SURFACE TEXTURE DEPTH MEASUREMENTS ON SOME BRITISH … · average texture depth for 50-metre road...

TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 143

SURFACE TEXTURE DEPTH MEASUREMENTS ON SOME BRITISH ROADS

by P G Roe, L W Tubey and G West

The views expressed in this Report are not necessarily those of the Department of Transport

Pavement Materials and Construction Division Highways Group Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1988

ISSN 0266-5247

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. The high-speed texture meter

3. The surveys carried out

3.1 The road networks

3.2 The testing programme

4. Method of analysis

5. Results

6. Discussion

6.1 General

6.2 Motorways

6.3 All-purpose roads

6.4 Concrete roads

6.5 Variation from one survey to another

7. Conclusions

8. Acknowledgements

9. References

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© CROWN COPYRIGHT 1988 Extracts from the text may be reproduced,

except for commercial purposes, provided the source is acknowledged

SURFACE TEXTURE DEPTH MEASUREMENTS ON SOME BRITISH ROADS

ABSTRACT Surface texture depth is an important factor in maintaining the high-speed skidding resistance of wet roads. The high-speed texture meter (HSTM), which uses a laser-based contactless sensor, has been developed to enable texture depth to be measured on a routine basis on large road networks at normal traffic speeds. The prototype HSTM has been used on networks in three English counties over several years, as part of a programme of work to relate texture depth, skidding resistance and accidents, with a view to identifying the implications of different levels of texture depth for in-service roads. This work has provided a mass of data on texture levels and this report presents a summary of this data to provide an indication of the general levels of texture depth to be found on roads in this country. The data are summarised in the form of histograms and cumulative frequency diagrams of the average texture depth for 50-metre road sections.

1 INTRODUCTION It has long been recognised (Sabey, 1966) that surface texture depth plays an important part in determining the skidding resistance of roads at high speeds when they are wet. Since 1979 the Laboratory has been making measurements of the surface texture depth of roads in order to:

(i) develop a machine that can be used by road engineers to monitor the surface texture depth of the network of roads for which they are responsible;

(ii) investigate the relationship of wet skidding accidents to the surface characteristics of a road as determined by its sideway force coefficient and surface texture depth so that appropriate levels of texture depth can be recommended.

Work is still in progress to realise these objectives, but in the meantime a considerable amount of data on the surface texture depth of roads has been obtained which is of interest in its own right. This Report brings this data to the attention of road engineers, together with some comments.

The key to this work has been the development of a mobile texture meter that can operate on public roads at normal traffic speeds and provide a continuous record of the texture depth. The

machine is known as the high-speed texture meter (HSTM) to dist inguish it from the hand- operated version (mini texture meter) which is already in use for measuring the surface texture of newly-constructed roads (Hosking, Roe and Tubey, 1987).

This report gives a brief descript ion of the HSTM and then discusses the data obtained during the course of several years' survey work in three counties. Some comments on the results are given but no attempt is made to give a detailed analysis, which must wai t for other work to be completed. However, the information is of interest since it gives, for the f irst time, an indicat ion of the levels of texture depth to be found on roads in Britain.

2 THE HIGH-SPEEDTEXTURE METER

The high-speed texture meter used in the present study was the original experimental model (Plate 1), designed for research purposes and used to evaluate the principles needed for a product ion machine. (Work to develop a product ion version is nearing complet ion at the t ime of wri t ing.)

Neg. No. B98/83

Plate 1. High-speed texture meter used in the study

The original version uses a contactless sensor of the type developed by Still and Winnet t (1975) mounted in a trailer and normal ly posit ioned so as to fo l low the nearside wheelpath of the towing vehicle. The principle of the sensor system is shown in Figure 1.

In th is arrangement a rapidly pulsing semiconductor laser, producing infra-red light (wavelength 906nm), is projected on to the road surface. Light reflected from the spot so formed is focussed by a receiving lens on to a linear array of 256 photodiodes. The posit ion of the diode receiving most l ight gives a measure of the distance to the road surface at that instant and the depth of the texture is computed from a series of such measurements•

Photosensitive Pulsing diode array laser l i g h t / /

s o u r c e / / /

,o,.o:.o, i

~ \ " I . - I \ \ t

\

Road surface

Fig. 1. Principle of the laser-based contactless sensor for measur ing texture depth.

The sensor system moves over the road surface in a plane normal to the page in Figure 1, and rays of laser l ight reflected f rom different points in the tex ture D1, D2 and D3 are detected by appropriate d iodes d l , d2 and d3 in the receiving array thus g iv ing a measure of the depth of the points. The laser pulses at approximate ly 3.5kHz and the number of the i l luminated diode is transmitted to the computer on board the towing vehicle. The tex ture depth is computed and expressed as a root-mean-square value wi th averages recorded for every 10 metres travelled.

