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5 ROAD IMPACTS AND NETWORK MANAGEMENT ISSUES

5.1 INTRODUCTION

This section of the report considers the predicted climate change trends associated with different weather events in relation to the design and operation of the Scottish road network. Measures are identified that may enable more effective management of the risks associated with occurrence of these weather events. The criteria for these measures are that they be practical, pragmatic and cost-effective. However, it is considered that some measures will require further and more detailed assessment before implementation. Where appropriate, recommendations for such further assessment have been made.

In particular, in connection with all of the assessments presented here, it is noted that the next update of the UK Climate Impacts Programme, UKCIP06, is understood to be due in 2006. It is therefore recommended that where this presents predicted trends in climate change that are appreciably different from those predicted in UKCIP02, the findings of this study are reviewed and either confirmed or amended as appropriate.

Consideration has also been given to network management practices associated with severe weather events. Consultation with maintaining agents has been undertaken to review their experience of the effects on the road network of such events. This consultation has also reviewed the degree of success achieved in recent years in mitigating potential impacts through preventative intervention, and any measures introduced to enhance road user awareness of severe weather risks.

Figure 5.1 - Snow clearance operations. Photograph courtesy of BEAR Scotland Ltd

The time horizons considered include the UKCIP02 scenarios for 2020's, 2050's and 2080's where given, and additional periods identified through subsequent work undertaken by the Met Office. Wherever predictions have been defined as scalable, figures have been interpolated from the modelled present day climate to the 2080's.

5.2 TEMPERATURE

5.2.1 Introduction

Three main effects of temperature on the road network are considered here, namely:

• Effects of High Temperatures on Bituminous Pavements
• Effects of Freeze-Thaw Action on Bituminous Pavements
• Growing Season
• Air Quality

The contribution of low temperatures to the formation of ice is discussed in relation to winter maintenance issues later in this section under the heading of 'Snow'.

Figure 5.2 - Surfacing operations on the A9 near Blackford. Photograph courtesy of BEAR Scotland Ltd

5.2.2 Effects of High Temperatures on Bituminous Pavements

A number of factors add to the influence of temperature on bituminous pavements. Traffic loading, speed and density, local ambient temperature, volume and frequency of rainfall and intensity of direct sunlight will all modify the effects of elevated temperatures.

The Transport Research Laboratory ( TRL) produced a working paper titled 'Maximum Road Temperatures in Relation to Surface Deformation in Scotland' in 1993. Mean maximum daily temperatures were recorded within asphalt wearing courses over three Scottish summers.

The highest maximum surface temperatures recorded in specific localities included two values of 43¡C and one at 44¡C. These indicate extreme conditions likely in normal summers but in general daily maximum temperatures rarely approached the DWT (Dry Wheel Tracking) test temperature of 45¡C. This is the temperature at which certain design mix asphalts may be liable to deformation. The temperatures within the asphalt wearing course, 25mm below the surface, were found to only rarely exceed 40¡C over the course of the three years of the investigation.

The length of time the road surface is at an elevated temperature is significant. In the DWT test the sample is preheated for a minimum of 4 hours at 45¡C and then tested for an hour at that same temperature. An interim report showed that the road surface temperature would be above 44.5¡C for one hour in 20 years and 3.5 hours once in 100 years. It is therefore probable that the potential for deformation due to prolonged high road surface temperature would occur once in 20 years. Concerns relating to an increase in temperature of bituminous pavements include adverse affects to the hardness of the road surface, fatting up of the road surface and thermal expansion and contraction affecting the integrity of the road surface.

The TRL paper concluded that within a 20 year period design mix asphalts are unlikely to deform due to prolonged high temperatures. It recommended that consideration should be given to future monitoring of road temperatures. This would provide valuable information in respect to any deformation failures occurring during the summer period.

Higher annual temperatures would also require an adjustment to current methods of pavement analysis. Assigned stiffness values are used for analysis and design. If the average temperature is warmer, then materials will display less stiff properties. The less stiff the bituminous component of the pavement is the thicker the pavement may have to be to provide the required lifespan. It should be further noted that at the time of the TRL report the typical design life of a bituminous pavement was 20 years, but many bituminous pavements on major roads are now designed with a 40-year design life and this could exacerbate weakness due to lower stiffness. In parallel with this development, current materials do, however, have typically improved stiffness properties over materials used in the past.

