Highways Agency

Composition and condition of the road surfacing

5.1 This chapter concerns measurements made on the surface course of the road at the material location during the course of this investigation in order to assess the general condition of the road surfacing. Two main aspects of the road were covered, i.e. skidding resistance and surface profile.

5.2 Skidding resistance is a complex technical field. The next section of this report gives a brief description of the important issues to set a context for the discussion of the measurements made.

General Comments Regarding Skidding Resistance

5.3 When a vehicle is braking, it slows down because the kinetic energy of the moving vehicle is converted into heat energy by the brakes which, in turn, are cooled by the passing air stream. So long as the wheel continues to rotate, this process continues until the vehicle stops or the brakes are released.

5.4 However, the above process relies upon friction between tyre and road surface to provide an opposing force against which the brakes can react. When a vehicle is cornering, side forces trying to make the vehicle continue in a straight line are generated and, again, friction between the tyre and the road is needed to generate an opposing force that enables the vehicle to follow around the curve.

5.5 If the combined braking and cornering forces required are more than the available frictional force, the tyre will slide over the road surface. In the extreme case, the rotating wheel may lock and the vehicle will skid or the tyre may slide sideways.

5.6 Tyre-road friction is dependent upon a number of factors, some of these relating to the road surfacing and some to the tyre itself. Friction can also be influenced by other factors such as weather conditions and localised surface contamination.

5.7 When a road surface is dry, the coefficient of friction is normally high and adequate for most normal manoeuvres. However, when the road is wet, tyre-road friction decreases significantly.

5.8 Due to the many factors involved, it is important to distinguish between the meanings of the terms "friction" and "skidding resistance":

• Friction refers to the forces that are developed between a specific tyre and a specific road at a particular time and under particular conditions. It is influenced by a large number of parameters, for example: the road, tyre and vehicle suspension characteristics, ambient temperature and presence of contaminants (including water).
• Skidding resistance is the term used to describe the contribution that the road makes to the development of friction. It is essentially a measurement of friction obtained under standardised conditions in which the various parameters are controlled so that the effects of the road surface characteristics can be isolated.

5.9 As skidding resistance (and hence friction) is high in dry conditions, the term is almost always used with reference to wet roads.

5.10 Skidding resistance depends upon two characteristics of the road surfacing known as microtexture and texture depth.

5.11 Microtexture is the name given to the fine "sand-paper-like" asperities on the surface of the road. On an asphalt road, such as this part of the M62 Motorway, microtexture comes either from the crystalline structure of the stone on the surface or from sand in the asphalt mortar.

5.12 The microtexture interacts with vehicle tyres to generate the adhesive forces needed to provide friction and is the major contributing factor to skidding resistance at low speeds.

5.13 However, when the surface is wet, the microtexture also has to penetrate through the water film and so this leads to a reduction in skidding resistance.

5.14 Microtexture is gradually polished away by the action of heavy traffic so, over time, the skidding resistance of a road will fall to an equilibrium level that depends upon the number of heavy vehicles using the road and the resistance to polishing of the aggregate used in the surfacing.

5.15 In temperate climates such as Britain, the polishing process is cyclic, with skidding resistance at its lowest during the summer, recovering to some extent during the winter.

5.16 The accumulation of fine deposits on the road during long dry periods also means that, the road may be unusually slippery after the first rainfall following a long dry spell.

5.17 A road surface usually has surface texture, formed by the shape of, and the spaces around, the particles making up the surfacing. The texture depth is a measure of this. Texture is important because it provides drainage paths to allow any water on the surface to move rapidly from the tyre-road contact patch. The tyre tread assists in this process, but the greatest contribution usually comes from the road.

5.18 Texture can also generate energy losses in the tyre when it is skidding. In this process, known as "hysteresis", some of the energy that is used to deform the tyre tread when it slides over the texture is absorbed by the tyre when it returns to its original shape. These energy losses can supplement the heat exchange process in the braking system and become very important when the wheel locks and the brakes can generate no more heat.

