolation.

Entry mats reduce slip risk by increasing traction on wet surfaces, but they simultaneously introduce new failure modes at the interface, including edge-driven trips and mat migration under shear. Risk therefore shifts rather than disappears, moving from the contact surface to the boundary condition.

In practical terms, this means a mat that improves footing in the center can still elevate overall entryway risk if its edges interrupt gait or if the mat moves during a corrective step. Evaluating mats in isolation obscures this tradeoff; slip and trip risk must be assessed together as a single, integrated system state.

VII. Downstream Propagation

Downstream propagation is defined as the immediate conversion of a traction shortfall into posture disruption, protective reactions, and secondary impacts. A slip event is rarely isolated; it changes foot placement, upper-body loading, and attention allocation, which can trigger collisions, seating instability, or door/handle grabbing. Propagation is fastest in entryways because the event occurs during task switching.

VBU calls this sequence the Entryway Slip Failure Cascade (ESFC): a short chain where friction loss propagates into corrective reactions and secondary impacts.

The mechanism is cascade timing: the first correction is often a rapid arm reach or a widened stance, which increases the chance of contacting furniture edges, benches, hooks, or doors.

Once a traction shortfall triggers a protective reaction, the event is no longer “about the floor” alone: friction instability, threshold geometry, attention switching, and nearby objects begin interacting in the same step window, which is why many incidents are best understood as entryway falls that emerge as system failures rather than isolated slip episodes.

The structured logic below shows when slip events transition into broader system failures. A typical slip onset sequence is mapped to predictable downstream effects, clarifying how early traction loss propagates into posture disruption, collision risk, and secondary injury—patterns that later guide diagnosis.

Failure sequence (stepwise):
IF wet film persists at heel strike → THEN available DCOF drops → RESULT: micro-slip begins.
IF micro-slip occurs during a threshold/turn → THEN required friction rises → RESULT: slip accelerates.
IF correction requires reach/grab → THEN collision/impact probability rises → RESULT: secondary injury risk increases.

Flooring instability amplifies failure risk in adjacent entryway layers by forcing emergency balance corrections during already complex tasks. When traction is lost, attention and control are rapidly reallocated from navigation to recovery, increasing the likelihood of unintended contact with seating, storage, doors, or handles.

A brief traction shortfall rarely remains isolated. Rapid reaching, widened stance, or abrupt direction changes commonly follow a slip onset, elevating collision and secondary impact risk even when a full fall does not occur. Flooring instability therefore acts as a system-level multiplier, allowing minor surface disruptions to cascade into broader entryway failures.

VIII. Metrics Feeding Transition Risk

Slip Risk Coefficient (SRC) is defined as a composite risk metric that quantifies how friction instability, threshold disruption, and mat interface geometry combine to increase slip probability during entry transitions. SRC is not a “material score”; it is a system score that increases when traction changes rapidly within the first steps, when thresholds force gait re-planning, and when mats introduce edge or migration hazards. Its purpose is consistent diagnosis across real entryway conditions.

The table shows which variables push the entryway system past a safe boundary. SRC rises when friction instability coincides with geometric disruption and interface asymmetry, reflecting how overlapping failures compound risk rather than acting independently.

SRC increases most sharply when two or more risk variables overlap within the same step window. Friction instability, geometric disruption, and interface asymmetry reinforce one another when they occur simultaneously, compressing balance recovery time. A wet step taken across a raised threshold onto a thick mat concentrates these effects into a single gait cycle, producing a disproportionate increase in transition risk.

IX. Risk Diagnostic

When multiple diagnostic checks trigger simultaneously, flooring interventions alone are insufficient to restore safety margin. Risk is no longer isolated to surface traction, but distributed across timing, geometry, and interface conditions that interact within the same step window.

In this state, instability is generated faster than it can be absorbed. Traction changes compress balance recovery time, geometric features interrupt normal gait, and interface boundaries distort foot placement at the same moment. Because these effects overlap rather than occur sequentially, stabilizing a single variable cannot fully interrupt the failure cascade; the entryway must be understood as a coupled system rather than a set of independent elements.

X. Engineering Criteria

Safe entryway flooring is governed by friction consistency, not surface roughness in isolation. What matters is whether traction remains predictable across the first steps indoors and through common transitions. Safety margin is preserved when friction, geometry, and interfaces behave as a coordinated system rather than as independent features.

