skid resistance modelling

Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 1 Linking Road Traffic Accidents With Skid Resistance – Recent UK Developments Authors: H E Viner1 , R Sinhal2 and A R Parry1 1 TRL Limited, UK 2 Highways Agency, UK ABSTRACT When, in 1988, the UK Department of Transport first introduced requirements for skid resistance on its trunk road network, it introduced the concept of “investigatory levels” to be compared with measurements from routine skid resistance surveys. At the heart of this process was a link between the risks of wet skidding accidents occurring and the levels of measured skid resistance on the road. Initially, this was based upon a survey of a sample of the network at which the time was limited by survey capacity and computing power. The skidding standards have recently been revised and as part of this process, a new assessment of the link between accident risk and skid resistance has been made. This has involved a study of the whole Trunk Road network. This paper will review the historic background and then describe in more detail the recent study and its findings, how the results compare with the historic work and the changes that were shown to be appropriate for application in the revised standard introduced in August 2004. Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 2 1. INTRODUCTION The skid resistance policy for trunk roads was introduced in January 1988 through the standard HD28 in the Design Manual for Roads and Bridges. This standard was based on research, carried out over a number of decades, that had pieced together the influence of road surface condition on accident risk and introduced a strategy for managing the skid resistance of trunk roads to deliver predictable levels of friction that are adequate for the majority of manoeuvres. While this approach was still valid, there have been a number of important developments since 1988 that meant it was appropriate to review how the detailed outcomes of the policy are achieved. The trunk road network has changed to some extent in terms of length and geometry, traffic flows have increased, new types of surfacing materials have been introduced and vehicle and tyre technology has improved. Research into skid resistance has also progressed (e.g. Roe, Parry and Viner, 1998) and it was recognised that there were areas in which the implementation of the policy needed to be improved. Therefore, the Highways Agency decided to review the policy and standard for today’s conditions. As part of this review, TRL was commissioned by the Highways Agency to conduct a network level analysis of the influence of skidding resistance on accident risk. As a result of this, some changes were recommended to the way the network is categorised and the required levels of skid resistance are determined. This paper outlines the historical background to the introduction of the standard, the results of the new accident study and the conclusions reached which led to changes to the standard that was introduced in 2004. 2. BACKGROUND Drivers need friction to accelerate, decelerate or change direction. During these manoeuvres, the friction generated between the vehicle’s tyres and the road surface provides the force necessary to change the speed or course of the vehicle. The manoeuvre being attempted by the driver and certain characteristics of the vehicle define the magnitude of the friction force that will be required to complete it successfully. If the friction generated is not sufficient, the tyre(s) will start to skid, with potential for subsequent loss of control. The friction available is influenced by several different factors, which makes analysis complicated. The condition of the road surface is one factor, which is particularly important in wet conditions. This led to the concept of measuring “skid resistance”, whereby factors such as the vehicle and tyre were standardised to provide a method of assessing the contribution of the road surface to the available friction. Early research in the 1930s showed that skid resistance: • was different on different types of road surface, • decreases with increasing speed, Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 3 • varies during the course of the year, and • was lower during the first rainfall after a long dry spell. Between the 1930s and the 1960s a range of different devices were developed in the UK for measuring skid resistance, culminating in the development of the SCRIM (Sideway-force Coefficient Routine Investigation Machine), by the early 1970s (Hosking and Woodford, 1976). This was the first device capable of large scale routine surveys, having been fitted with a water tank that greatly extended its range compared with the earlier test equipment. It had better facilities for entering markers for location referencing and an on-board paper tape punch to allow direct