SLIP RESISTANCE OF
TACTILE GROUND SURFACE INDICATORS
Rodney A. Hunter
Hunarch Consulting,
134 Johnston Street, Fitzroy, Vic., 3065, Australia

Tests of water-wet arrays of tactile ground surface indicators with a new,simulated-foot-slip tribometer identified three key slip distances: very short slip on
an indicator top; short slip to the second indicator(s); and full slip. It was concluded
that a full slip is more likely to cause overbalancing but that, because the arrays
were found to be from “very” to “extremely” slip resistant, the risk of imbalance onarrays appears to be extremely low, except for high-heeled shoes landing between
rows of indicators on a smooth substrate, for which the slip resistance was low andtherefore the risk of overbalancing high. The SFST provided a wealth of
information about foot slips on indicator arrays and revealed the complex
interaction of foot rotational behaviour with the interstices of the arrays.

Introduction
The benefit to individuals of reducing the risk of slips and falls on pedestrian surfaces is selfevident, and the socio-economic cost of not doing so is widely acknowledged. There is one typeof pedestrian surface that persistently evokes disquiet: arrays of tactile ground surface indicators(TGSIs1). This study is an exploratory investigation of the slip resistance of TGSI arrays as
measured by a new type of device: a simulated-foot-slip tribometer (SFST)2. The TGSIs
investigated are 5 mm high, 45°-sided truncated cones with a 25 mm diameter top and 35 mm
diameter base, spaced in a square grid of 50 mm pitch as used, for example, in Australia3.
A variety of tribometers have been used for TGSIs, including the non-portable inclining
platform, and portable devices such as the British Pendulum (Pendulum), the English XL, theBrungraber Mark 2 (BM2) and Mark 3 (BM3), and pull-sled devices 4. Like the BM2 and BM3,
the SFST is a mechanically operated portable inclinable articulated strut device. However, itapplies the amount and rate of loading of a person at footfall, as established by force platform
tests; its heel‒ground contact area at heel strike is very small; and it simulates the downward
rotation of the forefoot after heel strike in reflexive response to a slip as reported, for example,
by Redfern et.al. (2001). Weight distribution of the leg and foot, and foot and ankle proportions
are realistic, and the foot is articulated in the sagittal plane (like the BM2 and BM3) andpartially in the coronal plane. The centre of mass of the foot moves forward with its verticalrotation to simulate adaptive increased forefoot pressure. Amount and rate of loading, and footrotation and commencement of rotation are adjustable. Slip distance and velocity, and foot1

Known also, in various shapes, as “tactile paving”, “detectable warnings”, and “tenji blocks”.
Invented by the author
3
Intended for hazard warning, as distinct from elongated versions intended for direction indication.
4
For an example, see Rowe (2010).
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rotational velocity are electronically and synchronously recorded, including with video5. The
SFST was created after doubts about the suitability of the BM3 for this sort of testing, even withmodifications to it reported by Hunter (2010) and later more-elaborate ones.
The aim of this study was to investigate the: (1) slip resistance of the super-macro-texture of
TGSI arrays; (2) contribution of the different parts of TGSIs arrays to slip resistance; and (3)relationship between slip resistance of TGSIs and foot behaviour.

