PUBLISHED PROJECT REPORT PPR850 Optimisation of water flow depth for SCRIM S Brittain, P Sanders and H Viner
Report details Report prepared for: Project/customer reference: Copyright: Highways England, Network Services 359(1308)HALC TRL Limited Report date: November 2011 Report status/version: Quality approval: Stuart Brittain (Project Manager) Martin Greene (Technical Reviewer) Disclaimer This report has been produced by TRL Limited (TRL) under a contract with Highways England. Any views expressed in this report are not necessarily those of Highways England. The information contained herein is the property of TRL Limited and does not necessarily reflect the views or policies of the customer for whom this report was prepared. Whilst every effort has been made to ensure that the matter presented in this report is relevant, accurate and up-to-date, TRL Limited cannot accept any liability for any error or omission, or reliance on part or all of the content in another context. When purchased in hard copy, this publication is printed on paper that is FSC (Forest Stewardship Council) and TCF (Totally Chlorine Free) registered. PPR850
Table of Contents Executive summary 1 1 Introduction 2 2 Testing on TRL test track 3 2.1 Method of testing 3 2.2 Results 4 3 Supporting data from MIRA track 10 3.1 Method of testing 10 3.2 Results 10 4 Discussion and recommendations 15 4.1 Method of testing 15 4.2 Results and recommendations 15 Appendix A Plots of SC data against chainage 17 i PPR850
Executive summary Highways England, via its supply chain, undertakes annual surveys of skid resistance using Sideways-force Coefficient Routine Investigatory Machines (SCRIMs). During testing, these machines apply a controlled water flow onto the pavement directly in front of the test wheel to provide a nominal water depth of 0.5 mm. This report outlines a programme of testing to determine if it is possible, for devices fitted with a speed-controlled water flow system, to reduce this water depth without adversely affecting the results. If a lower water depth could be used it would reduce the water usage during surveys and therefore increase the length of the network that could be surveyed before the SCRIM needed to be refilled. As a result, survey costs could be reduced. The results for the test programme are presented and discussed. Analysis of the data shows that reducing the water depth to 0.4mm does not have a statistically significant effect on the survey results. The data at 0.3 mm were ambiguous and exhibited a greater degree of variability for individual measurements. It is therefore recommended that for testing on the HE network a water depth of 0.4mm should be adopted, subject to a satisfactory test during a SCRIM accreditation trial, as the standard water depth for SCRIMs fitted with speed-controlled water flow systems. Were such a change to be stipulated it should take place prior to the start of a survey season and apply to all SCRIMs with speed-controlled water flow systems. There should be no change to the 0.5 mm requirement for machines which do not use the speed-controlled system. These devices are set up to produce a water depth of 0.5 mm at the standard 50 km/h test speed and this water flow rate is used at all speeds. They therefore produce higher water depths at lower speeds, and lower depths at higher speeds. A reduction in water depth for these devices would be exaggerated at high speed, and this could affect the data quality on high-speed roads where the target speed is 80km/h. For these devices, the nominal water depth should remain at 0.5 mm It is recognised that this will result in different water depth requirements for different SCRIMs. However it has already been found that SCRIMs fitted with speed-controlled water flow use less water in comparison to the gravity-fed system and the application of a lower water depth requirement will further improve upon this efficiency, which includes reduced use of fuel and travelling time associated with refilling the water tank, as well as the reduction in the actual use of water. 1 PPR850