The sensor is shown mounted in the trailer in Plate 2.

The sensor is mounted in a trailer rather than di rect ly on the vehicle in order to overcome any prob lems that might be associated wi th the d isp lacement measuring range being affected by changes in vehicle loading, to isolate the sensor f rom any v ibrat ion from the engine and t ransmiss ion, and to avoid any problems associated w i th the vehicle suspension system and w i th w id th requirements for road vehicles.

The tow ing vehicle is a converted mini-bus which carr ies all the equ ipment needed to control the tex ture-measur ing process. The centre of the system is an Interdata 16-bit min i -computer wh ich is connected to the var ious elements of the system. These are: the sensor in the trailer, a d istance meter which generates pulses from a uni t attached to the gearbox final drive, a visual d isp lay uni t (VDU) wi th keyboard for immediate d isp lay of results, a paper-tape reader to al low

Neg. No. R482/87/6

The laser projector unit is hidden in the trailer wing to the left of the picture; the receiver section can be clearly seen. The camera is facing in the direction of travel.

Plate 2. The sensor mounted in the trailer.

rapid programming of the computer, a control box wi th push-buttons for starting and stopping texture measurements and inserting codes for location referencing, and a paper-tape punch which records the results for later processing and analysis on a mainframe computer. The equipment is powered via a 240V AC inverter driven by two large 12V DC batteries in series, ; . • batteries themselves being charged by an auxi l iary alternator driven by the vehicle engine. The equipment is positioned so that the operator can sit in the back of the vehicle (where the VDU and computer are accessible) or work from the front passenger seat using the control box and tape-punch during the course of a survey. Plate 3 shows the tape-punch and control-box in the front seat posit ion viewed from the driver's side.

An early version of this experimental HSTM was used to make the measurements of texture depth on Motorway M1, reported by Hosking and Still (1979).

The production versions of the HSTM will have a dedicated microprocessor-based computer unit mounted integrally in a compact control/display panel on the vehicle dashboard, and will record data on an ECMA34 standard magnetic tape cassette. All equipment will be powered by a 12V DC battery.

The experimental HSTM described above has been used for test surveys in three counties. The remainder of this Report describes and

Neg. No. R179/87/7

Plate 3. The vehicle cab set up for front-seat operation

summar ises the data obtained from these s u rveys.

3 THE S U R V E Y S CARRIED OUT

3.1 THE ROAD NETWORKS

Road networks in three counties, designated A, B and C in this Report, were used for the surveys. County A was chosen because it included a good sample of road types and because the County's Road Safety Unit expressed keenness to assist in the study. Sideway force coefficient measurements were already being made in County B for other purposes and it, too, included a good sample of dif ferent types of road and road usage. County C was included at a later stage in order to obtain data for concrete roads and because some of its roads carry very heavy commercia l traffic.

The networks were planned to include samples of all the motorways, t runk and other class A roads, some of the B roads and a small sample of unclassif ied roads. Because texture depth is pr imar i ly of importance in high-speed skidding, most of the roads chosen were non bui l t -up roads, ie those with a speed l imit exceeding 64 km/h (40mph). Most sample lengths were only included in one direction although some lengths were tested both ways. Dual carriageways were tested in the nearside lane only. The networks were div ided up on a simple node-and-l ink basis so that tested lengths could easily be identif ied

both in the f ield and in the data for later analysis. The approx imate total lengths tested in each ne twork were: County A, 720km; County B, 540km; County C, 830km.

3.2 THE TESTING PROGRAMME

The networks were tested by fo l l ow ing pro- def ined routes so that all the l inks wou ld be tested at least once dur ing the course of a survey. Wheneve r possib le surveys were completed w i t h i n a shor t per iod (about a week per network) a l though bad weather and equ ipmen t fai lures somet imes d is rupted this. Surveys were genera l l y carr ied ou t dur ing the summer months but one o r t w o were made in spr ing or early au tumn. Two complete surveys were carr ied out in County A dur ing 1984 in order to observe any d i f ferences between results obta ined ear ly in the season (soon after the win ter ) and those f rom later in the year (after any resurfacing or surface dress ing and a summer ' s traffic). The t imes at wh i ch the networks were surveyed are g iven in Table 1.

TABLE 1

Time of year for the surveys

Year Network Period of survey

1982 County A June/July County B September

County A August (part in October) 1983 County B August

County C October/November

County A April and September* 1984 County B July

County C July

*Two complete surveys were carried out; the bulk of the second survey was done in late September, and completed in early November.

