HD28/11

UK Skid Resistance Standard

HD28 Team

• Ramesh Sinhal - HA• Louise Caldwell - HA• Helen Viner – TRL• Stuart Brittain – TRL• John Donbavand - WDM

Purpose of the Standard

Describe how the provision of appropriate levels of skid resistance on in-service UK trunk roads will be managed.

Objectives of Standard

• Provide a level of skid resistance appropriate to the nature of the road environment at each location

• Maintain a consistent approach to the provision of skid resistance across the trunk road network.

Philosophical Changes

• It is acknowledged that it is not possible to provide a uniform risk across the network on all sites simply by adjusting the skid resistance.

• The term crash rather than accident is used throughout HD28/11. – This is because an accident can be

considered an act of God with no direct cause whereas most crashes have one or more causes.

Setting the Investigatory Level

Use of slippery road warning signs

Initial investigation

Site investigation and prioritisation

Desk study

Detailed investigations

Identify sites for investigation

Prioritisation of schemes

Measurement of skid resistance

Set or review ILs Conduct Surveys

Install signs where required/remove signs no

longer required

SCRIM - Only Machine able to be Used for Skid Survey’s Under HD28

Measuring Equipment - SCRIM

SCRIM Test Wheel Assembly

Test Wheel 20 degrees to direction of travel

200kg Load

Water Applicator

Survey Speed

• On Motorways and Dual Carriageway where the posted speed limit is greater than 50mph, the target survey speed is 80km/h.

• On all other roads, the target

survey speed is 50km/h.

Converting data to SC

• Converting data from SCRIM Readings to SCRIM Coefficient

• Correct for speed– SR(Sp.Corr) = SR(s) +(s*0.279 - 13.97)– SR(s) = SCRIM Reading at speed s– S = speed of SCRIM

• Apply index of SFC – SR*0.0078 = SC

Seasonally Correcting data

• Skid resistance varies throughout the season.

• Skid resistance can also vary from year to year.

• HD28 standard uses the Single Annual SCRIM Survey method to produce a Characteristic SCRIM Coefficient which takes into account within year and between year seasonal variation.

The CSC method.

CSC METHODCharacteristic SCRIM Coefficient (CSC) is a single-run SC value multiplied by an Equilibrium Correction Factor

The equilibrium correction factor is calculated for each area and is applied to individual 10-metre SC values

0.20

0.30

0.40

0.50

0.60

0.70

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

Month

SC

RIM

C

OE

FF

ICIE

NT

YEAR 1 YEAR 2 YEAR 3 YEAR 4

ESC for years 1 to 3 = mean ofas-measured values in years 1 to 3

E M

E

L

E = Early part of survey seasonM = Middle part of survey seasonL = Late part of survey season

Surfacing must have been trafficked for at least 12 months at time of Year 1 survey

SC measured in year 4

Correction factor is ESC divided by mean year 4 SC for area

SETTING THE INVESTIGATORY LEVELS 

Site Category and definition

IL for CSC data (SCRIM data speed corrected to 50km/h and seasonally corrected)

0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65

A Motorway

BDual carriageway non-event or single 1-way carriageway non-event

C Single 2-way carriageway non-event

QApproaches to and across minor and major junctions, approaches to roundabouts (see note 5)

KApproaches to pedestrian crossings and other high risk situations (see note 5)

R Roundabout

G1Gradient 5-10% longer than 50m (see note 6)

G2Gradient >10% longer than 50m (see note 6)

S1Bend radius <500m – dual carriageway (see note 7)

S2Bend radius <500m – single carriageway (see note 7)

Change to the Bend Criteria

• Categories S1 and S2 should be applied only to bends with a speed limit of >=50 mph,

• except if the radius of the bend is <100m, where the S1 and S2 categories shall be applied at all speeds

Change to Sharp Bends

This acknowledges that it is often not possible to travel > 50mph on bends with radii < 100m but these tight bends still provide a significant risk of wet skidding crashes at lower speeds.

Initial Investigation

• This section considers the validity of the data and provides a prioritisation process so that resources can be applied to the sites that have the greatest risk of wet skidding crashes occurring.

Data Validation

• Basic data validation checks are to be conducted for sites that are<= IL. – should include confirming that the IL has

been assigned correctly and that the skid resistance recorded is within the normal range expected.

• If anomalies are found it can result on the sites being removed from further analysis.

Prioritising Valid Sites

• TRL Crash Model• This model utilises crash trends from

historic HA trunk road network data to predict the expected crash saving from the application of a surface treatment.

• The inputs to the model are crash counts for the latest 3 years, CSC values, Site Category, IL and texture depth data. It then produces a score, which can be used to rank the particular site for a detailed site investigation.

Alternative Prioritisation Procedure

Parameters Scores and criteriaNumber of crashes

0 1 2 3+

Score 0 4 8 12

Likely impact of a crash

SlightSlight / Serious

SeriousSerious /

fatal

Score 1 2 3 4SCRIM Difference (SD)

>0>-0.05 and ≤0

>-0.10 and ≤-0.05

>-0.15 and ≤-0.10

≤-0.15

Score 0 1 3 6 12Site has SD≤0 and poor texture at the same point

No Yes

Score 0 1

SITE INVESTIGATION  

• Minimum data required on site includes:– mean CSC for the site, – 10m skid resistance data for bends and

roundabouts,– texture depth – last 3 years of crash data as a minimum.

• In addition the following may assist– Rut depth, longitudinal profile, gradient, crossfall and

curvature may be useful data if they are relevant

Signage

• Warning signs shall be erected at sites where the need for treatment to improve skid resistance has been identified

• The effectiveness of slippery road warning signs in reducing crash risk is unclear.

• It is generally accepted that a proliferation of signs reduces the impact of the message and too many signs can simply be seen as clutter.

Too Much Information in a Location where the driver needs to concentrate

Two slippery when wet signs within 100m

Avoiding a Proliferation of Signs

• Rule in the standard is that individual lengths requiring warning signs should be merged if they are separated by less than 1km

THANK YOU