MethodTests were in water-wet conditions. Most were with a male heel and outsole, each of Shore-A
84 rubber and 76 mm x 76 mm in size. The heel’s posterior edge had a 7.5 mm radius tosimulate the effect of wear (see Hunter, 2010). The outsole was convex in the sagittal plane andslightly convex in the coronal plane as typical of many shoes. Some tests were conducted with awomen’s heel of Shore-A 75 rubber, 14mm x14mm in size. The foot was inclined at 25° tohorizontal arrays, similar to the 26° used for worn heels tests by Sigler et.al. (1948), and 38.9°
to the sloped array (see below). Tests commenced with the heel 10 mm above the TGSI top(s).
TGSIs were of plastic, modified so that some had smooth tops and others had 400-grit
textured tops. The rims of most TGSIs were smoothly-sanded to a 1 mm radius, but the sharp
prominent rims of some were retained. The array substrates were of hardwood timber, somewith a hard shiny finish and some with a 400-grit coating. Most tests were conducted with thearray orthogonal to the direction of gait, but some were conducted obliquely to it (at 30°). Sometests were performed with the substrate only, and one test was performed with the array sloped
at 13.9°. Still and slow-motion-video photography was used to study the test-foot slip behaviour
for many tests. Tests were also conducted with the BM3 and the Pendulum.
Twelve combinations of four independent variables were tested: (1) substrate texture(smooth (Bs), textured (Bt)); (2) top texture (smooth (Cs), textured (Ct)); (3) heel strike positionon the first TGSI (6.25 mm, 12.5 mm and 18.75 mm from the rear of the rim (Strt1, Strt2 andStrt3 respectively)); and (4) heel strike location with respect to the array, that is, on the top of a
TGSI (Co-So), between them in the sagittal plane (Co-Sb), or between them in the coronal plane(Cb-So).
Four dependent variables were tested: slip resistance coefficient (SRC 6); duration (Drtn);
velocity (Vel); acceleration (Acc); and slip distance.
Three slip distances were used for analysis: (1) up to 18.75 mm (Slip1), being a slip of theheel on but not off the top of the first TGSI (for those tests that were on the axis of TGSI tops);(2) up to 42 mm (Slip2) corresponding with a slip off the first TGSI or from between adjacent
TGSIs and across to the next TGSI(s); and (3) a full slip of at least 300 mm (Slip3).
All strut angles for each of thirty-five test sets were recorded: a total of four hundred andthirty-eight strikes (plus eight sets of BM3 and Pendulum tests). Because slips on textured
TGSIs tops were difficult to generate, most tests with textured-top arrays were conducted withthe first two tops smooth. There were insufficient tests for reliable statistical analyses, butobservation of slips partly compensated for this. Not all combinations of variables were tested.

Results
Generally, greater force was required to generate longer slips than shorter slips, and resulted ingreater velocities. By far the greatest increase of velocity was between short slips and full slips.
The SRC for five of the thirty-five test sets exceeded the tribometer’s limit of 1.225.

5

Synchronisation of foot rotational velocity and video recording, and the varied commencement of footrotation were incorporated after data collection for this study.
6“Sip resistance coefficient” is used instead of “coefficient of friction” to convey the complexity of thetribometer foot's slips.

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Strike position of male heel on top of first TGSI (smooth)

SRC increased slightly with slip distance from Strt1; remained the same from Strt3 for Slip1
and Slip2; but increased dramatically for Slip3 (see Table 1). SRC from Strt1 was greater thanfrom Strt3 for Slip1 and Slip2, but less for Slip3. This corresponds with Slip1 and Slip2 having
greater Drtn and less Vel from Strt1 than from Strt3, but is the opposite for Slip3. In otherwords, initial slips are facilitated by heel strike on the closest part of tops and full slips by themost distant part.

Heel strike location in array

Generally, SRC was almost the same regardless of whether heel strike occurred on a TGSI top(CoSo), between adjacent TGSIs (CbSo) or between successive TGSIs (CoSb), although SRC
for CbSo was marginally the greatest.
Table 1.
Strike position on top
Cs
SRC
Strt1 Strt3
Slip1 0.612 0.471
Slip2 0.642 0.471
Slip3 0.692 1.0187

Table 2.
Texture; heel strike location
Co-So
Cb-So
Co-Sb Cb-So*
SRC
BsCs BtCs BsCt BsCs BtCs BsCt BsCs BsCs
Slip1 0.541 0.541 0.610 Slip2 0.556 0.572 1.099 0.471 0.735 0.806 0.642 0.557
Slip3 0.856 0.796! 1.221 0.950 1.049 0.930! 0.891 0.664

No
TGSI
0.519
Male heel, start on TGSI Male heel; horiz. orthog. array; Slip1 SRC is for top only (of first TGSI); BtCt
(Co-So); horiz. orthog. array not tested; first two tops of BsCt and Ct array were smooth; BtCs and
BsCt not tested with Co-Sb; rounded rims.* = Sloped array.
array; rounded rim.