As far as poss ib le the test ing was carr ied out under dry cond i t i ons because of the d i f f icu l ty of measur ing the tex ture when the road surface is we t and spray contaminates the sensor lenses. The l inks were tested in a con t i nuous pass, record ing results for each 10-metre length sequent ia l l y and inser t ing node-reference data as each node-po in t was passed. Other features, such as bends, br idges and secondary junct ions were also marked to assist in checking the data du r ing later processing.

All measuremen ts were made w i th the sensor moun ted to f o l l ow the lef t -hand wheel t rack of the carr iageway. Normal ly , test speeds were in the

range 45-65 km/h, but higher speeds (up to 80 km/h) were used on motorways; sometimes traff ic or road condi t ions necessitated lower speeds being used. Earlier studies wi th the machine had shown that texture results were sens ib ly constant over a speed range of 15-100 km/h.

The survey data recorded on paper tape was transferred to a mainframe computer at the Laboratory for processing, where the node- references, etc., were checked and the data converted into a standardised format ready for analysis as part of the main studies described in the Introduct ion. At this stage it was decided to carry out a pre l iminary analysis in order to gain an overal l picture of the distr ibut ion of texture depth on the networks. This would provide a useful datum against which the implications of

• main ta in ing dif ferent levels of texture depth could be assessed.

4 METHOD.OF ANALYSIS The three networks are each represented by a vast quant i ty of data; a single survey in County A alone amounts to over 72,000 individual ten- metre readings. It was therefore necessary to summar ise the data in some way to gain a useful p icture of the condi t ion of the roads. The data was analysed by means of a special ly-wri t ten Fortran computer program. This could select any group of roads, ind iv idual ly or by class, or an~ specified part(s) of selected links and then compute the average texture depth for 10, 20, 50, 100 or 250 metre sections. These section averages were then combined into a histogram and a cumulative f requency diagram to show both the overall d is t r ibu t ion of values and the proport ion of sect ions w i th a surface texture below any part icular value.

For those links which were tested more than once dur ing the course of a survey the program calculated the average of all the available runs for each section. Where the operator had indicated that the machine had deviated from the normal test l ine (to pass an obstruct ion for instance) the readings for that stretch were ignored and if a sect ion had less than half its length provid ing val id data it was not included in the histogram analysis.

All the data f rom the surveys in Table 1 were analysed using var ious sett ings for the section length and histogram class wid ths to decide on the most suitable combinat ion to display the results. The section length f inal ly chosen was 50m. This provided a suitable data reduction and at the same t ime represented a practical length of

4

road. The networks were divided up for analysis as fol lows:

1. Motorways

2. ClassA roads

3. ClassB roads

4. Unclassified roads

5. Concrete roads (Counties A and C only).

In the event, many of the analyses were similar and so they are not all reported here. Section 5 gives results from the most recent (1984) surveys in each county and includes some data from earlier surveys for comparison. The results are discussed in Section 6.

5 RESULTS All the results presented here are the sensor- measured texture depth (SMTD) in millimetres (defined by Cooper, 1974), measured with the HSTM. The traditional method of measuring surface texture is the sand-patch method (SP), in which a known volume of sand is spread into a round patch from whose diameter the average texture depth is calculated; this procedure is repeated at discrete intervals along the road. The SMTD is a root-mean-square value calculated from displacement measurements in a cont inuous narrow line. Because of the different principles involved the figures produced by the two methods are different, SMTD being about 0.6 of the equivalent SP value. It is recommended that in future SMTD values should be used for specif ication purposes wi thout converting them to SP values.

The size of the spot of laser light on the road surface is about 2mm x 0.2mm but this varies as it moves through the sensor's working range. This effectively limits the resolution of the system on very fine textures such as the surface of worn brushed concrete where the individual brush marks may have a spacing less than the width of the laser beam. The same may also be true of certain surface dressings made with fine chippings. Nevertheless, although the recorded SMTD might be under-estimated in such cases, the error is unlikely to be of practical significance.

All the measurements were made in the left-hand wheeltrack of the carriageway and the results, therefore, represent the minimum textu re depth for the roads studied. As the wheeltracks are the parts of the road likely to be involved when vehicles are skidding, it is there that the surface texture is most relevant.

TABLE 2

Average texture depth (SMTD) for various categories of road

Road Category

Motor- ways

ClassA

Class B

Unclass- ified

Concrete

Year of

test

1982 1983 1984

1982 1983 1984

1982 1983 1984

1982 1983 1984

1982 1983 1984

Network

County A County B

SMTD (mm) SMTD (mm) Length tested (km)

22.1 32.3 30.2 31.3

556.7 546.0 560.0 567.8

75.7 75.2 76.3 70.6

26.5 26.7 26.4 26.5

4.4 4.3 4.3 4.3

1,

mean s.d.