A wet DCOF of at least 0.42 along the primary walking path establishes a baseline for available traction, but consistency across adjacent zones is equally critical. Instability emerges when multiple stressors coincide, including:

• abrupt height changes that introduce clearance demand during traction uncertainty,
• patchwork traction zones that impose rapid shear-state changes,
• mats that migrate or curl under load, distorting the contact plane at boundaries.

Entryway risk is therefore rarely driven by a single deficiency. Slip and trip potential emerge when small inconsistencies accumulate within the same step sequence, progressively reducing the system’s ability to absorb disturbance during entry transitions.

A common example is an entryway where the primary walking surface meets wet DCOF targets in isolation, yet multiple secondary disruptions appear within the first steps:

• a narrow moisture strip forms just inside the door,
• a low but abrupt threshold interrupts normal gait,
• a mat introduces a secondary traction zone.

Individually, each condition may remain within acceptable limits. Encountered in consecutive steps, however, they compress balance recovery time and elevate transition risk.

Within this sequence, the body is required to manage several competing demands inside a single gait cycle:

• adaptation to reduced or variable traction,
• clearance adjustment over a vertical change,
• reorientation to a different surface response at the mat boundary.

When friction variability, geometry, and interface asymmetry overlap in this way, recovery demand exceeds available control, leaving little margin for corrective action if traction fluctuates unexpectedly.

Flooring that maintains a dominant, stable traction state and minimizes abrupt geometric or interface changes preserves balance recovery capacity during entry. Even as environmental conditions fluctuate, the foot encounters a predictable surface response, allowing normal corrective mechanisms to operate without enabling minor disturbances to escalate into slips, trips, or secondary collisions.

XI. VBU Matrix

The VBU Matrix summarizes how combinations of entryway conditions translate into overall system risk. Rather than evaluating flooring elements in isolation, it shows how Slip Risk Coefficient (SRC) escalates as friction instability overlaps with geometric disruption and interface asymmetry. As conditions compound, risk shifts from recoverable imbalance to unstable system behavior within the first steps indoors.

This matrix shows that entryway risk increases non-linearly as conditions stack. A wet surface alone may remain recoverable, but the addition of a threshold or a mat introduces new demands that compete for balance control within the same step window. Once friction variability, clearance requirements, and interface instability coincide, SRC escalates rapidly and system behavior shifts from correction-capable to failure-prone.

The practical implication is that entryway safety cannot be judged by any single feature or metric. Conditions that appear acceptable when assessed independently may become unstable when encountered in sequence during normal entry. Using the matrix as a diagnostic lens helps identify where risk is being created by overlap rather than severity, highlighting the importance of managing transitions so that traction, geometry, and interfaces do not demand simultaneous compensation from the body.

XII. VBU Audit Card

The VBU Audit Card is a rapid system-level diagnostic for identifying entryway flooring conditions that elevate slip and trip risk during everyday entry. Rather than evaluating materials or features in isolation, it focuses on how friction timing, geometry, and interface behavior interact within the first steps indoors.

Each audit item corresponds to a distinct failure pathway. Friction inconsistency increases timing instability, vertical changes introduce clearance demand during traction uncertainty, and mat edges or migration create interface asymmetry. When multiple checks trigger simultaneously, risk shifts from recoverable imbalance to system-level instability.

System-Aligned Responses

The audit is most useful when it informs system-level responses rather than isolated fixes. Effective responses reduce overlap between friction variability, clearance demand, and interface disruption, preserving balance recovery capacity during entry transitions.

1) Friction consistency in the first steps

The first two indoor steps should not require rapid adaptation to changing traction states. A common failure occurs when moisture is tracked just inside the door, creating a narrow wet strip at step one followed immediately by a dry surface at step two. System-aligned response separates moisture dissipation from the walking path, so traction does not collapse or recover abruptly under heel strike.

2) Vertical transition management

Vertical changes become disruptive when they coincide with uncertain traction. A low threshold may feel benign when dry, yet becomes destabilizing if the step occurs on a wet surface. System-aligned response places clearance demands where traction is already stable, allowing the body to manage elevation change without simultaneous friction compensation.

3) Interface geometry control

Mat edges should function as transitions, not as steps or obstacles. A thick mat may improve traction at its center while introducing an abrupt edge that elevates trip risk during corrective steps. System-aligned response maintains a continuous walking plane, so traction improvement does not introduce a competing failure mode.