1 Introduction The Sideways-force Coefficient Routine Investigatory Machine (SCRIM) (Figure 1.1) was introduced in the 1970s and has been used for routine surveys of the skid resistance on strategic roads in the UK since the late 1980s. The machine uses the sideways-force principle: a freely rotating wheel, fitted with a smooth test tyre, is angled at 20 to the direction of travel of the vehicle and applied to the road surface under a known vertical load. A flow of water wets the road surface in front of the test wheel so that when the vehicle moves forward the test wheel slides and the wet skid resistance can be measured. Figure 1.1 SCRIM Principle of operation Currently, water is applied to the pavement to create a nominal water film depth of 0.5mm. This report provides details of a study to investigate the effect on SCRIM results of reducing this water film thickness. If a lower depth could be used then this would reduce water usage and so increase the survey distance achievable before the water tank requires refilling. In turn this would also provide savings in the cost of routine surveys as down-time resulting from refilling the water tank would be reduced. Additional benefits derive from the reduction in use of fuel and water. The SCRIM used for this test programme was the one purchased by Highways England (HE) and operated on its behalf by TRL. This device controls the water flow rate to deliver a constant nominal depth of water film irrespective of vehicle speed and has the capability to deliver in increments of 0.1mm between 0.3 and 1.2mm. Currently the standard method for SCRIM testing requires a nominal water depth of 0.5mm to be provided. Therefore to evaluate the effect of reducing water depth, testing was conducted at 0.3, 0.4 and 0.5mm. Tests were conducted at three different speeds that aimed to cover the range of speeds found during routine testing. 2 PPR850
2 Testing on TRL test track 2.1 Method of testing The testing conducted on the TRL test track was carried out at 30, 50 and 70 km/h. The highest test speed would have preferably been 80 km/h, the speed for routine surveys on dual carriageways, but in practice this was not achievable due to the position of the test sections used. Tests were conducted on a selection of surfaces on the TRL test track that provided a range of texture depths and skid resistance levels. For the purposes of this analysis only data collected whilst the survey machine was travelling in a straight line were used (i.e. the bends were excluded). This resulted in the test circuit being split into four sections of different nominal skid resistance values. Following each test, the test machine remained stationary for 1-2 minutes in order to store the data and set up for the next test. This would allow time for the track to dry between runs. It was noted however, that the track remained visibly damp between runs. Due to the effect of water remaining after a pass, the tests at each water depth were conducted in ascending order; however the test speeds at each water depth were randomised within the programme. The test plan is shown in Table 2.1. Table 2.1 Test order TRL track Water depth (mm) Speed (km/h) Pass 30 1 70 1 50 1 0.3 70 2 50 2 30 2 50 3 70 3 30 1 70 1 50 1 70 2 0.4 50 2 30 2 30 3 50 3 70 3 30 1 70 1 50 1 70 2 0.5 50 2 30 2 30 3 50 3 70 3 3 PPR850
2.2 Results Three repeat runs of the test surfaces were conducted for the majority of the speed and water depth combinations. Unfortunately, due to operator error, only two passes were conducted for the 30 km/h, 0.3 mm water depth test. The data from each run were then plotted against distance and aligned so that the peaks and troughs in each dataset coincided. The graphs of the average SCRIM Coefficient (SC) values for each water depth and speed are provided in Appendix A.1. These graphs only show the data from the four test sections with the lengths between the sections removed. During the alignment exercise it was found that the third test section showed a significant difference in the pattern of the skid resistance data. The most likely reason for this variation would be due to differences in the test line and therefore the data from this section were removed from the subsequent analysis. If water depth does not have a significant effect on the SC values being recorded then it would be expected that the measurements at the alternative water depths would be similar to those at the standard depth of 0.5mm. In addition the spread of the individual readings in comparison to the average would be expected to be similar for each water depth (i.e. the between run variation is similar). 2.2.1 Difference between water depths To determine whether the tests at the different water depths were producing similar results, the average of the three runs at the 0.5 mm water depth for each 10 m length was taken as the reference value at each speed. Each test conducted at the other water depths was then compared to the reference value at the appropriate speed. A comparison of the data from the 0.3 and 0.5 mm water depth tests is presented in Figure 2.1 and those at the 0.4 and 0.5 mm water depths are compared in Figure 2.2. The data shown are SC values at the given test speed (s), referred to as SC(s), and have not been corrected to the standard test speed of 50 km/h. 4 PPR850