5.19 In wet conditions, as vehicle speeds increase, skidding resistance decreases. The extent to which this occurs depends upon the texture depth. Generally, the lower the texture, the greater the loss of friction. Research conducted by TRL has shown that, in general, skidding resistance reaches a minimum level at vehicle speeds of approximately 60 miles per hour (96.6 km/h).

Measurement of Skidding Resistance and Texture Depth

5.20 The skidding resistance of road surfaces is measured in a standardised manner. There are a number of different techniques that may be used, most of which involve sliding rubber over a wetted road surface and measuring the frictional force generated in some way. On a large scale, test vehicles fitted with a special test tyre designed to be sensitive to the microtexture of the surface are used.

5.21 With these devices, the test tyre (under a known vertical load) is forced to slip over the wetted road surface and the frictional forces generated are measured. From these measurements a value that represents the skidding resistance is calculated.

Sideway-force Coefficient Routine Investigation Machine (SCRIM)

5.22 SCRIM is the device used for this purpose routinely on UK trunk road and many local authority roads. The machine uses an angled wheel with a standardised, smooth tyre which is free to rotate as the contact patch slides over the surface and generates a "sideway force" along the axle of the test wheel.

5.23 SCRIM measurements are made at a standard vehicle speed of 50km/h although, because the test wheel is rotating at an angle, the slip speed at the contact patch is 17 km/h.

5.24 After correcting for the influence of factors such as test speed, the skidding resistance as measured by SCRIM is reported as a value called SCRIM Coefficient (SC).

5.25 Due to the seasonal variation of skidding resistance, it is normal practice to characterise roads in the UK by measuring the SCRIM Coefficient three times during the summer (when the skidding resistance is likely to be at its lowest) and reporting a Mean Summer SCRIM Coefficient (MSSC).

5.26 On motorways and other trunk roads, this is currently undertaken on one third of the network each year, so that the whole network is tested on a three-year cycle.

5.27 SCRIM results obtained for a site are viewed against a pre-determined Investigatory Level (IL) for that site that is related to the risk of wet skidding accidents on that type of site.

5.28 The process of determining appropriate ILs and a table showing appropriate ILs for sites with average risk ratings is provided in HD 28/94 (Skidding Resistance), which forms part of Volume 7 of the DMRB.

5.29 Should the SCRIM results be at, or below, the IL, an investigation is required, with slippery road warning signs required to be erected whilst the investigation is conducted. A MSSC obtained above the IL requires no immediate action.

Pavement Friction Tester (PFT)

5.30 SCRIM is designed specifically for long-distance surveys at constant speed to monitor the general skid resistance levels on large networks. It measures low-speed skid resistance and essentially assesses the condition of the microtexture of the surfacing during the summer months.

5.31 Unfortunately, due to seasonal variation in skidding resistance, SCRIM is considered to be "out of season" during the months between October and April inclusive. Therefore, no standard test can be undertaken by SCRIM during these months. Also, skidding resistance changes markedly when the surface is contaminated in any way. Such contamination includes rock salt, which is commonly used to prevent ice from forming on main road carriageways during the winter months. If a road surface is salty, then once again, no standard SCRIM test can be undertaken.

5.32 It is possible to allow for such non-standard testing using SCRIM. However, the correction factors that must be applied are, by necessity, somewhat subjective in nature. When standard SCRIM testing cannot be undertaken, alternative equipment using a different measurement principle can be used to assist more detailed research or site investigation. The Pavement Friction Tester uses the locked-wheel principle to measure skidding resistance.

5.33 The PFT comprises a special trailer on which one wheel can be rapidly braked under computer control until it locks, while the vehicle continues to move at a steady speed. It is held in this state for a short time before being released ready for another test.

5.34 A standardised smooth tyre is used on the test wheel.

5.35 The load and drag forces (see Figure 5.1 below) are measured throughout the braking cycle and once the wheel has locked and settled into a skid (this normally takes about 1 second) the average friction coefficient during a further 1 second interval is calculated. The value is known as a "Friction Number" (FN).

Figure 5.1 The principle of locked-wheel skid resistance measurements

5.36 Whilst unsuited to long-distance monitoring, which is one of the reasons why this technique is not used by the Highways Agency to routinely monitor the skidding resistance of trunk roads, the above principle is useful for more detailed investigations of certain sites because it allows skidding resistance to be assessed over a range of speeds, since the value measured relates directly to the vehicle speed at the time of the test.

