North
Yorkshire County Council
Business and
Environmental Services
Executive
Members
23
April 2021
Skid
Resistance Investigatory Levels
Report of the
Assistant Director – Highways and Transportation
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1.0
Purpose Of
Report
1.1
To
seek authorisation to update the skid resistance investigatory
levels on the County’s Category2, 3a and 3b carriageway
network.
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2.0
Background
2.1
The maintenance of adequate levels of skidding resistance on
carriageways is an important aspect of highway maintenance given it
contributes significantly to the safe usage of the
network.
2.2
The existing Skid Resistance Policy was developed in 2006 and
subsequently updated in October 2017. This policy was
established in line the Design Manual for Roads and Bridges (DMRB)
note HD28/04 - the standard for Skidding Resistance on the National
Strategic Road Network (Trunk Roads and Motorways). This standard
was removed in 2015 and has been superseded in the DRMB by document
CS228, outlining the standards for skid
resistance. As with all documents within the DMRB, the
standards outlined are based on Motorways and All Purpose Trunk
Roads. However they provide a useful reference point from
which Local Highway Authorities can develop appropriate local
policies and standards.
2.3
In order to measure skid resistance of the network a SCRIM* survey
(Sideways Force Coefficient Routine Investigation Machine –
Industry Standard skid resistance survey) is used. SCRIM
surveys are carried out annually in each direction on all Cat 2, 3a
and 3b roads (a length of about, 2184km or about 25% of the total
NYCC network length). This coverage includes all the A roads in the
County.
3.0
Investigatory Levels
3.1
The requirements for skidding resistance vary across the network,
dependent upon local factors, surface characteristics and the road
geometry. As such, sites are categorised based on their
characteristics in to a site category. Each site category is
then assigned an investigatory level (IL).
3.2
ILs are a pre-defined limit of minimum acceptable skid resistance.
SCRIM measurements greater than the limit are considered
satisfactory, while those equal to or less than the limit will
trigger further investigation in line with the existing NYCC skid
resistance policy.
3.3
An IL must be assigned to every part of the surveyed network by
selecting an appropriate Site Category and associated
IL.
3.4
A recommendation within CS228 states that “A procedure shall
be put in place for reviewing the IL at least every three
years”. The three yearly reviews only determine that the Site
Category is appropriate and is generally completed using video and
GIS based data. The NYCC network requires an update of site
categories and ILs to ensure that they remain relevant.
Informal reviews of the ILs and site categories have been carried
out, however there is a need to carry a full formal
review.
3.5
It is proposed to review the existing Site Category ILs to ensure
that they are in line with those outlined within CS228. Using
collated network information and survey data, the appropriate site
category and associated IL would then be allocated to individual
10m lengths of the Cat2,3a and 3b network. Given changes to the
network since the adoption of the skid resistance policy, we are
seeking to ensure that site categories and associated ILs have been
amended to reflect new road layouts and infrastructure, for example
new junctions, speed limit changes, new pedestrian crossings
etc.
3.6
It is proposed to carry out this review of ILs ahead of the 2021
surveying season to allow for collated results in 2021 to be
compared against updated site category and IL data. The revised IL
and site category information would then feed in to a wider review
of the NYCC Skid Resistance Strategy.
4.0
Existing IL Information
4.1
The site category and IL thresholds were established in 2006 when
the existing skid resistance policy was introduced as part of the
Highway Maintenance Plan. See table below illustrating the
existing ILs:
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Site
Category
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Situation
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SCRIM CSC/Grip
Number Investigatory Levels at 50km/hr
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0.30
0.35
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0.35
0.41
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0.40
0.47
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0.45
0.53
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0.50
0.59
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0.55
0.65
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0.60
0.71
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0.65
0.76
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A
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Motorway
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B
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Dual Carriageway
non-event
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C
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Single
Carriageway non-event
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Q
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Approaches to and
across minor/major junctions, approaches to roundabouts
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K
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Approaches to
pedestrian crossings and other high risk situations
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R
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Roundabout
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G1
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Gradient
5–10% longer than 50m
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G2
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Gradient > 10%
longer than 50m
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S1
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Bend radius
<500m Dual Carriageway
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S2
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Bend radius
<500m Single Carriageway
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5.0
Revised Investigatory Levels
5.1
It is proposed that the following ILs, which are in line with
CS228, are adopted by NYCC. They are consistent with the
approach taken by other local highway authorities. These
proposals have been prepared by NYCC officers alongside our
network-surveying contractor (Perfect Circle) and have been
reviewed against information prepared by WSP.