Individual SC(s) value at 0.4mm Individual SC(s) value at 0.3mm 0.80 0.70 0.60 0.50 30km/h 50km/h 70km/h Equality 0.40 0.35 0.30 0.30 0.40 0.50 0.60 0.70 0.80 Average SC(s) value at 0.5mm Figure 2.1 Summary results from TRL test track comparison of SC(s) values at 0.3 and 0.5mm water depths 0.80 0.70 0.60 0.50 30km/h 50km/h 70km/h Equality 0.40 0.35 0.30 0.30 0.40 0.50 0.60 0.70 0.80 Average SC(s) value at 0.5mm Figure 2.2 Summary results from TRL test track comparison of SC(s) values at 0.4 and 0.5mm water depths On first examination of the results there appears to be a good correlation between the data collected at 0.5 mm and the other water depths. However it can be seen that there is a tendency for the points in the 0.3/0.5 mm plot (Figure 2.1) to lie below the line of equality, particularly for the 30 km/h tests. This translates to a slightly lower SC value for the 0.3 mm tests in comparison to the 0.5 mm tests. To determine if these variations are statistically 5 PPR850
significant the mean difference and the standard deviation of the differences were compared. From this data the standard error (standard deviation divided by the square root of the number of results) was calculated. The standard error is used to provide the upper and lower confidence limits for the mean value calculated (the upper limit is the mean + standard error, and the lower limit is the mean the standard error). These confidence limits define the range of values that the mean can be expected to lie between if the test was repeated. All of these statistics are shown in Table 2.2. In the table the i next to the water depth refers to individual readings and the a refers to the average reading. Table 2.2 Statistics on difference between water depths TRL track Water depths Speed km/h Mean difference Standard deviation of difference Number of results Standard error Confidence limits on mean difference Lower Upper 0.3i -0.5a 0.4i 0.5a 30-0.02 0.022 152 0.002-0.03-0.02 50 0.00 0.024 228 0.002-0.01 0.00 70 0.00 0.028 218 0.002 0.00 0.00 30-0.01 0.015 228 0.001-0.01 0.00 50 0.00 0.016 228 0.001 0.00 0.00 70 0.00 0.023 228 0.002 0.00 0.00 The results suggest that the SC values generated with a water depth of 0.4 mm are not significantly different from the values generated with a water depth of 0.5 mm for the speeds tested (a mean of zero no difference - is within the confidence limits). The results produced with a water depth of 0.3 mm at 30 km/h are significantly lower than those at 0.5 mm at 30 km/h. It is possible that the structure of the testing (all of the 0.3 mm water depth tests followed by 0.4 mm and 0.5 mm in turn) allows for conditioning of the surfaces during the testing and may have influenced the results. It has been observed at the annual SCRIM correlation trials that the repeated passage of the test machines causes a polishing effect which lowers the SC values recorded. Prior to this polishing effect an initial cleaning effect can also cause an increase in SC values. The number of passes required for a polishing effect was unlikely to occur during this test programme (it usually requires 10 or more SCRIM machines in line, each conducting the same amount of testing as in this programme). However it is possible that a cleaning effect may have been occurring. To investigate if this was the case, the statistics for each individual run were compared with the appropriate 0.5mm average (this is done to normalise the data) and are shown in Table 2.3. The runs are ordered to match the order they were carried out in. 6 PPR850