As the TRL report suggests, failure of bituminous carriageways due to high temperatures in Scotland is not a common occurrence. Although rutting, which is possible evidence of this method of failure, has been observed, this tends to have other causes. However, some recent instances of deformation of bituminous materials where heat may have been a factor are listed below:

• 1999, the binder content of surface dressing on certain local roads in a central belt district exhibited excessive fluidity. Gritters were sent out to sand the roads
• 1995, a sudden wearing course rutting failure occurred on a section of southern Scotland trunk road following elevated temperatures. Analysis showed 100pen binder rather than 50pen binder had been used by the surfacing contractor, indicating the potential susceptibility of this material to deformation associated with elevated temperatures.
• 2003, a section of trunk road in the central belt suffered basecourse failure. Material was within specification and no reason for failure was apparent. The hypothesis of the Managing Agent was high temperatures on a heavily trafficked section of carriageway where vehicles were also braking caused the failure.

Managing Agents consider that most rutting problems on the trunk road network are the result of pavement failure due to the volume of heavy goods vehicles. In particular, rather than the effects of high temperatures, it is considered that there is an increasing number of vehicles that are single-wheeled on each side as opposed to double-wheeled.

Raising the temperature in the upper layers of pavement can cause the binder to lubricate the aggregate where the materials are binder-rich. However, this is less of a concern in Scotland, as Scottish aggregate tends to be more angular, thus promoting mechanical interlock.

The introduction of stiffer pavement materials, even with a longer life design, together with the likely rarity of prolonged periods of high temperatures, should result in effects due to high temperatures on recently constructed major roads being very limited. However, older sections of road, and much of the local road network, which is largely composed of less stiff materials, or materials of unknown stiffness, may be more susceptible to failure.

It is recommended that bituminous materials with appropriate stiffness characteristics are specified in road construction or maintenance works on the road network, in order to provide greater confidence that pavement deformation due to high temperatures does not occur. It is not considered cost-effective to replace sections of roads constructed with less stiff materials specifically to address this issue. However, should any pattern of failures emerge in the future this position should be reviewed.

5.2.3 Effects of Freeze-Thaw Action on Bituminous Pavements

During the winter months moisture in certain types of road surfacing material may freeze and thaw in cycles. This cycle of freezing and thawing can cause a volume change within the material, with changing stresses resulting in a loosening effect. This may be exacerbated by vehicles displacing or disturbing surfacing materials, which in turn allows more moisture to ingress.

Once water has entered the road pavement structural damage is initially caused by hydraulic pressure, with vehicles passing over the road pavement impacting considerable and sudden pressure on the water, thus forcing it further into the fabric of the road. Water that has already entered the road pavement and is then subjected to the process of freezing and thawing during the winter further increases this internal pressure. The recent practice of specifying smaller aggregates and requiring intermediate bond or tack coats may be expected to reduce the voids within road pavements, and thus provide greater durability under freeze/thaw conditions.

Surface dressing reacts poorly to freeze/thaw conditions as it has a highly exposed binder. A combination of this, its susceptibility to freeze/thaw action and its thickness may result in embrittlement of the surface. This is then no longer able to provide the function of sealing the upper layers of the underlying bituminous carriageway.

All four climate change scenarios suggest that there will be a significant reduction in the number of days per year when the daily minimum temperature is at 0 ^oC, therefore the annual number of freeze-thaw cycles may be expected to reduce, thus reducing the extent of damage associated with this process.

The experience of surface dressing failure on the trunk road network varies depending upon geographic area, with some areas experiencing large scale failures and others performing satisfactorily. The Managing Agents considered that the failure mechanism is generally a two stage chip-loss, the first soon after application and the second during the winter months, leaving a deeply exposed substructure.