0.71 0.25 0.80 0.24 0.91 0.30 0.87 0.26

0.77 0.30 0.79 0.30 0.87 0.32 0.81 0.32

0.69 0.29 0.70 0.28 0.74 0.28 0.67 0.25

0.77 0.29 0.68 0.25 0.71 0.24 0.78 0.24

0.48 0.07 0.47 0.09 0.46 0.07 0.46 0.06

Length tested (km)

80.4 83.4 82.9

275.8 277.0 272.2

80.7 77.7 70.1

4.3 4.2 4.3

mean s.d.

0.67 0.17 0.75 0.19 0.76 0.19

0.67 0.26 0.70 0.27 0.71 0.29

0.64 0.29 0.65 0.28 0.66 0.35

0.59 0.19 0.47 0.14 0.44 0.12

*Much of the motorway tested length in County C was concrete

Length tested (km)

78.0* 80.2*

637.6 630.5

101.8 103.8

12.4 14.8

48.7 46.7

County C

SMTD (mm)

mean s.d.

0.58 0.22 0.58 0.22

0.67 0.25 0.68 0.28

0.63 0.23 0.63 0.27

0.66 0.22 0.66 0.22

0.48 0.14 0.48 0.13

For each group of roads the analysis results are presented as a series of figures giving the histogram and cumulative frequency diagram for each of the counties in turn. The histogram is the most useful way of showing the range and distribution of surface texture for a group of roads, whilst the cumulative frequency diagram can be used to estimate the percentage of a group of roads that has a surface texture below any particular level.

All the diagrams are based on the same scale and class interval; all refer to 50-metre section averages. A dashed line has been drawn on each histogram to show where the mean texture level falls. The means and standard deviations for the histograms are summarised in Table 2. This also includes average results for the surveys not illustrated, to facilitate comparison.

Figu re 2 gives the results for motorways, Figu re 3 gives the A roads, Figure 4the B roads and Figure 5 the unclassif ied roads. Results for County C's motorways are separated into b i tuminous and concrete surfaces in Figure 6. Results for all the concrete roads tested in Counties C and A are in Figure 7. Most concrete roads have short lengths of b i tuminous surfacing occurring within them, usually on bridge decks. These lengths were excluded from the analysis wherever possible. Figures 8 and 9 have been included to illustrate some of the var iat ions between surveys. Figure 8 compares the frequency distr ibut ions of the A and B roads in County B for three years whi lst Figure 9 shows the equivalent County A data for the summer 1983 to autumn 1984 period.

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15

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(a) County A

0.~20 0 :60 ' 1.00 1.40 1 . 8 0 ' 2 :20 ' 2 :60 ' 3.~)0 SMTD for 50-metre sections Class interval = .050

Analysis covers 625 sections of 50m (Total tested length=31.25km) 100

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~ 70 0

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MeanSMTD= 37 SD= .26

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SMTD for 50-metre sections Class interval = .050

(b) County B Analysis covers 1657 sections of 50m (Total tested length = 82.85km) Mean SMTD = .76 SD = ,19 1 0 0

I I I I I I I I I

i i ! , = | i -~

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(C) C o u n t y C Analysis covers 1602 sections of 50m (Total

0.~20 0.60 1.00 1.40 1. 2.~20 2.60 3.00 SMTD for 50-metre sections Class interval = .050

tested length = 80.10km) Mean SMTD = .58 SD = .22

100 ,0 j / ~ 80

~ 70 ..Q ~ 60 0

'~ 50 g

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0.20 0.60 1.60 1.70 1.80 2.?0 2.60 3.00 SMTD for 50-metre sections Class interval = .050

(SMTD in mil l imetres)

Fig. 2 Texture depth distributions on motorways in three counties in 1984

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(a) C o u n t y A Analysis covers 11356 sections of 50m (Total

o . ~ o " o . ~ o J ' ' ' . . . . . 1.00 1 .~ ,0 '1 .~0 2.20 2.60 3 b o SMTD for 50-metre sections Class interval = .050

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tested length = 5 6 7 . 8 0 k m ) Mean S M T D = .81 S D = . 3 2 100

90 / ~ -

80

70

60

50

40

30

20

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0.20 0.60 1.00 1.40 1. 0'2. 0 2. 0 3.bo • SMTD for 50-metre sections Class interval = .050

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(b) C o u n t y B Analysis covers 5440 sections of 50m

, , . , , , 7 i , ,

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00 SMTD for 50-metre sections Class interval = .050