entry of the data to a computer for processing. Apart from changes to the vehicle chassis and updates of the data recording system, SCRIM has remained essentially the same until the development of the dynamic vertical load measurement system reported in a separate paper (Roe and Sinhal, 2005). In parallel to the development of measuring equipment, research was also carried out to determine the levels of skid resistance appropriate. Again, in early work it was recognised that different levels of skid resistance might be appropriate, depending on the location and the local traffic conditions (Bird and Scott, 1936). These concepts were developed through the 1950s to 1970s (e.g. Giles, 1957 and Salt and Szatkowski, 1973) to the idea of different levels of skid resistance for “Most difficult sites” (e.g. roundabouts and sharp bends), “Average sites” (e.g. motorways and high speed roads; heavily trafficked urban roads) and “Other sites” (e.g. mainly straight roads with easy gradients and curves and no junctions). This also introduced the ideas of different levels of risk for different sites within the same category and the concept of equalising accident risk across the network. A larger-scale study of the link between skid resistance and personal injury accidents, based on 1000km of road network (Rogers and Gargett, 1991), confirmed the different levels of accident risk for different types of road site and the increase in risk for sites with lower skid resistance. Some results from this work are reproduced in Figure 1. Figure 1 Risk of accidents on wet roads and Mean Summer SCRIM Coeffcient (MSSC) from Rogers and Gargett (1991) To avoid the misinterpretation of a skid resistance value below a threshold level as implying that a dangerous situation existed, Gargett introduced the concept of and “investigatory level” (IL) of skid resistance. Skid resistance measured at or below the Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 4 IL would trigger a detailed examination of the site and an assessment of the need for remedial works. This approach also recognised that at that time it was expected that the skid resistance levels on the network would be found to be lower than the proposed ILs in a significant proportion of cases; it would take a programme of improvements, carried out over a number of years, to remedy this. This work led to the definition of the 13 “site categories” and corresponding suggested ILs that were introduced in the standard for skid resistance in January 1988. 3. NETWORK ACCIDENT ANALYSIS As part of the recent review of skid resistance policy, a database of pavement condition was constructed to support a new analysis of the link between site characteristics, skid resistance and accident risk. The analysis drew data from a number of sources: • A Highways Agency database containing pavement condition data from routine machine surveys, specifically measurements of skid resistance and texture depth, geometric parameters (gradient, crossfall and curvature), rut depth and longitudinal evenness. The site categories assigned to each part of the network under the existing skid resistance standard were also included. The data used in the analysis were the most recent data available during 2001 and were, therefore, as concurrent with the accident records as was practical. • A network inventory database including carriageway and hard shoulder widths and details of junctions and other accesses. • Junction locations and types obtained from Ordnance Survey maps. • A Department for Transport database containing information on traffic flow and composition. • A subset of the STATS19 data, used by police forces to record information about personal injury accidents, covering accidents on the trunk road network in the period between 1994 and 2000. Details such as incidence of skidding, number of vehicles, number and severity of casualties, road speed limit and road condition (wet, dry etc.) were extracted. In the database, the network was divided into lengths of around 500m (motorways) or 200m (other roads) to which the other data were assigned. These lengths represented a compromise between needing a long length, to be reasonably confident of assigning accidents to the correct length, and preferring short lengths, so that the surface condition was reasonably homogeneous. Shorter lengths were created where necessary, e.g. around features such as junction approaches or bends. A summary of the data available for analysis is given in Table 1, broken down by the site categories defined in the 1988 policy. Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 5 Table 1 Summary of data available for analysis Site category Number of lengths with data Median length (m) Total length (km) Data coverage (% of whole network) Motorway 3979 500 1901 56 Dual c/way non-event* 8246 200 1648 59 Single c/way non-event* 9026 200 1711 67 Dual c/way minor junction 359 93 41 40 Single c/way minor junction 2096 70 202 73 Major junction 909 57 80 49 Gradient 5 to 10% 708 200 126 82 Gradient steeper than 10% 14 190 3 100 Bend zero accidents 1995-97 Percentage of sites Zero accidents 1998-2000 >Zero accidents 1998-2000 Figure 6 Random vs. systematic variability in accident risk This indicates at least an element of systematic behaviour within the overall variability, which supports the case for retaining an Investigatory Level and basing the judgement on the need for treatment partly on the observed accident history. A further argument in favour of retaining the Investigatory Level is that fixing an Intervention Level, even as an underpinning level, would distort decisions about budget prioritisation that would otherwise be made based on a risk assessment. This is contrary to the ethos of a risk-based approach. It should be noted that improving the skid resistance may not be the most appropriate solution to reducing accident risk in all cases. Figure 6 also indicates that some sites where no accidents were recorded in the first period did have accidents recorded in the second. This means it will also be important to determine whether there are identifiable risks at individual sites that might lead to accidents in future in spite of a current good accident record. From these considerations, TRL recommended that the Investigatory Level should continue to be considered as a trigger for further investigation, as opposed to automatic intervention, and that a range of Investigatory Levels be specified for each site category to take into account the variability in accident risk observed. Furthermore, that to achieve this in practice, it would be necessary to strengthen the application of engineering judgement through improving the advice on setting Investigatory Levels and on site investigation contained in the standard. 6. NEW SITE CATEGORIES The changes to the site categories and Investigatory Levels recommended as a result of the accident analysis are shown in Table 2. Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 11 Table 2 Old and new recommended site categories and Investigatory Levels from HD28/88 (and subsequently HD28/94) and HD28/04 for trunk roads in Great Britain Investigatory level (at 50km/h) Site category and definition HD28/88 (preceding) HD28/04 (current) A Motorway 0.35 0.35 B Dual carriageway non-event 0.35 0.35-0.40 C Single carriageway non-event 0.40 0.40-0.45 Q Dual Carriageway (all purpose) - minor junctions 0.40 0.45-0.55 Single Carriageway minor junctions & approaches to and across major junctions (all limbs) 0.45 Approach to roundabout 0.55 K Approaches to pedestrian crossings and other high risk situations 0.45 0.50-0.55 R Roundabout 0.45 0.45-0.50 G1 Gradient 5-10% longer than 50m 0.45 0.45-0.50 G2 Gradient >=10% longer than 50m 0.50 0.50-0.55 S1 Bend radius <500m – dual carriageway 0.45-0.50 0.45-0.50 S2 Bend radius <500m – single carriageway 0.50-0.55 Table notes: 1. Category R and some sites in new categories S1 and S2 were previously tested at 20km/h. 2. A reduction in Investigatory Level of 0.05 is permitted for categories A, B, C, G2 and S2 in low risk situations, such as low traffic levels or where the risks present are well mitigated and a low incidence of accidents has been observed. Exceptionally, a higher or lower Investigatory Level than indicated in the Table may be assigned if justified by the observed accident record and local risk assessment. For most categories a range of Investigatory Levels is specified and it was recommended that the Investigatory Level set would normally be the lowest value in the band. Circumstances that would warrant setting a higher Investigatory Level include: • Notable potential for conflict between road users, particularly at speed or where the outcome is likely to be severe. • Road geometry departing substantially from current standards. • Known incidence of queuing where the traffic speed is otherwise high. • The presence of accesses onto the main carriageway, if they are busy, have poor advance visibility or create conflict between leaving or joining traffic. Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 12 • Low texture depth. An attempt was made to estimate the financial costs and benefits that would accrue as a result of changing the skidding resistance standard for the English trunk road network in line with the recommendations of the report. The length of the network likely to be affected by the changes was estimated based on up to date records of the existing site categories, the current distribution of measured skid resistance and the percentage of the categories in the analysis database with geometry or accident risk that would imply a higher Investigatory Level would be selected. The cost estimates are based upon likely treatment lengths, the cost of resurfacing and traffic management and road user costs associated with delays at the works. Benefits are based upon the financial value assigned to accident reductions by the Department for Transport. Further details of this process are given in Parry and Viner (2005). Depending on the assumptions made about the accident savings, it was found that the realisation period, i.e. the time at which the benefit associated with the accident saving would match the cost associated with treatment, varied from less than a year (best case) to 16 years (worst case). However for most site categories even the worst-case realisation period was within the normal lifetime of the surfacing, assumed to be around 10 to 12 years. Based on this albeit simple analysis, it appears that, in addition to assisting Highways Agency meet its targets for accident reduction, the costs of applying the recommended changes to the skidding resistance standard will be recovered in the financial value of the accident reductions that are estimated to result. 7. SUMMARY AND CONCLUSIONS As a result of this work, recommendations were made for a rationalisation of the site categories and investigatory levels. A key finding was the extent of variability of the accident risk for individual sites within a site category, indicating that the effectiveness of the standard could be maximised by identifying and targeting maintenance treatment to improve skid resistance at sites where there is greatest potential to reduce the accident risk. It was recommended that this could be achieved through better procedures for setting investigatory levels and conducting site investigation. These changes were implemented in the revision to the skid resistance policy for UK trunk roads that came into force in 2004. It is believed that the revision will result in more robust decision-making, leading to more effective prioritisation of maintenance budgets. Furthermore, it is expected that, although the revision to the site categories and investigatory levels would result in higher Investigatory Levels for some sites, and that a proportion of these would require maintenance to be brought forward under the new standard, the costs will be recovered through the reduced accident costs within the lifetime of the surfacings. TRL are currently working with the Highways Agency to monitor the implementation of the new standard in England and to assess its effectiveness in delivering the desired outcomes detailed above. Research is also being undertaken to determine whether there are areas of the policy which could be further improved. Linking road traffic accidents with skid resistance – recent UK developments by H E Viner, R Sinhal and A R Parry 13 8. ACKNOWLEDGEMENTS The work in this paper was carried out in the Infrastructure division of TRL Limited as part of projects carried out for the Highways Agency. 9. REFERENCES Bird G and W J O Scott (1936). Measurement of the non-skid properties of road surfaces. Department of Scientific and Industrial Research and Ministry of Transport Road Research, Bulletin No. 1 HMSO, London. Giles C G (1957). The skidding resistance of roads and the requirements of modern traffic. Proceedings of the Institution of Civil Engineers, 1957, Vol 6 (Feb) pp216-42, ICE, London. Highways Agency, Scottish Executive, Welsh Assembly Government and the Department for Regional Development, Northern Ireland. Design Manual for Roads and Bridges, Volume 7. The Stationery Office, London. HD28 (1988 and 2004). Skid Resistance. DMRB 7.3.1. HD36 (1999). Surfacing materials for new and maintenance construction. DMRB 7.5.1 Hosking J R and G C Woodford (1976). Measurement of skidding resistance: Part I. Guide to the use of SCRIM. Department of the Environment, TRRL Report LR 737, TRRL, Crowthorne. Maher M J and I Summersgill (1996). A comprehensive methodology for the fitting of predictive accident models. Accident Analysis and Prevention, Vol 28, No 3, pp281-296. Parry A R and H E Viner (2005). Accidents and the skidding resistance standard for strategic roads in England. TRL Research Report TRL622, TRL Limited, Wokingham, Berkshire. Roe P G, A R Parry and H E Viner (1998). High and low-speed skidding resistance: the influence of texture depth. TRL Report TRL367, TRL Limited, Wokingham, Berkshire. Roe P G and R Sinhal (2005). Recent developments to the SCRIM measurement technique in the UK. International Conference on Surface Friction of Roads and Runways, Christchurch, New Zealand, May 2005. Rogers M P and T Gargett (1991). A skidding resistance standard for the national road network. Highways and Transportation, Vol 38, No 4, pp10-16. Salt G F and W S Szatkowski (1973). A guide to levels of skidding resistance for roads. TRRL Report LR510. Department for the Environment, Transport and Road Research Laboratory, Crowthorne, Berkshire. © TRL Limited 2005 Paper reference PA/INF/4520/05