It is also observed that bituminous pavement materials other than surface dressing tend to deteriorate more quickly during the winter months than at other times of the year, although this may relate to winter wetness rather than freeze-thaw action. However, the Managing Agents also considered that temperature changes observed at present tend to result in a greater extent of thawing and refreezing, possibly following a day/night pattern, than observed previously. If so, this would suggest that greater damage may occur from freeze-thaw action in the future.

The porosity of pavement layers is likely to remain an issue of some concern. However, it is recognised that surface dressing is a cost effective short term solution which goes some way to protecting the underlying materials and restoring necessary skid resistance. In order to maximise the effectiveness of this treatment it is recommended that local experience of the durability of surface dressings be reviewed to consider whether these or another intervention measure is appropriate for the location concerned.

5.2.4 Growing Season

Landscaping of roads may entail both significant planting that matures over time to meet visual and ecological objectives, and seeding of verges and side slopes to provide a simple finished form. In order to maintain an appropriate appearance of the soft estate and to maintain a safe road it is essential that cyclic maintenance is undertaken to ensure that vegetation does not obscure signage or visibility splays.

Figure 5.3 - Example of grass-cutting in verges.

It is generally considered that a minimum of two cuts per year is adequate to control the height of grassed roadside verges, centre reserves and junction areas on the Scottish trunk road network. However, in recent years this has increased to 3 or more cuts. This maintenance activity requires traffic management at most locations and hence increased cutting is likely to result in greater disruption and delay to the travelling public.

The approach to landscaping design should recognise the potential effects of a longer growing season, and it is recommended that slow-growing elements are used where appropriate, in order to minimise the extent of cyclic maintenance required. It should be noted that an increasing extent or frequency of cyclic maintenance will require a consequential larger annual budget to achieve the same quality of appearance.

5.2.5 Air Quality

The predicted average temperature increase in Scotland of 1¡C suggests that the future temperature regime in Scotland will not change to be significantly different from that observed elsewhere in the UK at the present time. As the air quality assessment guidance is used throughout the UK, it is therefore not considered necessary at this time to recommend any change to current practices of air quality assessment and prediction.

5.3 RAIN

5.3.1 Introduction

Five main effects of rain on the road network are considered here, namely:

• Road Surface Drainage
• Pavement Deterioration as a Result of Wet Conditions
• Watercourse Flooding
• Ground Water
• Soil Moisture

5.3.2 Road Surface Drainage

Road drainage design has two major objectives:

• the rapid removal of surface water to provide safety and minimum nuisance for the road user
• provision of effective sub-surface drainage to maximise longevity of the pavement and protect its associated earthworks

Traditional methods of draining paved carriageways involve the use of a network of gullies or filter drains to collect surface water runoff, which is then discharged to a watercourse. Due to the closed nature of these systems, the capacity available is limited and can be further hindered by blockages, leading to localised flooding on the road surface.

The effects of the predicted climate changes in precipitation in 8 locations across Scotland with good geographical coverage were selected for analysis. Calculations were undertaken to examine how the return period of the 1-year rainfall, for 3 durations - 15, 30 and 60 minutes, would change according to these statistics. By the 2020's the 1-year event was found to change to a 1.4 +/- 0.2 year event when assessed using the present depth-duration-frequency statistics, with rainfall increases of between 4% and 13%. By the 2080's the 1-year event was found to change to a 2 +/- 0.6 year event when assessed using the present depth-duration-frequency statistics, with rainfall increases of between 10% and 30%.

A number of locations on the trunk road network have been identified as particularly at risk of drainage 'failure', and inspections by Managing Agent Area Teams are consequently prioritised during periods of heavy rainfall. The purpose of these inspections is to ensure unimpaired drainage. The most common cause of flooding in areas where drainage is present is due to detritus being washed into the system, resulting in partial or complete blockage.

Figure 5.4 - Damage to A9 Raigmore Slip Road, Inverness, 2002, following heavy rainfall. Photograph courtesy of BEAR Scotland Ltd.

Filter drains can also be susceptible to failure through root invasion. Serious problems have been observed within 10 year old plastic filter drain pipes that have become inundated with roots. In addition, a major maintenance scheme was delayed when it was found that the sub-base was water-logged. Inspection showed that the adjacent filter drains had consisted of porous concrete pipes, which through root ingress had disintegrated and collapsed.