Texture of TGSI tops (male heel)

Except as indicated by the two exclamation marks in Table 2, SRC for textured tops andsubstrates was greater than for smooth tops and substrates, although not significantly (theexceptions might be attributable to the small number of tests). There were no consistent orsignificant SRC differences between each of the texture conditions and slip distances for maleheel strikes, that is: on TGSIs (Co-So); between adjacent TGSIs (Cb-So); and between
successive TGSIs (Co-Sb). However, for the texture conditions generally, including for sharprimmed TGSIs, SRC for Co-So was less than for Cb-So and Co-Sb for Slip2 and Slip3. That is,male heel slip resistance along rows was approximately the same as between them.

Sloped array (male heel)

For smooth substrate and tops (BsCs), SRC between TGSI rows (Cb-So) of the sloped array was
greater than for the horizontal array for Slip2, but less for Slip3 (see Table 2). This is reflectedin the negligible difference of Vel for Slip2 but the larger difference for Slip3 (see Table 6).

Array alignment (male heel)

Generally, the male heel SRC on the oblique alignment was slightly greater than on theorthogonal alignment for smooth-topped TGSIs, but the opposite for textured-topped TGSIs.
The result for Slip2 is anomalous because the first two TGSI tops in the textured-top array weresmooth. Discrepancy was also evidenced by slip Drtn and Vel. There was weak correspondence
between AccRsq and SRC variances, with slips tending to be “rougher” on the orthogonal thanon the oblique smooth-topped TGSI array, but rougher on the oblique than on the orthogonal
textured-topped TGSI array, but this was opposite to expectation. It is concluded that for slips
along TGSI rows, there is no difference between orthogonal and oblique arrays.

Rim shape of TGSI top (male heel)

Sharp rims produced greater SRC than rounded rims for Slip2, but less for Slip3 (see Table 4).
SRC increased substantially from Slip2 to Slip3 for the rounded rims, but negligibly for the
sharp rims. For sharp-rimmed TGSIs, textured collars provided slightly greater SRC thansmooth collars.

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Heel typeSRC of the high-heel was less than the male heel, except as indicated by the three exclamation
marks in Table 5. SRC differences were not significant.
Table 3. Array alignment
SRC
Slip1
Slip2
Slip3

BsCs
Orth
0.556
0.856

Oblq
0.868
0.883

BsCt
Orth
1.099
1.221

Oblq
1.061
1.080

Male heel; on TGSI top (Co-So); smooth
substrate; 1st two tops of BsCt arraysmooth; rounded rims; Slip1 not applic..
Orth = orthogonal; Oblq = oblique

Table 4. Rim profile
Rim profile
SRC
Rnd
Shrp
Slip1
Slip2
0.556 0.653
Slip3

Table 5. Collar texture
SRC
Slip1
Slip2
Slip3

0.856 0.675

Male heel; horiz. orthog.
array; Co-So, BsCs only;
Slip1 not applic.. Rnd =rounded; Shrp = sharp

TextureSmooth0.636
0.667

Textured
0.686
0.692

Sharp rims; male heel; horiz.
orthog. array; Co-So, BsCs
only; Slip1 not applic..

Temporal variables

High-heel Vel for all slips was greater than male heel Vel, with three exceptions indicated byexclamation marks in Table 6. The difference of Vel between the high-heel and male heel was
not significant for Slip1 and Slip2 generally, but it was significant for Slip3 for heel strike
between TGSIs (Cb-So) and borderline significant for heel strikes on TGSIs (Co-So).
Table 5. High heel compared with male heel
SRC
Slip1
Slip2
Slip3

Co-So
BsCs
BtCs
MH
HH
MH
0.612 0.413 0.612
0.642 0.471 0.673
0.692 0.808! 0.690

Cb-So
BsCs
HH
MH
HH
0.413 0.972! 0.471 0.348
0.981! 0.986 0.368

BtCs
MH
0.606
0.823

No TGSIs
Bs
MH
HH
-

HH
0.582
0.787 0.519

0.388

Horiz. orthog. array; all tops smooth; Slip1 SRC is for TGSI top only; Strt1 only; rounded rims.

