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3. Factors affecting results
Like all skidding machines, the portable tester can only be
used to the best advantage with a full knowledge of the
factors influencing skidding resistance, and results must
be interpreted with due consideration for all conditions
obtaining at the time of the tests. Full details of the
development of the tester are given elsewhere.1 The main
factors influencing 'skid resistance' are outlined here.
1.
The quantity measured with the portable tester has
been termed 'skid-resistance' and is intended to
correlate with the performance of a vehicle with
patterned tyres braking with locked wheels on wet
road at 50 km/h.
The order of merit of road surfaces can change
substantially between 50 km/h and 130 km/h.3 Thus,
'skid-resistance' values, which represent the 50 km/h
value, cannot alone be expected to give an indication
of high speed performance. The fall-off in skidding
resistance with increased speed on wet roads
depends on the roughness of the surface macro-
texture, and is considerably less on rough surfaces
than on smooth ones. If the tester is used on high
speed roads, an additional criterion indicating texture
is required.
Suggested minimum values of 'skid-resistance'
Category
Difficult sites such as:
A
1.
Roundabouts
2.
Bends with radius less than 150 m on
unrestricted roads
3.
Gradients, 1 in 20 or steeper, of lengths greater
than 100 m
4.
Approaches to traffic lights on unrestricted roads
Motorways, trunk and class 1 roads and heavily
B
trafficked roads in urban areas (carrying more than
2000 vehicles per day)
All other sites
C
Note: For category A and B sites where speed of traffic is high (in excess of 95 km/h) an
additional requirement is a minimum 'texture depth' of 0-65rnm
Table 1
(measured with the portable tester)
Type of site
Minimum
'skid-resistance'
(surface wet)
65
55
45
page 7
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