Given the expected change in rainfall events, it is recommended that consideration be given to revising the parameters for the design storm. This could be done on an immediate basis by simply changing the design storm from the 1 in 1 year and 1 in 5 years events to a 1 in 2 years and 1 in 10 years events respectively, whilst continuing to take account of the available historical information. Alternatively, further assessment could be carried out using climate change modelling to provide guidance on the extents of future 1 in 1 year and 1 in 5 years events. In either case it is important that drainage systems are designed to meet the desired performance level and there is a risk that at present the drainage systems being designed under current guidelines may not achieve that objective.

Some types of drainage systems have increased available capacity for storage of surface water runoff in comparison to gully drainage systems. It is also noted that filter drain systems can also provide environmental benefit through partial filtration of the surface water runoff. It is recommended that where a choice of drainage system is available preference is given to systems that provide additional capacity and take account of sustainable drainage techniques.

5.3.3 Pavement Deterioration as a Result of Wet Conditions

The durability of a mixed material depends on either its ability to keep the weather out if it is intended to be an impermeable material, or the ability of its components to resist the weather, if it is permeable. Certain elements of bituminous pavements can be permeable, and the pavement will deteriorate if moisture remains within the bound or sub-grade layers. It is therefore essential for pavement durability that effective drainage is present to remove both surface and sub-surface water.

The climate change predictions suggest wetter winters, in which circumstances the greatest extent of pavement deterioration is likely to occur. This would indicate that the need for effective drainage to maintain pavement durability is likely to be greater in the future.

The Managing Agents noted that recent summers appear to have been wetter than average and this has coincided with the need for more structural trunk road maintenance than in previous years. It may be that the level of deterioration experienced is as a result of the high levels of this precipitation. The sections of carriageway which have been most badly affected have in general been older sections of carriageway, sections already showing signs of surface deterioration and sections with limited or no formal drainage system.

It is recommended that appropriate formal surface and sub-surface drainage systems are introduced to the road network during maintenance operations where these are not in existence at present. It is noted that for many of the rural roads in Scotland this will involve land purchase to accommodate the extra width required for drainage measures. However, it is considered that the long-term benefits will usually justify the additional investment.

5.3.4 Watercourse Flooding

Whether major river or minor watercourse, flooding from catchment response to storm events is a significant risk, with the potential to impact on the safe operation of the road network. Examples of the issues that may arise include:

• Bridge/culvert capacities exceeded, causing upstream flooding to occur
• Overtopping and scouring problems to structures
• Roads and any properties on flood plains becoming inundated

The climate change predictions suggest that in addition to the generally wetter winters that are expected, the intensity of precipitation is predicted to increase. In addition, rising temperatures resulting in fewer snow days and a shorter period prior to snowmelt occurring through fewer days where the temperature is below 0¡C, may create a situation where catchment responses are significantly higher than previously observed.

The view of the Managing Agents is that generally there are very few areas of the trunk road network in Scotland that are susceptible to river flooding and it is not seen as a major concern in terms of the frequency of incidents. In addition, emergency response plans are in place to cover events including flooding. In one area the Managing Agent is linked to a flood early warning system run by the local authority, and it is understood that consultations are currently taking place to discuss recent flooding events with the Operating Company, Council Emergency Planning Officers, SEPA, Police and Scottish & Southern Electricity in an effort to further improve the system of communication.

Culverts are generally considered by the Managing Agents to represent a more recurrent, if less extensive, concern, with flood events regularly affecting particular locations. This is due, in part, to culverts at these locations being unable to accommodate flood-borne detritus, which then reduces the available capacity of the culvert and hence exacerbates the impact of the flood event. To address this there are programmes for culvert inspections, focussing attention in specific months, when areas of particular concern are identified and monitored by the Managing Agent for additional attention.

It is noted that that while major watercourses rarely affect the trunk road network, they can be severely damaging to local roads. Such roads often follow historic routes that pre-date significant developments within the river catchment. As a result, at the present time they are at a much higher risk of flooding than at any time previously.