(Total tested length = 272.00km) Mean SMTD = .71 1 oo

9 0 121

I.-- ~ 80

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~ 60 O

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~ 10

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SD = .29

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(c) C o u n t y C Analysis covers 12610 sections of 50m (Total

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tested length = 630 .50km) Mean SMTD = .68 SD = .28

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90

80

~ 70 O

~ 60

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40

30

20

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0 .20 0 .60 1.00 1.40 1.80 2 .20 2 .60 3 .00 SMTD for 50 -me t r e sec t ions Class interval = .050

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Fig. 3 Texture depth distributions on A roads in three counties in 1984

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(a) C o u n t y A Analysis covers 1411 sections of 50m


(Total tes ted len( th=70.55km) MeanSMTD=.67 SD =.25 100

_ 9o / £3

~ 8 0

~ 7 0 O

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(b) C o u n t y B Analysis covers 1401 sections of 50m

' ' r i i f t , 7 , " 1 i a

0.2, 0.60 ~.00 140 180 2.~0 2.~0 3 ; 0 SMTD for 50-metre sections Class interval = .050

(Total tested length = 70.05km) Mean SMTD = .66 SD = .35

100

90 80 60 70 5o

u~

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0 i i w i r ~ i

0 . 2 0 0 . 6 o 1 . 0 0 1 . 4 0 1 . 8 0 ' 2 . 2 0 ' 2 . 6 0 ' " "

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20

15

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(c) C o u n t y C

= i i | i i J i i i , i

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00 SMTD for 50-metresections Class interval = .050

Fig. 4

Analysis covers 2075 sections of 50m (Total tested length= 103.75km) Mean SMTD= .66 SD=.27

100

o _ 9o / F- 8 0

~ 7 0 O

x~ 6 0

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"6 30

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( S M T D in m i l l i m e t r e s )

Texture depth distributions on B roads in three counties in 1984

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(a) C o u n t y A

I

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Analysis covers 530 sections of 50rn (Total tested length = 26 .50km) Mean SMTD = .78 SD = .24 100 _ 90 j /

a i..- ~ 8 0

~ 70 O

~ 6 0

.9 ~ 50

E 4 0 O •

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1 0 ' n

0 i = i i i , , , f , w i .

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(b) C o u n t y B

20

a i - ~ 15

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g ~ 5 g

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Analysis covers 86 sections of 50m (Total tested length = 4 . 3 0 k m ) Mean SMTD = .44 SD = .12

100

- - 9 0 D I.-- ~ 80

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~ 6 0 O

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~, 50

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1 | i | = i i , , , ' ~ - i , = • , , | , | • | , ! 0.20 0.60 ' .00. 1~40' 1.80 2.20 2.60 3.00 0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.(~0 SMTD for 50-metre sections Class interval = .050 SMTD fo r 50-met re sections Class interval = .050

-$

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(c) C o u n t y C Analysis covers 296 sections of 50m (Total tested leng th= 14 .80km) M e a n S M T D = . 6 6 S D = . 2 2

100

n ~ = , i , = • | ,

0.'20 0.60 1.00 1.40 1.80 2.20 2.60 3.00 SMTD for 50-metre sections Class interval = .050

90 £3

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"~ 30 g ~ 20 g u

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0.½0 . . . . . . . . . . . . . . 0 .60 1.00 1.40 1.80 2.20 2.60 3.00 SMTD for 50-met resec t ions Class interval = .050

(SMTD in mi l l imetres)

Fig. 5 Texture depth distributions on unclassified roads in three counties in 1984

9

(a) B i t u m i n o u s su r fac ings Analysis covers 673 sections of 50m (Total tested length = 33.65km) Mean SMTD = .70 SD = .23

\ , , I , , , I ~ , ~ r 3 , , , , ,

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00

S M T D f o r 5 0 - m e t r e s e c t i o n s C l a s s i n t e r v a l = . 0 5 0

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1 O0

-~ 9 0 £3

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~ 1 0 Q.

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/ i ' " i = i i i i i [ i w i i i I

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00

SMTD for 50-metresections Class interval = .050

(b) C o n c r e t e

2O

£3 1-

15

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g 5 g

Analysis covers 864 sections of 50m (Total tested length =43 .20km) Mean S M T D = . 4 8 SD = .13

\ i i i , l i i i = i i | i

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00

SMTD fo r 50-metre sections Class in te rva l= .050

I00

~ 90 a b- ~ 8 0

~ 7 0 0

x~ 6 0

o

~ 5 0

E 4 0 0

-5 3 0

g ~ 2 0

lO Q_

0

J

i j , i i . . . .