5.37 The test is based on an American (ASTM) standard and is widely used to assess roads in the US. In the UK, the HA acquired their PFT primarily for use in their own research programme, although the equipment is well suited for special investigations such as the subject of this report. TRL maintain and operate the PFT on behalf of the HA.

Texture Depth

5.38 Texture depth is measured either as the average level below the peaks in the surfacing, or as the root mean square deviation about a notional datum level.

5.39 The best-known method for the former approach uses a known volume of sand that is spread into a circular patch; from the average diameter of the patch, the average texture depth is calculated. This method is commonly referred to as the sand patch test. The tests are repeated along the road as required.

5.40 The second method involves calculating the texture depth from a sequence of displacement measurements of the road surface profile, usually measured using a laser device such as those fitted to the Highways Agency Road Research Information System (HARRIS) machine used in this particular investigation.

5.41 One way of processing the data is to calculate the root mean square deviation using an algorithm that filters out of longer-wavelength displacements due to movement of the vehicle chassis. Such measurements are known in the UK as Sensor Measured Textured Depth (SMTD) to distinguish them from sand patch measurements.

5.42 The values obtained from the two methods are different and their relationship varies with the surfacing type.

Surface Nature of the M62 Westbound Carriageway Around Little Heck Bridge and Visual Inspection

5.43 The road surfacing at the material stretch of the M62 Motorway comprises standard hot-rolled asphalt with pre-coated chippings, laid across hard shoulder and all three running lanes. As discussed earlier in this report, the pertinent section of the westbound carriageway appears to have last been resurfaced in 1993.

5.44 Visually, the surfacing appears to be in good order, with a good coverage of chippings and no evidence of significant chipping loss or marked wheel-path rutting.

5.45 A visual comparison of the hard shoulder with the running lanes showed a clear difference in the character of the surfacings in the running lanes, due to trafficking, as would be expected. The chippings on the main carriageway (especially in Lane 1) showed obvious signs of polishing action and have been rounded by the abrasive action of traffic over the years.

5.46 In temperate climates, such as the UK, the process by which heavy traffic polishes the road surface material is cyclic, with skidding resistance being normally at its lowest during the summer months, recovering to some extent during the winter. Fine dust collects on the surface during the drier summer months, which then acts as a medium to accelerate the polishing of the aggregate by vehicle tyres. During the winter, more frequent rain disperses the fines and increases the quantity of "gritty" particles on the road surface. Combined with the action of vehicle tyres, these tend to roughen the surface again.

5.47 This effect is known as "seasonal variation" and gives rise to a "polishing cycle". Although the extent of the fluctuation in skidding resistance can change depending on the actual prevailing conditions, under constant traffic levels the skidding resistance eventually reaches an equilibrium cycle which is maintained for many years of service.

5.48 The accumulation of fine deposits on the road during long dry periods also means that the road may be unusually slippery after the first rainfall following a long dry spell. This effect is in addition to the seasonal effect and is caused by the contaminating effect of the dust and oil that mixes with the water to form a thin slurry on the surfacing, until it is washed away by the rain.

5.49 The month of March each year is towards the end of the winter recovery period of the polishing cycle. As would be expected at this time of year, the chippings looked, and felt to the touch, less "polished" than they might in mid-summer.

5.50 On the hard shoulder, the angular "crusher-run" edges of the chippings remained and the binder film, although well weathered, was still in evidence. This was most noticeable on sections that had been wetted during the testing, where the natural colouring of the aggregate showed through on the more-trafficked areas.

5.51 At the time of the tests there was a scattering of undissolved salt crystals on the surface of the road. Any effect that this might have had on the measurements would have been very small.

Measurements of Skidding Resistance

5.52 Skidding resistance measurements were made on 4 and 5 March 2001 by specialist teams from TRL. The Highways Agency's Pavement Friction Tester was used for these tests. Texture depth was measured using laser sensors on the HARRIS vehicle.

