DEPARTMENT OF GEOGRAPHY
UNIVERSITY COLLEGE LONDON
THE PREDICTION OF ICE FORMATION
ON MOTORWAYS IN BRITAIN
by
John Edward Thornes
August 1984
A Thesis Submitted for the Degree of Doctor of Philosophy
for the University of London
ABSTRACT
Each winter, Britain spends up to £120 million spreading approximately 2 million tonnes of rock salt on our roads to keep them free of ice and snow. This thesis shows that it would be possthle to significantly reduce the amount of salt spread, by improving the accuracy of the Road Danger Warnings issued to Highway Authorities. Each day in winter, the maintenance engineer receives a Road Danger Warning from his local weather centres. Unfortunately these Warnings are not very accurate because they are based on forecasts of minimum air temperature alone, rather than using road surface temperatures. During the winter of 1982/83, of 102 Road Danger Warnings issued to Hereford and Worcester County Council, only 32 were correct in predicting icy conditions on the MS motorway.
This thesis presents a computer model to predict ice formation on roads up to 24 hours ahead. During the winter of 1978/79 instruments were installed in the M4 motorway to measure road surface temperature and wetness. The computer model has been tested retrospectively for 30 nights when the road surface temperature fell below 5°C. The predicted minimum road surface temperature has a root mean square error of 0.9°C. During the winters of 1982/83 and 1983/84, the model was tested in 'real time' against road surface temperatures measured automatically on the MS and MG motorways, giving a root mean square error of 1.5°C for 80 nights during 1.982/83, and 1.3°c for 120 nights during 1983/84.
The form of the issued Road Danger Warnings has been changed from a simple sentence issued over the telephone or using telex, to a graph of predicted road surface temperature and wetness. An optimistic and a pessimistic graph is issued to give the maintenance engineer an idea of the certainty of the forecast.
The thesis proposes a national network of automatic road surface monitoring sites. Each site would be linked to microcomputers in local weather centres, which would then run the prediction model and issue Road Danger Warnings accordingly. The information could then be sent to maintenance engineers using Prestel.
CONTENTS
Abstract II
Acknowledgements III
List of Contents Iv
List of Figures VII
List of Tables x
Chapter 1 Introduction to Winter Maintenance 1
1.1 Introduction 1
1.2 Atmospheric Management 4
1.3 Atmospheric Hazards 8
1.4 The Ice/Snow Hazard and Traffic Flow/A.ccidents 12
1.5 The History of Winter Maintenance in Britain 16
1.6 International Research into Winter Maintenance 26
1.6.1 Finland 27
1.6.2 Sweden 29
1.6.3 Other European Countries 30
1.6.4 North America 30
1.6.5 Other Countries 33
1.7 Aims of the Thesis 34
Chapter 2 Motorway Winter Maintenance in England 35
2.1 Introduction 35
2.2 The Rayner Scrutiny of Winter Maintenance 36
2.2.1 The Need for Winter Maintenance 38
2.2.2 Financial Analysis 43
2.2.3 The Recommendations 55
2.3 Road Danger Warnings and Winter Maintenance on Motorways 61
Chapter 3 Modelling the Atmospheric Boundary Layer above a Motorway 63
3.1 Introduction 63
3.1.1 Introduction to Planetary Boundary Layer Models (PBL) 63
3.2 The Boundary Layer over a Motorway G7
3.3 One-Dimensional Modelling of the Boundary Layer over a Motorway 68
3.3.1 Flux Profiles in a Non-Neutral Boundary Layer 72
3.3.2 Operational Forecasting 73
3.4 Modelling the Energy Blance of a Road-Air Column 74
3.4.1 Outcalt's 1971 Model 75
3.4.2 The Heat Balance Model for Predicting Road Surface
Temperatures - 78
3.5 Net Radiation 78
3.5.1 Solar Radiation 78
3.5.2 The Effect of Cloud 80
3.5.3 Outgoing Terrestrial Radiation 82
3.6 The Sensible Heat Flux 86
3.7 The Latent Heat Flux 93
3.8 The Road Heat Flux
3.9 Summary 100
Chapter 4 Instrumentation of the M4 Motorway 102
4.1 Introduction 102
4.2 Instrumentation 102
4.2.1 Temperature 107
4.2.2 Wind 109
4.2.3 Surface Wetness 112
4.3 Data Collection 112
4.4 Data Analysis 113
4.5 Slab Temperatures 129
4.6 Sub-Road Temperatures 133
4.7 Maintenance Depot Action 136
Chapter 5 The Fortran Model 141
5.1 Introduction 141
5.2 Alterations to the Fortran 141
5.3 Inputs to the Model 145
5.4 Sensitivity Analysis 21-22 March 1980 148
5.4.1 Damping Depth Temperature at 72 cm Below Road Surface 148
5.4.2 Wind Speed 152
5.4.3 Air Temperature and Humidity 152
5.4.4 Cloud Cover 153
5.4.5 Time of Year 155
5.4.6 Road Wetness 155
5.4.7 Forecasting Errors 157
Chapter 6 Actual Versus Predicted Road Surface Temperatures 160
6.1 Introduction 160
6.2 Night by Night Analysis of Model Performance 160
6.3 Overall Performance of the Model 206
6.3.1 Use of Average Input Values 206
6.3.2 The Likely Effect of Traffic on Road Surface Temperatures 209
6.3.3 The Effect of Precipitation 211
6.3.4 Latent Heat Release with the Formation of Ice 212
6.3.5 Hoar Frost Formation 212
6.4 Potential of the Model for Real Time Forecasting 213
Chapter 7 Real Time Forecasting for the M5 Motorway 215
7.1 Introduction 215
7,2 The Three West Midlands Motorway Sites 215
7.2.1 thapman's Hill MS 218
7.2.2 Ray Hall M5/M6 Interchange 221
7.2.3 Coleshill M42 222
7.3 Comparison of Observations Between the Three Sites 223
7.4 The Winterof 1982/83 Road Danger Warnings and Saltings in Hereford and Worcester 235
7.4.1 Salting Records at Lydiate Ash 238
7.4.2 The Scan 'Freeze Factor' 239
7.5 Road Danger Warnings Issued to Hereford and Worcester from Elmdon 244
7.6 The Performance of the Model in Predicting MS Temperatures 254
7.7 Overall Performance of the Model 256
Chapter 8 Current and Future Developments of the Model 260
8.1 Introduction 260
8.2 Data Collection 260
8.3 Data Processing and the Production of the Road Danger Warning 263
8.4 Disseminatiou of the Graphical Road Danger Warning 265
8.5 Future Developments of this System 266
8.6 Final Coimnents 270
Chapter 9 Conclusion 272