0.20 0. 0 1.00 1.40 1.80 2. 0 2. 0 3. 0

SMTD for 50-metresections Class interval = .050


Fig. 6 T e x t u r e d e p t h d i s t r i b u t i o n o n t w o su r face t ypes f o r m o t o r w a y s in C o u n t y C in 1 9 8 4

10

20

(a) C o u n t y A Analysis covers 86 sections of 50m (Total tested length = 4 .30km) Mean SMTD = .46 SD = .06

O

O

~ 10 E

O m

g ~ 5

n

7 , , i i , i i i ! ! i ! i

0.20 0,60 1.00 1.40 1.80 2,20 2.60 3,00

SMTD for 50-metre sections Class interval = ,050

100

9 0 o ~- 8O

~ 70 O

.o 6 0

O ~ so

E 4 0 O

~ 30 g ~ 20 g ~ 10

0

S

I ! i ! ! i I

0.20 0.60 1 . ; 0 1.40 1.80 2.20 ' 2 .60 3.00

SMTD fo r 50-metre sections Class interval = .050

20

a

.~ 15 U3

.O_

lO

E Q

"6 g 5 E

n

(b) C o u n t y C Analysis covers 934 sections of 50m (Total tested length = 46 .70km) Mean SMTD = .48 SD = .13

0 .20 0 .60 1 .00 1 .40 1 .80 2 .20 2 .60 3 . 0 0

SMTD for 5 0 - m e t r e sec t ions Class interval = .050

100

} 90 121 I'-- ~ 8 0

~ 70 O

x~ 60

O

"- 50

E 4 0 O

.6 30 g ~ 20

g_ 0

]JY

i I i i ! i i , , ! • , . • ,

0.20 0.60 1.00 1.40 1 3 0 2.20 2.60 3.00


(SMTD in mi l l imetres)

Fig. 7 Texture depth distributions on concrete roads in two counties in 1984

11

¢g => E3

03

O

E Q Lt3

O.

20

15

10

0

1982

0.20 0 .60 1.00 1.40 1.80 2 .20 2 .60 3.00

S M T D fo r 50-metre sections Class interval = .050

20

a

15 09

0

E O

g ~ 5

1982

0.20 0 .60 1.00 1.40 1.80 2.20 2.60 ' 3.00


m

>= a F-

O

E Q LO

"5

(3-

20

15

10

1983

| • , , , , • , , , ° , • , ,

0.20 0 .60 1.00 1.40 1.80 2 .20 2 .60 3.00

S M T D fo r 50-met re sections Class interval = .050

20

E3

:~ 15 O9

O

~ ~0 E

O

g 5

Q.

1983

i

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00

SMTD fo r 50-metresect ions Class interval = . 0 5 0

(:3

(/3

O

E O L¢3

"5

co

(3.

20

15

10

1984

0.20 0.60 ~.00 ~.40 ~.80 '2..';0 '2.(~0 '3.60

S M T D f o r 50 -met resec t ions Class in terval = .050

(a) Class A roads (SMTD in mi l l imet res)

2O

(:3

15 {/3

0 -.~,

~' 10 E

0

g ~ 5

1984

, , i , i , ! " w

0.20 0.60 1.00 1.40 1.80 2 .20 2.60 3.00

SMTD fo r 50-metresect ions Class interval = . 0 5 0

(b) Class B roads (SMTD in mil l imetres)

Fig. 8 Comparison of texture distributions over 3 surveys in County B

12

20

£3 F-

15

O

= 10 E

Q

g ~ 5

S u m m e r 1983

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00

SMTD for 50-rnetre sections Class interval = .050

20

a

15 o~

O

lO E

Q

g 5

o_

S u m m e r 1 9 8 3

i i | , , i

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.()0


20

>= a F-

15 o~

O 4 - ,

o= 10

E O L¢3

"6 5

== L )

Q .

Spring 1984

| , ¢ , ! , , , | , • , i i •

0.20 0 60 .00 1.40 1.80 2.20 2.60 3.00


20

E3 1-

15

0

10

E 0

g 5

D-

S p r i n g 1 9 8 4

0.20 0 6 0 1.00 1.40 1.80 2.20 2.60 3.00


20

E3

:} 15 09

O

g ~0 E

g 5

g.

20

A u t u m n 1 9 8 4

I

i , , I , , , , , ~ , , , • i

0.20 0.60 1.00 1.40 1.80 2.20 2.60 3.00


(a) Class A roads (SMTD in mill imetres)

=> £3 F- 15 09

0

g 10

E 0

g 5 e -

A u t u m n 1 9 8 4

I

[

J . . . . . . . .