5.53 The section of surfacing included in the assessment of skidding resistance was a 500 metre length of the westbound carriageway of the M62 Motorway leading to the bridge over the East Coast Main Line (ECML). The sections studied were identified by means of the motorway marker posts, the measurements reported here being made at various points between Marker Posts 146.6 and 146.1.

5.54 The marker posts are used here to define the position of the measurements on the road, often referred to as a "chainage". The chainage is usually measured from a fixed point at one end of the road and so it usually increases in one direction of travel and decreases in the other.

5.55 On motorways, chainages are shown by marker posts placed every 100 metres at the carriageway edge. Each marker post has a number giving the number of kilometres from a reference point and, underneath it, the number of 100 metre sections along the current kilometre.

5.56 On the M62 Motorway, the chainage is measured from its western end, so the numbers on the marker posts on the westbound carriageway decrease in the direction of traffic. For example, the parapet of Little Heck Bridge begins at MP146.1. Chainage 146.200 to 146.100 represents the 100 metres immediately before the bridge parapet.

5.57 It has been established that the nearside wheels of the Land Rover initially left the hard shoulder and mounted the kerb road approximately 50 metres from the start of the safety fence protecting the bridge parapet. This point is approximately 90 metres from the start of the bridge parapet, i.e. at a chainage of 146.19.

5.58 The marks on the grass verge indicate that the vehicle left the road at a shallow angle, approximately 5 degrees to the line of the kerb. Assuming the vehicle had followed an approximately straight path before arriving at this point it is possible to estimate the distance that it would have covered between leaving Lane 1 and hitting the kerb.

5.59 It is reasonable to assume that the vehicle had (at least for a time) travelled in the normal wheel paths in Lane 1. This would position its nearside wheels approximately 3.5 metres from the kerb. Therefore, for an angle of approach of between 3 and 6 degrees, it would have travelled approximately between 30 and 70 metres along the road while moving to the left.

5.60 Therefore, the main area of interest from a skidding resistance viewpoint lies between chainages 146.26 and 146.19. However, it is also important to consider this length in the context of the general condition of the road and therefore, measurements were made over a rather longer length than this.

5.61 During the period when the tests were made, a system of traffic management was in place, diverting live traffic into one or both of the overtaking lanes (Lanes 2 and 3). For this reason, measurements with the PFT were confined to Lane 1 and the hard shoulder.

5.62 A police incident room was established on the railway bridge, obstructing the hard shoulder and limiting the lane width in Lane 1. Furthermore, the section of the hard shoulder immediately adjacent to the point where the material vehicle left the road and mounted the grass verge was cordoned off from MP 146.2 onwards.

5.63 For these reasons, it was not possible to make skidding resistance measurements using the PFT on the hard shoulder throughout the whole length of the area of specific interest, especially at the higher speeds.

5.64 However, despite the above, it is considered that the results obtained using the PFT provide sufficient coverage to provide a detailed opinion on the condition of the road surface at the location around the material time.

Test Methodology

5.65 The pavement friction tester (PFT) was used to measure locked-wheel friction at two speeds, nominally 20 km/h and 100 km/h respectively, in the nearside wheel path of both Lane 1 and the hard shoulder.

5.66 The slower speed represents a similar slip speed to that used by SCRIM when operated at 50 km/h and is known to relate to those measurements. This type of low-speed measurement can be used to assess the underlying condition of the microtexture.

5.67 The higher speed gives a direct indication of skidding resistance at higher vehicle speeds and can be used to confirm the influence of texture depth on skidding resistance.

5.68 Before making any skidding resistance measurements, the calibration of the distance recording system on the PFT was checked by measuring the distance recorded between marker posts over three different 1 km lengths on the eastbound carriageway.

5.69 The computer system on the PFT was set to operate in "continuous lock-and-release" mode. Using this approach, a pattern of tests can be initiated at a fixed reference point (MP 146.6 was used) and the system locks the test wheel, records the forces, releases the brake to allow the test wheel to rotate again before automatically repeating the braking cycle.