Table 2.3 Statistics changes through testing TRL track Water depths Speed km/h Mean difference Standard deviation of difference Number of results Standard error Confidence limits on mean difference Lower Upper 0.3r1-0.5a 30-0.03 0.024 76 0.003-0.03-0.03 0.3r1-0.5a 70 0.00 0.026 72 0.003 0.00 0.00 0.3r1-0.5a 50-0.01 0.029 76 0.003-0.01 0.00 0.3r2-0.5a 70 0.00 0.026 70 0.003-0.01 0.00 0.3r2-0.5a 50-0.01 0.020 76 0.002-0.01-0.01 0.3r2-0.5a 30-0.02 0.019 76 0.002-0.02-0.02 0.3r3-0.5a 50 0.00 0.022 76 0.003 0.00 0.00 0.3r3-0.5a 70 0.00 0.032 76 0.004-0.01 0.00 0.4r1-0.5a 30-0.01 0.016 76 0.002-0.01 0.00 0.4r1-0.5a 70 0.00 0.028 76 0.003 0.00 0.01 0.4r1-0.5a 50 0.00 0.015 76 0.002 0.00 0.00 0.4r2-0.5a 70 0.00 0.021 76 0.002-0.01 0.00 0.4r2-0.5a 30-0.01 0.015 76 0.002-0.01-0.01 0.4r2-0.5a 50 0.00 0.013 76 0.001 0.00 0.00 0.4r3-0.5a 50 0.00 0.020 76 0.002 0.00 0.00 0.4r3-0.5a 30 0.00 0.012 75 0.001 0.00 0.00 0.4r3-0.5a 70 0.00 0.020 76 0.002 0.00 0.00 It can be seen from the statistics that there does not appear to be any consistent increase or decrease in the values within each water depth with time. This suggests that the changes in mean difference are not a result of a cleaning effect on the surface of the test sections. 2.2.2 Between run variability To investigate the variability of individual tests at each water depth the results were compared to the average value for that water depth. These data are presented in Figure 2.3, Figure 2.4 and Figure 2.5 for the 0.3, 0.4 and 0.5 mm water depths respectively. The 30 km/h tests are excluded from the 0.3 mm graph as there were only two runs at this speed which would not provide a consistent comparison with the other data. 7 PPR850
Individual SC(s) value at 0.4mm Individual SC(s) value at 0.3mm 0.80 0.70 0.60 0.50 50 km/h 70 km/h Equality 0.40 0.35 0.30 0.30 0.40 0.50 0.60 0.70 0.80 Average SC(s) value at 0.3mm Figure 2.3 Summary results from TRL test track comparison of the individual 0.3mm SC(s) values to the average 0.3mm value 0.80 0.70 0.60 0.50 30 km/h 50 km/h 70 km/h Equality 0.40 0.35 0.30 0.30 0.40 0.50 0.60 0.70 0.80 Average SC(s) value at 0.4mm Figure 2.4 Summary results from TRL test track comparison of the individual 0.4mm SC(s) values to the average 0.4mm value 8 PPR850
Individual SC(s) value at 0.5mm 0.80 0.70 0.60 0.50 30 km/h 50 km/h 70 km/h Equality 0.40 0.35 0.30 0.30 0.40 0.50 0.60 0.70 0.80 Average SC(s) value at 0.5mm Figure 2.5 Summary results from TRL test track comparison of the individual 0.5mm SC(s) values to the average 0.5mm value The standard deviation of the differences between the individual readings and the average were calculated and are shown below in Table 2.4. The mean difference is not shown, as by definition this was zero for each pairing. Table 2.4 Statistics on between run variability TRL track Water depths Speed km/h Standard deviation of difference Number of results 0.3i 0.3a 0.4i 0.4a 0.5i 0.5a 30 - - 50 0.014 225 70 0.015 198 30 0.011 228 50 0.013 225 70 0.015 228 30 0.009 228 50 0.012 225 70 0.013 225 The data suggests that there may be a slightly lower variability for the 0.5 mm water depth tests. However, there does not seem to be a major difference between the values at the different water depths, especially considering that SC is reported to only two decimal places. 9 PPR850
3 Supporting data from MIRA track 3.1 Method of testing The method of testing used at the TRL test track was repeated on the Twin Horizontal Straights facility at MIRA. However, it was decided that this time the higher speed testing would be conducted at 80 km/h (the standard survey speed on dual carriageways) rather than 70 km/h (80 km/h was originally planned for the testing on the TRL track but the positioning of the test sections meant that a maximum speed of only 70 km/h was achievable). As before, only the data collected whilst the survey machine was travelling in a straight line were used in the subsequent analysis (i.e. the bends were excluded). This approach resulted in the test loops being split into four sections, two for lane 1 and two for lane 2. The testing length (including bends) was approximately 8.9 km, and equates to 17.8, 10.7 and 6.7 minutes per pass (for the 30, 50 and 80 km/h tests respectively). As before the test machine remained stationary for 1-2 minutes following each test to save the file and set up the next test. A similar test pattern to that on the TRL track was used (i.e. the speeds were randomised and the water depths were conducted in ascending order). 3.2 Results Due to the time available and the longer length of the test sections on the twin horizontal straights, in comparison to the TRL track, the original test programme involved two passes at the high and low speeds (30 and 80 km/h) and three at the standard speed (50 km/h) for each water depth. Due to a code red incident on another part of the MIRA complex which required all testing to stop, not all of the planned tests were completed. The number of passes achieved for each test condition is shown in Table 3.1. Table 3.1 Number of passes conducted at MIRA track Water depth Speed (km/h) Number of passes 0.3 mm 0.4 mm 0.5 mm 30 2 50 3 80 2 30 1.5 50 2 80 2 30 1 50 2 80 1 Due to the restricted number of runs it was not possible to investigate the between run variability with this dataset. However it was still possible to look at the differences in SC 10 PPR850