9.1 Introduction 272
9.2 Fulfilment of the the Aims of the Thesis 272
9.3 Summary of Conclusions 274
Reference 275
Appendix 1 284
Appendix 2 302
8.6 Final Comments
This thesis attempts to cut across the traditional boundaries of meteorology and municipal engineering. In doing so many assumptions have had to be made to realise a final useable model for the production of more accurate Road Danger Warnings. Research does not stop here however. Although the model has and is performing remarkably well considering its primitive physical base, more research is required to narrow the gap between optimistic and pessimistic forecasts. Also thermal maps of all main roads are required to ascertain the micro-. climatic differences in road surface temperature around the region. Road Danger Warnings issued. by the Meteorological Office have hardly altered in format in 30 years. It is time for change.
CHAPTER 9 CONCLUSION
9.1. Introduction
The data used in this thesis to test the road surface temperature prediction mode]., has been collected over a period of five years from 1979 to 1984. During this time the model has evolved and been adapted to produce an output suitable for use by a practicing maintenance engineer. The results have been encouraging to the extent that the model is now to be used by the Meteorological Office for producing Road Danger Warnings at Elmdon Weather Centre, starting with the winter of 1984/85. The recommendations outlined in Table 8.1 have been fully approved by the Department of Transport and the Meteorological Office. The equipment used at Birmingham University to test the model in real time, will be transferred to Elindon Weather Centre during the Autumn of 1984.
Birmingham University will stay involved in the project, acting as consultants to the Meteorological Office, and also running a company entitled 'Thermal Mapping International' which will start to'thermally map'the roads of Britain and Europe. During September 1984 the University is host to the 'International Road Weather Conference' which will bring together the latest developments in winter maintenance practices. It can be summised from these statements that the research is still ongoing. Nevertheless sufficient has already been learned to significantly imnrove Road Danqer Warnings in Britain and also with the aid of thermal mapping it is possible to give more accurate .regional Road Danger Warnings.
9.2 Fulfilment of The Aims of the Thesis
The road surface temperature prediction mode], developed, tuned, tested and reported in this thesis represents a useful framework for future developments of Road Danger Warnings. The model can still be improved to provide more accurate optimistic and pessimistic forecasts. More development is required to disseminate the forecast information to maintenance engineers over a medium such as Prestel. A faster microcomputer is needed to enable the model to be run quickly for many sites at once.
The gap between . research and operational use has yet to be finally closed. County Councils are spending a great deal of money to put sensors into their roads. GoUand (t984) the Principal Engineer for Maintenance Management for West Midlands County Council states:
'The installation of an ice early warning system is fairly expensive and has, to some extent been an act of faith by the few Highway Authorities who. have taken the plunge. However, evidence is accumulating which would suggest that the investment is economically justifiable due to the potential savings which are available once the operational users establish their faith in it and develop experience in using it effectively.
The maximum potential at present lies in improving the accuracy of forecasts, which can only be achieved by reducing the area covered by one forecast. The computer model developed by the Meteorology Department of Birmingham University on behalf of the TRRL has proved to be a :very promising development, and there would be considerable benefits available to all Highway Authorities with ice warning systems if the model could be extended in operation.'
The aims of this thesis set out in Chapter One, have been largely fulfilled,, The success of the model will now depend upon the Meteorological Office, and the speed with which they can gear up to the use of microcomputers in regional weather centres.
9.3 Summary of Conclusions
This thesis has shown that the meteorological dimension of winter maintenance in Britain has been largely ignored. In order to fill this gap the following findings of this thesis are presented:-
(1) Current Road Danger Warnings issued to Highway Authorities are inadequate.
(2) An extensively modified version of the heat balance model proposed by Outcalt (1971) can be used successfully to predict road surface temperatures up to 24 hours ahead, despite it s primitive physical base.
(3) The form of the Road Danger Warning needs to be changed. A graphical presentation, with 'optimistic' and 'pessimistic' scenarios is presented. This form seems to -be prefered by maintenance engineers.
(4) Sensors embedded in the road surface are necessary to measure road surface temperature, wetness and salinity. This information can then be fed directly into the heat balance model, and used to check the model's performance in real time.
(5) Thermal mapping is needed to locate the sensors in their optimum location, and to provide an accurate 'fingerprint' of road surface temperatures between sensors.
(6) A national network of sensors is -required to feed into the local weather centres around Britain. This is already under discussion with the Meteorological Office, and a trial is being mounted at Elmdon airport for the winter of. 1984/85.
(7) Expenditure on winter maintenance can potentially be reduced by up to 50%; a considerable saving on- the current annual budget of up to £120 million in Britain.
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