Table 6. Mean velocity
Vel
(m/s)
Slip1
Slip2
Slip3

Co-So
BsCs
MH
HH
0.015 0.051
0.093 0.475
0.711 0.836

Cb-So
BtCs
BsCs
BtCs
MH HH
MH HH
MH
0.015 0.051 0.022 0.351 0.132 0.094! 0.352
0.648 0.582! 0.651 1.026 0.790

Cb-So*
BsCs
MH
0.136
0.980 0.922 1.097 0.851
No TGSIs;
Bs
HH
MH HH
0.269! -Orthog. array; TGSI tops smooth; Slip1 Vel is for top only; Strt1 only; rounded rims.* = Sloped array.

For all array conditions, slip Drtn increased with slip distance: 0.201 s, 0.241 s and 0.506 s
for Slip1, Slip2 and Slip3 respectively. The increments were not proportional with variations ofslip distance. The mean Vel for Slip1, Slip2 and Slip3 was 0.080, 0.190 and 0.661 m/s
respectively. In other words, Slip3 was over eight-times faster than Slip1 and three-and-a-halftimes faster than Slip2. Mean Acc for Slip1, Slip2 and Slip3 was 0.138, 0.591 and 2.321 m/s 2
respectively. AccRsq decreased from 0.132, 0.067 to 0.017. That is, slips became marginally“rougher” with increasing distance. Greatest Vel was on the smooth substrate without TGSIs.

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Comparison with BM3 and Pendulum results

As indicated in Table 7, tests of sharp-rimmed, smooth-topped TGSIs yielded an extremely low
coefficient of friction (CoF) when measured with the BM3 (0.03), and a low CoF with thePendulum (0.26). This contrasts with 0.653 and 0.675 respectively with the SFST. The smooth
substrate without TGSIs had an extremely low CoF when measured with the BM3 (0.05); a very
low CoF with the Pendulum (0.25); a low CoF as reported by the manufacturer for sharp rims
and textured tops (0.49); a low SRC with the SFST and the high heel (0.388), and moderately
high SRC with the male heel (0.519). There was close correspondence for the texture-topped
array between the BM3 (0.98) and the SFST for Slip3 (0.930), but the Pendulum results of 0.33
and 0.45 are approximately one third and one half respectively of the SRC with the SFST (0.856
and 0.883). The BM3 CoF for smooth top and substrate arrays was 0.43 compared with 0.572
for Slip2 and 0.796 for Slip3 with the SFST.
Table 7. BM3 and Pendulum test results
1

BM3
Co-So
BsCs4
CoF 0.03

Cb-So
BsCs BsCt
0.43 0.98

Pendulum2
No TGSIs Co-Sb
Bs
BsCs BsCs4
0.05
0.33
0.26

Co-Sb3BsCs
0.45

No TGSIs
Bs
0.25
1
With Neolite test pad. 2With Four-S rubber slider; CoF=3*BPN/(330-BPN). 3 Oblique array.
4
Sharp rims. 5 Sharp rims; texture comprised of fine concentric rings; result given by Supplier