Individual SC(s) value at 0.3mm values between the three water depths to add supporting data to the work discussed in section 2.2.1. As before, the data from each pass were plotted and aligned and are presented in Appendix A.2 (lane 1) and A.3 (lane 2). During the alignment exercise it was found that the 2nd half of the 2nd section (on lane 1) had variations due to the test line and so these data were excluded from the subsequent analysis. The individual values for the alternative water depths were then compared to the average (or only) values from the tests conducted at the 0.5 mm depth, and are shown in Figure 3.1 and Figure 3.2. 0.90 0.85 0.80 0.70 0.60 30km/h 50km/h 80km/h Equality 0.50 0.40 0.4 0.5 0.6 0.7 0.8 0.85 0.9 Average SC(s) value at 0.5mm Figure 3.1 Summary results from MIRA test track comparison of SC(s) values at 0.3 and 0.5mm water depths 11 PPR850
Individual SC(s) value at 0.4mm 0.90 0.85 0.80 0.70 0.60 30km/h 50km/h 80km/h Equality 0.50 0.40 0.4 0.5 0.6 0.7 0.8 0.85 0.9 Average SC(s) value at 0.5mm Figure 3.2 Summary results from MIRA test track comparison of SC(s) values at 0.4 and 0.5mm water depths As with the testing conducted at the TRL track, on first inspection there appears to be a good correlation between the data collected at 0.5 mm and the other two water depths. However this time there appears to be a tendency for the values at 0.3 mm to be slightly higher than those recorded at 0.5 mm (Figure 3.1), i.e. to lie above the 1:1 line. As before to determine if these variations are statistically significant the mean difference and the standard deviation of the differences were compared. These data are shown in Table 3.2. Table 3.2 Statistics on difference between water depths MIRA track Water depths 0.3i -0.5a 0.4i 0.5a Speed km/h Mean difference Standard deviation of difference Number of results Standard error Confidence limits on mean difference Lower Upper 30 0.01 0.020 1066 0.001 0.01 0.01 50 0.01 0.026 1599 0.001 0.01 0.01 80 0.02 0.030 1061 0.001 0.02 0.02 30 0.00 0.014 683 0.001 0.00 0.00 50 0.00 0.019 1066 0.001 0.00 0.00 80 0.00 0.024 1045 0.001 0.00 0.00 These data suggest that there is no significant difference between the SC values generated with a water depth of 0.4 mm and those generated at 0.5 mm. However, the results for the 0.3mm water depth are significantly higher than those at 0.5 mm. 12 PPR850
Individual SC(50) value at 0.3mm For the 0.4mm water depth, this agrees with the findings from the tests on the TRL track. However the results at the 0.3 mm water depth are the opposite of what was found at the TRL test track (0.3 mm was significantly lower). During testing at MIRA it was noticed that on some lengths (outside of the test sections) very high SCRIM readings were being recorded. Due to these high readings the condition of the tyre was inspected periodically throughout the testing to determine if a replacement was necessary. Following the majority of the testing at 0.3 mm water depth (2 passes at each speed) it was observed that a tyre change would be required soon. In order to obtain data to allow a comparison of the two tyres the final pass at 0.3 mm was postponed and the initial 0.4 mm test was conducted. The tyre was then changed and the final 0.3mm test undertaken followed by the remaining 0.4 mm tests (and the 0.5 mm tests). A comparison of the data obtained from the two tyres is shown in Figure 3.3 and Figure 3.4, and summary statistics are provided in Table 3.3. Note in all cases the 0.5mm data are from tyre 2. 0.9 0.85 0.8 0.7 0.6 Tyre 1 Tyre 2 Equality 0.5 0.4 0.4 0.5 0.6 0.7 0.8 0.85 0.9 Average SC(50) value at 0.5mm Figure 3.3 Summary results from MIRA test track comparison of the two tyres at 0.3 and 0.5 mm water depths (50 km/h) 13 PPR850