0 20 0 60 1.00 1.40 1.80 2.20 2.60 3.00


(b) Class B roads (SMTD in mil l imetres)

Fig. 9 C o m p a r i s o n o f t e x t u r e d i s t r i b u t i o n s o v e r 3 su rveys in C o u n t y A

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6 DISCUSSION This section discusses some features of the results, i l lustrat ing how large surveys of this nature could be used to monitor the levels and changes of texture over a network. No attempt has been made to perform a critical analysis of the detai led condit ion of the roads. It should be borne in mind that, as yet, there are no general requirements for the texture depth of in-service roads. Therefore, the information given in this report should in no way be taken as implying any shor tcomings in the networks studied; the quest ion of what are appropriate levels of texture depth in given si tuat ions has yet to be decided.

6.1 GENERAL A str ik ing feature of the results generally, inc luding those not il lustrated in Figures 2 to 5, was the simi lar i ty between the three counties in each of the road categories. Obvious exceptions were the motorways in County C (Figure 2c) and B roads in County B (Figure 4b); these are discussed below. Most of the histograms are skewed,w i th a longer right-hand tail. This isto be expected since texture generally reduces slowly w i th the effects of traff ic whi lst routine surface dressing or resurfacing work creates a high initial texture quickly, but on a comparatively small part of a network.

The exact shape of a distr ibut ion depends on the dominan t surface type and the t iming of the HSTM survey relative to any resurfacing or surface dressing - programme being carried out. Because results have been averaged over 50- metre sections, the short lengths of very low or very high texture which occasionally occur do not show in these f igures unless they dominate a 50-metre section.

For all the counties there was a trend for the average texture-depth level to fall wi th the class of road (Table 2) wi th the most marked dif ferences being between the moto rway /A road categories and the B roads. This is probably due to the dominance of chipped rolled asphalt on the h igher class roads, w i th well-trafficked surface dressings and macadams being more common on B roads. It would be unwise to attempt to read too much into the average f igures for the unclassif ied roads because of the limited amount of data; County B, in particular, is represented by a very small sample.

6.2 MOTORWAYS The moto rway results for County C (Figu re 2c) are lower, w i th a more skewed distr ibut ion, than those for the other two counties. This is because about half of the motorways tested in County C were of concrete construct ion and brushed concrete has a much lower texture depth than

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bi tuminous surfaces. Figure 6, which compares the two surfacing types, makes this clear.

The bituminous surfacings on the motorways of all three counties had a more consistent texture depth on average than those for the other road categories. This reflects the fact that rolled asphalt was the only bituminous surfacing material used on the lengths tested. County A had a more marked right-hand tail because the lengths tested were extensively resurfaced during the period of this study.

The average results for the (bituminous) motorways were the highest of all the major road categories.

6.3 ALL-PURPOSE ROADS The results for the A roads (Figure 3) show that the average texture depth for both County B and County C was about 0.7mm. County A had a higher average, 0.8mm. Some of the lower texture depths on A roads in Counties A and C may be due to the presence of some lengths of concre te but in both cases the proportions are small compared with the total lengths tested (less than 1 per cent and about 7.5 per cent respectively).

The average texture depth for the B roads was practically the same in all three counties. The shape of the distr ibution for County B (Figure 4b) was noticably more skewed and almost bimodal. This feature was not observed in other years when the results were very similar to the other counties; this is discussed further in Section 6.7 below. The County C results (Figure 4c) show a bulge in the right-hand tail which was not apparent in the Autumn 1983 survey.

The unclassified road samples in each county, although fairly typical, were too small for the results to be regarded as truly representative. The County A results (Figure 5a) were, untypically, skewed to the right. This was not true of the three earlier surveys, but part of this length was surface dressed between the two 1984 tests, which would also account for the relatively high average texture depth. In County B (Figure 5b) the length tested was mostly bitumen macadam, which accounts for the narrow range and lower average textu re depth.

6.4 CONCRETE ROADS The concrete roads gave lower texture depths on average, with a much lower standard deviation, than the bituminous surfacings; this is to be expected, because of the way in which the texture is formed on concrete surfaces (by transverse brushing). It should be borne in mind that textures needed to maintain high-speed wet skidding resistance on concrete roads are lower

than for bituminous surfacings (Salt and Szatkowski, 1973). Although the County A results refer to only about 4 km of road, they compare well wi th the results for the much larger sample of concrete roads in County C.

6.5 VARIATION FROM ONE SURVEY TO ANOTHER

As mentioned earlier, in general terms there was comparatively little variation in the results from one survey to the next, apart from the small changes to be expected as some surfaces wear and others are surface dressed or resurfaced. This general 'dynamic equil ibrium' meant that average textures for the different road classes remained fairly constant. However, within this general framework there were many detail variations; Figures 8 and 9 illustrate some of these.