4) Interface stability under shear

Surfaces intended to improve grip must behave as fixed planes under lateral and rotational forces. A mat that slides or twists during turning or reaching converts recovery effort into instability. System-aligned response ensures traction zones remain stationary, preserving trust in the surface during balance correction.

When the audit reveals overlapping deficiencies, flooring interventions alone are insufficient. The entryway must be evaluated and adjusted as a coupled system, where timing, geometry, and interface behavior together determine whether normal balance recovery remains possible during everyday entry transitions.

XIII. Cross-System Intelligence

Entryway flooring friction does not fail in isolation. It follows the same system-level failure logic observed across other furniture and movement systems: a stable state operates safely until a rapid change compresses the available margin for correction.

This pattern is well documented in TV Stand Stability Engineering , where structures remain upright under normal loads, yet fail suddenly when a load shift exceeds the stability reserve. Entryway flooring behaves similarly. When friction collapses abruptly—such as during wet first contact— the body’s balance system is pushed beyond its recovery window before corrective forces can engage.

A parallel mechanism appears in Coffee Table Geometry & Movement . There, collision risk is not driven by average table height, but by how edge geometry intersects disrupted gait paths. In entryways, friction loss alters step timing and length, making feet more likely to meet thresholds, mat edges, or floor transitions at unstable angles.

These failures become significantly more severe within Aging-in-Place systems , where reduced balance reserve and slower recovery mean that small design imperfections compound jointly. A minor friction drop, a subtle height change, and a slightly unstable mat—each tolerable in isolation— can synchronize into a single destabilizing event that exceeds the user’s remaining stability margin.

XIV. Common Entryway Flooring Mistakes & Engineered Fixes

XV. The Engineered Standard

Design intent: engineered flooring preserves friction consistency and continuous geometry across the first steps, without transferring slip risk into new trip hazards.

A properly engineered entryway floor is designed around predictable foot–floor interaction, not ideal conditions. Its role is to maintain a stable margin of safety as the surface transitions between dry, damp, and wet states, without introducing secondary failure modes through geometry, edges, or surface discontinuities.

1) Predictable Traction Across Wet + Dry States

The engineered standard prioritizes traction stability rather than peak friction. Large swings between dry and wet friction force sudden gait adjustments and increase slip probability during entry.

• Wet friction remains within a controlled range of dry friction
• Surface finishes avoid trapping continuous water films
• Slip resistance is evaluated at first contact, not steady-state walking

2) Continuous Geometry Without Abrupt Height Changes

From an engineering perspective, the entryway floor should behave as a single continuous plane. Abrupt height changes become destabilizing exactly when friction is reduced.

• No sudden lips, step-ups, or rolled edges within the entry zone
• Thresholds are treated as load-bearing transitions, not trim elements
• Floor continuity is preserved under wet conditions

3) Mats as Integrated Floor Elements

In an engineered floor system, mats are not accessories. They are temporary surface extensions that must behave predictably under shear and compression.

• Mat surfaces provide friction without excessive thickness
• Edges remain flat under repeated foot loading
• Under-mat friction prevents migration across the floor

4) No Failure Transfer Within the Floor System

The engineered standard enforces a single rule: slip reduction must not introduce a new floor-based hazard. Fixes that trade sliding for toe-catching or edge instability are considered failures.

• Slip control does not increase trip probability
• Friction improvements do not rely on added thickness or unstable edges
• Floor behavior remains predictable under changing moisture conditions

Engineered Floor Standard — Pass / Fail Checklist

Use this checklist to assess compliance. A single failure indicates elevated Slip Risk Coefficient (SRC) during wet entry events.

XVI. People Also Ask (PAA)

1) Why do floors feel slippery right after you come in from rain?
Because a thin water film can survive heel strike and reduce available DCOF faster than gait can adapt, especially in the first-step window near doors and thresholds.

2) Is slip risk highest on the mat or beside it?
Slip risk often drops on the mat footprint but can reappear at the mat’s edge, where raised profiles, bevels, or curl introduce toe-catch risk and shift the hazard from slip to trip.

3) Do small thresholds really matter?
Yes—height changes force micro re-planning during the same step window where friction can be unstable, increasing SRC when clearance demand overlaps with traction uncertainty.

4) Why do some tiles seem fine dry but dangerous wet?
Microtexture that grips when dry can lose interlock when water fills texture valleys, creating a lubricating film and widening the wet–dry friction gap.

5) Can shoe soles change the outcome?
Yes—tread geometry and rubber compound affect how water films disperse and how quickly traction returns after heel strike, changing available friction in the step window.