Individual SC(30) value at 0.4mm 0.90 0.85 0.80 0.70 0.60 Tyre 1 Tyre 2 Equality 0.50 0.40 0.4 0.5 0.6 0.7 0.8 0.85 0.9 Average SC(30) value at 0.5mm Figure 3.4 Summary results from MIRA test track comparison of the two tyres at 0.4 and 0.5 mm water depths (30 km/h) Table 3.3 Statistics on difference between Tyres MIRA track Test condition Tyre for 0.3/0.4 mm test Mean difference Standard deviation of difference Number of results Standard error Confidence limits on mean difference Lower Upper 0.3i 0.5a at 50 km/h 0.4i 0.5a at 30 km/h 1 0.02 0.019 1066 0.001 0.02 0.02 2-0.01 0.022 533 0.001-0.01-0.01 1 0.01 0.014 533 0.001 0.00 0.01 2 0.00 0.011 150 0.001 0.00 0.00 The results from this analysis suggest that the SC values measured using tyre 1 were higher than those produced with tyre 2. If this effect is taken into account in interpreting the results from the MIRA tests, it suggests that the tests at a water depth of 0.3 mm produced results significantly lower than at 0.5 mm, i.e. the results now agree with those found from the tests on the TRL track. The influence of the tyre has not altered the conclusion that the results at water depths of 0.4 and 0.5 mm are comparable. 14 PPR850
4 Discussion and recommendations 4.1 Method of testing The test programme reported herein was undertaken to determine whether reducing the water depth below 0.5mm would have a significant impact on the level of skid resistance measured during SCRIM surveys. Testing was carried out at two test tracks (the TRL test track and the twin horizontal straights at MIRA), and involved tests at three speeds (30, 50 and 70 or 80 km/h) and three water depths (0.3, 0.4 and 0.5 mm) on each track. While the structure of the testing (i.e. all of the tests at a water depth of 0.3mm followed by 0.4mm and 0.5 mm in turn) meant that it was possible that conditioning of the track may have taken place during the testing and altered the results, investigation found no significant change in the SC values over the course of the testing on the TRL track. During the testing on the MIRA track it was necessary to change the SCRIM test tyre part way through the testing. The degree of tyre wear was unexpected and is likely to be due to the particularly high levels of friction generated on sections outside of the test sections, especially during the 30km/h testing. As the change in test tyre appeared to have an effect on the results, this was taken into account when reviewing the results and in drawing conclusions. The test surfaces were visibly damp on most repeat tests so it is possible that the water depths tested were not precisely the target depths of 0.3, 0.4 and 0.5 mm. Further testing would be required to fully eliminate this effect. However, an elapsed time of several minutes between runs should mean that the effect of water remaining between runs was minimised in the test programme being reported. 4.2 Results and recommendations The results from the TRL track suggest that SC values measured by a SCRIM with speedcontrolled water flow and a water depth set to 0.4 mm are consistent with those measured at the standard water depth of 0.5 mm. This conclusion is supported by the results of the MIRA testing: although these data are complicated by the change of tyre, the confidence limits of the data from both tyres indicate no statistically significant difference between the water depths (Table 3.3). The results at 0.3 mm are less clear cut. The TRL track data indicate a statistically significant change of 0.02 units SC, with lower skid resistance measured for the lower water depth. This result is counter intuitive, as higher skid resistance measurements are produced from dry tests in comparison to wet tests. A trend in the opposite direction was observed at MIRA for 0.3 mm water depth, but the two tyres used produced inconsistent results. For both test sites, the 0.3 mm data exhibited considerably more noise in the individual data points than was the case for the higher water depth. It is therefore recommended that for testing on the HE network a minimum water depth of 0.4mm should be adopted as the standard depth for SCRIMs fitted with speed-controlled water flow systems. At 0.3 mm the current data are ambiguous and do not support a 15 PPR850