5.70 Operating in this mode, the watering system delivers a controlled flow just in front of the test tyre. The flow rate is regulated by the vehicle speed and is designed to deliver a nominal water film thickness of 1 mm.

5.71 This process results in a sequence of locked-wheel wet skids at approximately five second intervals. The length of the skid over which measurements are made lasts for 1 second, covering a distance of approximately 5 metres at 20 km/h and 28 metres at 100 km/h.

5.72 Eight passes were made, two at each target speed in each lane. The actual speeds achieved were slightly different, but close to, the target speeds. Measurements in Lane 1 were started at MP146.4. Due to the restrictions in the hard shoulder, measurements were started there at MP 146.6.

Test Results

5.73 The results of the tests are shown in graphical form in Figure 5.2 below. This figure shows the individual skids and, in addition, the texture depth recorded by HARRIS.

Figure 5.2 - PFT Test Results

Click to enlarge graph

5.74 As can be seen in Figure 5.2, the skidding resistance at higher speeds (depicted by the longer "dashes" on the graph) is well below that at lower speeds (depicted by the shorter "dashes" on the graph) both in Lane 1 and on the hard shoulder. This relationship is normal and to be expected.

5.75 The repeat passes at each speed can also be seen to be very similar to one another.

5.76 At low speed, the hard shoulder had slightly higher skidding resistance than Lane 1. This is as would be expected from the relatively untrafficked lane.

5.77 As also would be expected on a road of this type, skid resistance is consistent along the road.

5.78 There is rather greater variation in the hard shoulder; this is to be expected from the untrafficked nature of the surfacing.

5.79 The hard shoulder shows a greater fall in skidding resistance at the higher speed than Lane 1. This is consistent with the lower texture depth observed on the hard shoulder, although, because it starts from a higher level, the general level of high speed skid resistance is similar to Lane 1.

5.80 The texture depth in Lane 1 is considered to be at a good level (1.53 mm on average) while the hard shoulder exhibits a lower, but still acceptable level (0.74 mm on average). Routine texture depth measurements taken on a trafficked lane of motorway would need to be less than 0.5 mm in order for it to warrant investigation (DMRB Volume 7, HD 29/94, Structural Assessment Methods, refers).

Comparison of Results with Standards

5.81 The requirements for the skidding resistance of motorways are set out in Chapter 3 of Volume 7 of the Highways Agency Design Manual for Roads and Bridges. This defines the Investigatory Level for the MSSC in Lane 1 of a main line motorway as 0.35.

5.82 The road at the material location was last tested by SCRIM during the summer of 1998 (therefore, it is due for a repeat test in the summer of 2001). The results from the 1998 tests have been obtained and the measurements for the relevant section of road have been extracted.

5.83 Utilising an empirically derived conversion process obtained from research work carried out at TRL it is also possible to make an estimate of the SCRIM coefficient represented by the slow-speed measurements from the PFT.

5.84 These results, expressed as average values over successive 50 metre lengths have been calculated and are given in Table 5.1 below.

Table 5.1 - Summary of SCRIM and PFT Measurements

5.85 The SCRIM measurements obtained in 1998 were all above the Investigatory Level, which was appropriately set at 0.35, and therefore fully complied with the requirements of the Highways Agency standards at that time.

5.86 In 1998, the surfacing was approximately five years old and would have been expected to have reached its equilibrium skid resistance level.

5.87 The measurements made with the PFT in March 2001 show that the skidding resistance of the road at the material time remains at a good level and exceed the appropriate Investigatory Levels by a considerable margin. The estimated values for the SCRIM coefficient at the site are higher than the MSSC measured in 1998, as is to be expected with a late winter measurement. The results obtained are considered to be consistent with what would be expected on this type of road at this time of year.

5.88 It should be noted that there are currently no specific in-service requirements for texture depth on this type of road. However, the texture depth measurements indicate a level that past research has shown to be acceptable.

5.89 The PFT measurements at higher speed confirm that the texture is performing as would be expected. The results from the surfacing at the material location are considered to be entirely consistent with those found from research on similar surfacings elsewhere in the UK.

5.90 In summary, the skidding resistance of the road at the material location met or exceeded all the requirements of the Highways Agency standards.