6) Why are slips clustered near doors?
Doors concentrate wet→dry transitions, thresholds, and mat interfaces within a few steps, increasing friction variability and making traction loss more likely early in the entry sequence (see Section III winter amplification for seasonal effects).

XVII. FAQ — Entryway Flooring Slip Physics

1) What does “friction consistency” mean in practice?
It means the floor does not swing between high and low traction states across adjacent zones during the first 1–2 steps, keeping available friction predictable.

2) What is the fastest way to spot a high-SRC entryway?
If a wet strip, a threshold, and a mat edge occur within the first two steps, SRC is elevated because friction instability and geometry disruption overlap in the same step window.

3) How should I interpret persistent wet footprints past the first two steps?
Persistent prints indicate the moisture film is not being broken or absorbed quickly, increasing the chance that heel strike will land on lubricated contact conditions repeatedly.

4) When does a mat become a net hazard instead of a net help?
When it introduces edge height, curl, or migration under shear that converts slip mitigation into slip–trip coupling at the boundary.

5) Does polishing or wear change wet safety even if the floor looks “fine”?
Yes—polish and wear can reduce effective microtexture in the walking lane, widening the wet–dry friction gap and increasing friction instability under load.

6) Why are entryway floors especially slippery in winter?
Salt and slush create viscous, uneven films that delay microtexture engagement and cause abrupt traction swings across steps; for the mechanism details, see Section III winter amplification.

XVIII. Conclusion

Entryway flooring failures are not cosmetic problems. They are timing failures—events where friction, geometry, and foot–floor interaction change faster than the body can respond during the first steps indoors.

When traction collapses abruptly, balance recovery is forced into a losing race. The outcome is not bad luck or clumsiness, but a predictable mechanical consequence of unstable friction interacting with disrupted gait.

Engineering the flooring layer therefore means more than selecting a “grippier” surface. It means designing for friction consistency, continuous geometry, and controlled interfaces that preserve stability exactly when conditions change.

In entryway safety, the first step decides everything.

Glossary

• DCOF (Dynamic Coefficient of Friction): A measure of sliding resistance under motion that represents the friction available at the foot–floor interface during dynamic contact.
• Available vs Required Friction (AV/REQ): A biomechanics framework in which slip occurs when the friction the surface can provide is less than the friction demanded by the gait task.
• Friction Margin: The difference between available friction and required friction, which determines how much balance recovery capacity remains.
• Step Window (First-Step Window): The first one to two indoor steps where wet–dry transitions, thresholds, and mats concentrate and friction can change faster than gait can adapt.
• SRC (Slip Risk Coefficient): A VBU system metric that increases when rapid friction change, geometric disruption, and interface instability overlap within the same step window.
• Microtexture: Fine surface roughness that generates friction through mechanical interlock and water-film breakup at the contact interface.
• Water Film: A thin liquid layer that reduces shear resistance and can temporarily collapse available friction under heel-strike load.
• Threshold Transition Event: A gait re-planning moment caused by a vertical change that increases risk when it coincides with friction instability.
• Interface Asymmetry: A condition where adjacent walking zones have different traction or geometry, forcing rapid adaptation across consecutive steps.
• Slip–Trip Coupling: A failure mode where a slip-mitigation measure reduces sliding but introduces trip risk through edge height, bevel, or instability.
• ESFC (Entryway Slip Failure Cascade): A VBU term describing the sequence in which wet contact causes friction collapse, triggering micro-slip, corrective reaction, and secondary impact risk.

References (Standards & Research Anchors)

• ANSI A326.3 — American National Standard Test Method for Measuring Dynamic Coefficient of Friction (DCOF) of Hard Surface Flooring Materials: Technical bulletin overview (A326.3)
• ADA Standards for Accessible Design — Section 303: Changes in Level (threshold height + bevel guidance): Access Board guide (Chapter 3, includes 303)
• TCNA (Tile Council of North America) — DCOF / slip-resistance context used in tile specification: Slip & fall risk resource (TCNA)
• NFSI (National Floor Safety Institute) — Position/guidance documents on slip resistance & floor safety: NFSI STF policy (floor safety guidance)
• Representative biomechanics anchors — required friction in gait + corrective response timing (conceptual anchors; response timing often discussed in the ~150–250 ms neighborhood depending on task and perturbation): Open-access physiology review (latency phases; timing framework)