further reduction. There should be no change to the 0.5 mm requirement for machines which do not use the speed-controlled water flow system. These SCRIMs are set up to produce a water depth of 0.5 mm at the standard 50 km/h test speed and this water flow rate is used at all speeds. They therefore produce higher water depths at lower speeds, and lower depths at higher speeds. At 80 km/h these machines provide approximately 0.3 mm of water depth which is already lower than the recommendation made above for devices with speed-controlled water flow. Prior to implementing this change, it is recommended that its implications are tested at the annual SCRIM accreditation trial, where the number of devices and varied conditions on the network route would provide an effective additional test, particularly of whether an acceptable degree of consistency is observed between the machines that do or do not have the speed-controlled system fitted. The speed-controlled water flow system has already been found to provide a reduction in water usage in comparison to the gravity-fed system, and the lower water depth proposed here will improve upon this efficiency, which includes reduced use of fuel and travelling time associated with refilling the water tank as well as the reduction in the actual use of water. 16 PPR850
Average SC(50) of three runs Average SC(30) of three runs Appendix A Plots of SC data against chainage A.1 TRL Track 0.80 0.70 0.60 0.50 0.3mm 0.4mm 0.5mm 0.40 0.35 0.30 0 200 400 600 800 1000 1200 Chainage in metres (length between sections removed) Figure A.1 TRL track Average SC values at 30km/h 0.80 0.70 0.60 0.50 0.3mm 0.4mm 0.5mm 0.40 0.35 0.30 0 200 400 600 800 1000 1200 Chainage in metres (length between sections removed) Figure A.2 TRL track Average SC values at 50km/h 17 PPR850
Average SC(70) of three runs 0.80 0.70 0.60 0.50 0.3mm 0.4mm 0.5mm 0.40 0.35 0.30 0 200 400 600 800 1000 1200 Chainage in metres (length between sections removed) Figure A.3 TRL track Average SC values at 70km/h 18 PPR850
Average SC(50) of repeat runs Average SC(30) of repeat runs A.2 MIRA horizontals lane 1 0.9 0.85 0.8 0.7 0.6 0.3mm 0.4mm 0.5mm 0.5 0.4 0 500 1000 1500 2000 2500 3000 Chainage in meters (length between sections removed) Figure A.4 MIRA lane 1 Average SC values at 30km/h 0.9 0.85 0.8 0.7 0.6 0.3mm 0.4mm 0.5mm 0.5 0.4 0 500 1000 1500 2000 2500 3000 Chainage in meters (length between sections removed) Figure A.5 MIRA lane 1 Average SC values at 50km/h 19 PPR850
Average SC(80) of repeat runs 0.9 0.85 0.8 0.7 0.6 0.3mm 0.4mm 0.5mm 0.5 0.4 0 500 1000 1500 2000 2500 3000 Chainage in meters (length between sections removed) Figure A.6 MIRA lane 1 Average SC values at 80km/h 20 PPR850
Average SC(50) of repeat runs Average SC(30) of repeat runs A.3 MIRA horizontals lane 2 0.9 0.85 0.8 0.7 0.6 0.3mm 0.4mm 0.5mm 0.5 0.4 3000 3500 4000 4500 5000 5500 6000 Chainage in meters (length between sections removed) Figure A.7 MIRA lane 2 Average SC values at 30km/h 0.9 0.85 0.8 0.7 0.6 0.3mm 0.4mm 0.5mm 0.5 0.4 3000 3500 4000 4500 5000 5500 6000 Chainage in meters (length between sections removed) Figure A.8 MIRA lane 2 Average SC values at 50km/h 21 PPR850
Average SC(80) of repeat runs 0.9 0.85 0.8 0.7 0.6 0.3mm 0.4mm 0.5mm 0.5 0.4 3000 3500 4000 4500 5000 5500 6000 Chainage in meters (length between sections removed) Figure A.9 MIRA lane 2 Average SC values at 80km/h 22 PPR850
This report outlines a programme of skid resistance testing, undertaken using a SCRIM fitted with a speed-controlled water flow system, to determine if it is possible to reduce the standard nominal water depth without adversely affecting the results. It is recommended that reducing the nominal water depth from 0.5mm to 0.4mm should be implemented, subject to satisfactory testing at a SCRIM accreditation trial. (No change should be made to devices not fitted with the speedcontrolled water system.) This change would reduce water usage during surveys and reduce the fuel use and travelling time associated with water refills. As a result, survey costs could be reduced. Other titles from this subject area PPR768 Performance review of skid resistance measurement devices. P D Sanders, S Brittain and A Premathilaka. 2015 PPR587 Speed correction for SCRIM survey machines. S Brittain. 2011 PPR729 Highways Agency skid resistance current policy 2014: a review. A Dunford, P D Sanders, S Brittain, N Sidaway and R Smith. 2014 PPR815 Better understanding of the surface tyre interface. P D Sanders, M Militzer and H E Viner. 2017 TRL Crowthorne House, Nine Mile Ride, Wokingham, Berkshire, RG40 3GA, United Kingdom T: +44 (0) 1344 773131 F: +44 (0) 1344 770356 E: enquiries@trl.co.uk W: www.trl.co.uk ISSN 0968-4093 ISBN 978-1-912433-08-7 PPR850