BsCt
0.49

5

Discussion
The effect of the male heel strike position on the first TGSI top (Co-So) highlights the criticalinteraction of foot rotation with heel strike on TGSI arrays. The greater distance of Strt1 allows
the foot to rotate more than for the shorter distance of Strt3 before launching off the TGSI. With
greater rotation, the heel posterior becomes more elevated and the foot more co-planar with thetops of the TGSIs. Not only does the heel posterior tend to avoid the substrate but the wholeheel tends to avoid the collar and rim of the next TGSI, landing instead on the top of it withconsequently less slip impediment to continued slipping than for Strt3. There would tend to beless avoidance of the substrate if the heel was new and its posterior edge therefore sharp.
The slightly greater SRC for heel strike between the first and second TGSI (CoSb) than forthe other two locations might be due to the very short distance to the second TGSI and theconsequent limitation of the foot’s rotation before it engages the second TGSI. As a separate
comment, whereas for heel strike on a TGSI or between successive TGSIs the heel engages thenext TGSI at the heel’s mid-width, for heel strikes between adjacent TGSIs, the heel
engagement is at each of its sides and it tends to become wedged.
The greater Slip2 SRC for the sloped array than for the equivalent horizontal arraycorresponds with the perching of the heel on the tops of the second and third TGSIs, not on therim of the second as for the horizontal array. This might be due to the greater angle throughwhich the foot rotated to engage the TGSIs, exerting greater pressure immediately uponcompletion of rotation, greater compression of the heel, and greater SRC. Consistent with thiswas the observed compression of the anterior corners of the heel by the TGSIs at each side of it.
The similarity of results for the orthogonal and oblique arrays might be partly attributable tothe small number of tests, to the use of only one heel strike location within the array, and to thefirst two TGSIs in the textured-topped array being smooth. If tests were also conducted with thehigh-heels, SRC differences would result because there would be no opportunity for full slips
between adjacent rows of TGSIs.
The greater SRC of sharp rims compared with rounded rims for Slip2 is attributable to theaggressive indentation of the plane of the heel in its inclined pose by the sharp rim of the second
TGSI(s). The greater SRC of rounded rims for Slip3 is attributable to decreased contact area ofthe prominent sharp rims compared with the rounded rims (consistent with results with thePendulum). Only one test was conducted for collar texture (heel strike on a sharp rimmed TGSI

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array). Compared with smooth collars, SRC differences between smooth and textured collars
would be greater for rounded rimmed TGSIs, and for heel strikes between TGSIs.
The lesser SRC for the high-heel strike than for the male heel between adjacent TGSIs (CbSo), for Slip2 and initially Slip3, is attributed to the high-heel’s sides not engaging adjacent
TGSIs as do those of the male heel. An explanation of the exceptions noted for heel strikes on
TGSI tops (Co-So ) is frustrated by the absence of video recording of high heel tests and by thevery small number of the tests; nor do temporal results assist in clarification. However, for
Slip1, the lesser SRC of the high-heel might be attributable to its more-compressible rubber andits sharp posterior edge─allowing it to creep forward. The lesser SRC of the high-heel mightalso be due to the slight increase in contact area of the male heel as it rotates downward.
With respect to Slip2 and Slip3, the SRC differences (but not necessarily the order of them)between the high-heel and male heel are attributable to the following: (1) the male heel rotateson its posterior at heel strike on the TGSI top and then launches across to the rim of the second
TGSI, which it strikes with its underside and whereupon it perches (see also above); progression
to Slip3 entails the heel rotating about the fulcrum constituted by the rim, and the sole therefore
becoming co-planar with the array tops; (2) the high-heel also rotates on its posterior at heel
strike but, because of its smallness, after launching it lands on the substrate, completes its
vertical rotation, on its posterior edge, and strikes the collar of the second TGSI with its anterior
tapered face (the angle to the vertical of the TGSI collar and the high-heel’s anterior face wereunintentionally similar); progression to Slip3 therefore entails the heel’s anterior face sliding up
the TGSI collar, with the sole already co-planar with the array tops.
A conspicuous characteristic observed during some tests on oblique and orthogonal arrays
was zigzagging of the foot during full slips, especially for the high-heel. For orthogonal arrays,
this is attributed to the foot being slightly offset from the centre of successive TGSIs in the samerow or from the centre between adjacent rows, or to slight angular misalignment of the array.
This behaviour has implications for maintenance of balance following a slip, particularly forwearers of high-heel shoes.
Of the three slip distance categories: (1) slip confined to the top of a first TGSI, (2) slip
extending to the second TGSI(s), and (3) full slip, it is predicted that the first one is very
unlikely, the second unlikely, and the third likely to cause loss of balance. Loss of balance fromfull slips is predicted because of their speed, and possibly also their “roughness” in thehorizontal and vertical planes. The predicted improbability of loss of balance from the other two
slips is based on their very short distances.
A goal of the SFST was to determine whether slips were arrested or retarded by thereflexive downward rotation of the foot. In this study, this occurred in less than 2% of all tests,
with stopping distances ranging from 13 mm to 82mm beyond the start of the second TGSI; asreported above, full slips were accompanied by substantial acceleration. If the SFST accurately
simulates reflexive foot rotation, slip arrest and balance recovery prior to completion of fullslips would depend upon other reflexive responses, such as with the other leg.
Based on the results here, it is predicted that the slip resistance of TGSI arrays would begreater for treaded and compressible soles than for the smooth firm soles used in this study.
TGSI arrays of different grid pitch and TGSI shapes are also predicted to yield different results
to the arrays tested here.
The substantial difference in results between the SFST and the BM3 and Pendulum is
attributable to the following: (1) the interaction of the BM3 test foot with the TGSI array islimited to the TGSI tops; (2) the Pendulum’s test foot interacts with the substrate and collar of
TGSIs, but does not replicate the vertical rotation of the foot; (3) the Pendulum tends to bounceon macro-textured surfaces (Fwa et.al., 2005) and even on smooth surfaces, as found by Sigler(1948) and observed by the author.
Hallas and Hunwin (2009) recommended the use of a 76 mm rather than a 38 mm slider for
the Pendulum when testing macro-textured surfaces to minimise interaction with interstices ofprotrusions, the same reason that Australian slip resistance test standards7 now stipulate that7