Figure 8 shows the distr ibutions from three consecutive mid-summer surveys in County B for the A and B roads. For both classes the average texture remained sensibly constant in spite of changes taking place in the distribution. The A roads changed little over the two-year interval, but on the B roads there were marked changes. From 1982 to 1983 the basic shape was retained but in 1984 the distr ibution became almost bimodal. From the stretching of the right-hand tail there had obviously been some surface dressing on parts of the network but there was also a sharp increase in the height of the lower- texture peak. The most likely reasons for this are hot weather causing rapid deterioration of some surface dressings and resurfacing with bitumen macadam.

Figure 9 covers a shorter period of about 15 months but includes the effects of a winter and summer between the surveys. The 1983 results are very similar to the previous summer's values (not shown), reflecting the gradual wearing ofthe surfaces. The 'step' to the right of the mean for the B roads in 1983 is due to the highest textures from the previous year, then appearing in the right-hand tail of the distribution, wearing down to boost the number of sections in the 0.9mm to 1.2mm range. During the latter part of the summer of 1983 the general wearing process continued, reducing the 'step' effect on the B roads and steepening the left-hand side of both A and B road distributions.

However, during the winter of 1983/4 the average texture depth recovered a little due to the effect of weathering and the scouring action of snow, salt and grit; the A roads increased from 0.79mm to 0.87mm whi ls t the B roads moved from 0.70mm to 0.74mm. In the fol lowing summer the dual effects of gradual wear and resurfacing or surface dressing continued: the 'step' in the B roads was

fur ther worn down and a peak appeared around the 1.5mm level for the A roads. The average texture depth fell again during the summer, from 0.87mm to 0.81mm for the A roads and from 0.74mm to 0.67mm for the B roads.

The concrete surfaces showed barely any differences f rom one survey to the next. The mean values (Table 2) did not change. There was some evidence of sl ight changes in the d is t r ibut ions due to surface wear; there was a very small reduction in texture on the short County A stretch, which was barely a year old at the start of the study, but nothing of practical signif icance.

7 C O N C L U S I O N S

ItVl'easu rements of sensor-measu red textu re depth have been obtained on road networks in three English counties using the prototype high-speed texture meter, i ne aa~a nave been analysed to examine the frequency distr ibut ions of texture depth values for 50-metre sections to gain an overall picture of the values of textu re depth likely to be found. The fo l lowing general conclusions may be drawn:

(a) The results from the three counties were broadly similar.

(b) The average texture depth was higher on motorways and A roads than on class B roads; motorways showed less variat ion in levels of texture depth, reflecting the dominance of one surface type (rolled asphalt) on this class of road.

(c) The texture depth fell dur ing the summer and recovered a little dur ing the winter months.

(d) The concrete roads had a lower texture depth than their b i tuminous counterparts with, on average, a smaller range of textures.

8 A C K N O W L E D G E M E N T S The work described in this report was carried out in the Pavement Materials and Construction Div is ion (Mr G F Salt, Divis ion Head) of the Highways Group of the Transport and Road Research Laboratory. Special thanks are due to the three County Surveyors and their staffs and to the Department of Transport 's Regional Offices for their co-operation in al lowing the measurements to be made on their roads.

Thanks are also due to Mr D C Webster and Miss J Fricker for their assistance in the data-processing.

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9 REFERENCES COOPER, D R C (1974). Measurement of road surface texture by a contactless sensor. Department of the Environment TRRL Report LR639, Transport and Road Research Laboratory, Crowthorne.

DEPARTM E NT OF TRANSPORT (1986). Specifica- t ion for highway works (Sixth Edition). Her Majesty's Stationery Office, London.

HOSKING, J R, ROE, P G and TUBEY, LW (1987). Measurement of the macrotexture of roads. Part 2: a study of the TRRL mini texture meter. Department of Transport, TRRL Report RR 120. Transport and Road Research Laboratory, Crowthorne.

HOSKING, J R and STILL, P B (1979). Measure- ment of the macrotexture of roads. Part 1. Trials wi th the contactless sensor on Motorway M1. Department of the Environment Department of Transport, TRRL Report SR498. Transport and Road Research Laboratory, Crowthorne.

SABEY, B E (1966). Road surface texture and the change in skidding resistance with speed. Ministry of Transport, RRL Report No. 20. Road Research Laboratory, Crowthorne.

SALT, G F and SZATKOWSKI, W S (1973). A Guide to levels of skidding resistance for roads. Department of the Environment TRRL Report LR510. Transport and Road Research Laboratory, Crowthorne.

STILL, P B and WlNNETT, M A (1975). Development of a contactless displacement t ran sd u ce r. Department of the Environment TRRL Report LR690. Transport and Road Research Laboratory, Crowthorne.

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