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Site
Category
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Definition
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Investigatory
Level
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0.30
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0.35
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0.40
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0.45
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0.50
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0.55
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A
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Motorway
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BR
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Non-event
carriageway with one-way traffic
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BU
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CR
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Non-
event carriageway with two-way traffic
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CU
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QR
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Approaches
to and across minor and major junctions, approaches to roundabouts
and traffic signals
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QU
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KR
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Approaches
to pedestrian crossings and other high-risk situations
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KU
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KS
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Extents
of school warning signs*
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RR
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Roundabout
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RU
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G1R
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Gradient
5-10%, longer than
50m
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G1U
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G2R
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Gradient
>10%, longer than
50m
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G2U
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S1R
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Bend
radius <500m –
carriageway with one-way traffic
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S1U
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S2R
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Bend
radius <500m –
carriageway with two-way traffic
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S2U
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Note - Sites
with R = Rural (speed limit greater than 40mph). Sites with U
= Urban (speed limit 40mph or lower)
*School warning
signs is a proposed new category
5.2
The main differences from the existing ILs, is that site
categories are split into Urban/Rural sub categories to
differentiate between speed limits above or below 40mph. Rural (R)
carry a higher investigatory level in all site categories.
Approach lengths for site categories Q and K to be 50m.
5.3
If more than one site category is appropriate, then the site
category with the highest recommended IL will be selected. If
the highest recommended IL for the site categories are the same,
then the category highest up the table shall be selected (A being
the highest on the table and S2 the lowest).
5.4
An additional sub category for parts of the network within the
extent of school warning signs is also proposed. This an
enhancement of the existing ILs and brings routes close to schools
in line with the IL for pedestrian crossing approaches on urban
roads.
5.5
This proposed approach is easier to understand and provides a clear
distinction between site categories and urban / rural roads within
the same site category, reflecting the increased skid risk
associated with increased vehicle speeds.
5.6
In line with the existing Skid Resistance Policy updated in 2017,
following a site investigation, it may be necessary to amend the IL
at a specific location. Should this be needed, a plan of the
specific location alongside the proposed amendment as part of the
site investigation recommendation will be supplied to our SCRIM
contractor for them to update the IL to the specified
level.
6.0
Financial Implications
6.1
There is an additional cost of £54K to carry out the IL
review. It is proposed that this is funded from the existing
network condition survey budget in 2021/22 and the additional cost
is managed as part of the wider programme management
process.
6.2
By carrying out a full review in 2021/22, subsequent three yearly
reviews of the IL network will be less onerous and as a result will
be delivered at a lower cost.
7.0
Equalities Implications
7.1
Consideration has been given to the potential for any adverse
equality impacts arising from the recommendations. Officers
consider that there are no adverse impacts arising from the
recommendations in this report.
7.2
A copy of the ‘Record of Decision that Equality Impact
Assessment is not required’ form is attached as Appendix
A.
8.0
Legal Implications
8.1
The County Council, in its capacity as the Local Highway Authority,
Street Authority and Local Traffic Authority must act in accordance
with a wide range of statutory powers and duties imposed by
legislation.
8.2
The proposed amendments to the Skid resistance policy have been
developed in line with the relevant legislation such as the
Highways Act 1980, the New Roads and Street Works Act 1991, the
Road Traffic Regulation Act 1984, the Transport Act 2000, the
Traffic Management Act 2004 and the Flood and Water Management Act
2010.
9.0
Climate Change Implications
9.1
A climate change impact assessment has been carried out, see
Appendix C. We do not envisage any climate change impacts as
a result of the recommendations to this report.
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10.0
Recommendation(S)
10.1
It is recommended that the Corporate Director BES in consultation
with the BES Executive Members:
i.
Approve the
revised Skid Resistance Investigatory Levels in line with DMRB
guidance note CS228
ii.
Approve addition
of an additional site category for parts of the network within the
extent of school warning signs
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BARRIE
MASON
Assistant Direct
Highways and Transportation
Author of Report;
James Gilroy
Background
Documents: CS228 Skidding Resistance – Standards for Highways
-
50d43081-9726-41e8-9835-9cd55760ad9e
(standardsforhighways.co.uk)