AS 4586-2013: Slip resistance classification of new pedestrian surface material; AS 4663-2013: Slip
resistance measurement of new pedestrian surfaces. Standards Australia, Sydney, Australia.

- 126 -TGSI arrays be tested obliquely (at 30°). The present study considers that interaction of the heelwith interstices is inherent to foot slip behaviour for “heel-down” gait and that measurement of
it is necessary. It seems evident that this also applies to other macro textured surfaces.

Conclusion
When water-wet arrays of truncated-conical TGSIs were tested with a new, simulated-foot-slip
tribometer, three key types of slips were identified: very short slip on a TGSI top of up to 18.75mm; short slip to the second TGSI(s) of up to 42 mm; and full slip of 300 mm. The full slip isconcluded to be the most likely to cause imbalance. However, results suggest that TGSIs are“very” to “extremely” slip resistant (and even “non-slip”), and much more so than indicated bytests with the Brungraber Mark 3 and the British Pendulum tribometers. In other words, the riskof imbalance on TGSIs appears to be extremely low, except for high-heeled shoes landing
between rows of TGSIs on a smooth substrate, for which the slip resistance was low andtherefore the risk of overbalancing high.
It is tentatively concluded that for the male heel:x Greater force was required to generate longer slips than the short slips, and resulted ingreater velocities and acceleration, by far the greatest of which was for full slips.
x For heel strike on the top of a first TGSI, a short slip is more likely for strikes on the closest
part of TGSI top, and a full slip is more likely on the most distant part.
x Slip resistance along the tops of rows was approximately the same as between rows; andslightly greater for obliquely than for orthogonally aligned arrays for smooth-topped TGSIs,
but the opposite for textured-topped TGSIs. In other words, the direction of gait andlocation of heel strike on the array substrate seems to be irrelevant.
x A sloped array, for heel strike between TGSI rows of an array with smooth substrate, wasgreater than for the equivalent horizontal array for the short slip but less for the full slip.
x Sharp rims produced greater SRC than rounded rims for Slip2, but less for Slip3. SRC
increased substantially from Slip2 to Slip3 for the rounded rims, but negligibly for the sharprims.
x Textured collars of sharp rim TGSIs provided slightly greater SRC than smooth collars.
x Slip resistance was greater than for the high-heel
The new tribometer provided a wealth of information about foot slips on TGSI arrays andrevealed the complex interaction of foot rotational behaviour with the interstices of the arrays;
this enabled the explanation of seemingly inconsistent test results.

References
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surfaces. Research Report RR726, HSL, Harpur Hill, Buxton, Derbyshire.
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