Where a known problem with regard to flow capacity exists, it is recommended that assessments should be made of the implications of improving or replacing the structure concerned. In order to target this work, it is recommended that a schedule of watercourse structures that have been affected by flood events is prepared and those that have seen repeated occurrences be treated as the highest priority.

Figure 5.5 - Flooding on the A90 at Boddam, 2003.

In addition, when developing the designs for watercourse structures, it is recommended that the design return period be reviewed, to take account of the predicted change in intensity of rainfall event and the other factors that may affect catchment response. At present the design of such structures is based on a return period between 1 in 50 years and 1 in 100 years. As for the surface water drainage systems, the change could be implemented on an immediate basis by simply amending the design storm from a return period of between 1 in 100 years to 1 in 200 years. Alternatively, further assessment could be carried out using climate change modelling and reviewing flood estimating procedures to provide guidance on the extents of future 1 in 50 years and 1 in 100 years events. In either case it is important that the structures are designed to meet the desired performance level. At present there is a risk that the systems being designed may not achieve this objective.

It is also recommended that consideration be given to extending the flood warning systems that have been developed by other public bodies and agencies to identify potential conflicts with the road network. This could include, for example, integrating a Geographical Information System for known watercourse areas of concern with systems showing predicted catchment responses to anticipated rainfall events. This could also usefully include pre-agreed proposed diversion routes for local or trunk road traffic should it become necessary to close the affected section of road.

This system would also be enhanced through improved communication between all parties with responsibilities arising from major flood events. This would also facilitate improved communications with NADICS, affording the opportunity to improve the dissemination of information to road users.

It is noted that clarification is being prepared on the requirements for inspections of watercourse structures on the trunk road network that are potentially susceptible to scour. It is recommended that this address both periodic and post-flooding event requirements, in order to provide early warning of any potential problems.

The effective maintenance of watercourses and ditches is essential to the operation of culverts and it is recommended that measures to target areas where known problems exist through pre-emptive clearing of detritus in advance of predicted heavy rainfall should be considered by all maintaining authorities.

5.3.5 Ground Water

Ground water is one of the critical elements affecting the design of cutting slopes. Parameters used in design include the height of ground water and the degree of movement to which it is susceptible. Changes in these parameters can materially affect the design or operational effectiveness of the cuttings concerned. The presence of effective surface and sub-surface drainage previously discussed for the road pavement, together with well maintained pre-earthworks drainage at the top of slopes, also enables cuttings to remain stable. In some instances counter-forte slope drainage is also required to maintain slope stability.

Ground water is also one of the factors that is a potential contributor to landslide events. Further information on this subject may be found in the Landslides Study, as referred to in Section 1 of this report.

The climate change trends tend to suggest that potential evaporation is likely to increase across Scotland. This factor, coupled with the reduced summer rainfall, will tend to result on average in greater and more persistent soil moisture deficits, thus reducing the summer and autumn recharge to the groundwater. However, with slightly increased winter soil moisture it is possible that winter conditions will increase the recharge to the ground water. This may yield greater variability in ground water in soils that are reasonably permeable. It is possible that in winter conditions the ground water levels may be higher than allowed for in the design of the cuttings concerned.

The Managing Agents expressed no opinion in relation to observation of any impacts associated with general changes in ground water level or moisture content.

While no formal recommendation can be made without an appropriate climate change model being developed for this issue, it is recommended that consideration be given to carrying out earthworks inspections under the principles of HD 41/03 'Maintenance of Highway Geotechnical Assets' of the Design Manual for Roads and Bridges by parties responsible for maintaining the road network.

Figure 5.6 - Debris flow being cleared from the A82, 2002. Photograph courtesy of BEAR Scotland Ltd

5.3.6 Soil Moisture

Soil moisture is one of the factors that affects catchment response, with soils holding greater moisture being less capable of absorbing additional rainfall and therefore contributing to an increased surface water runoff.

The climate change trends tend to suggest that potential evaporation is likely to increase across Scotland, which coupled with the reduced summer rainfall will tend to result on average in greater and more persistent soil moisture deficits. However, it is considered winter soil moisture will increase slightly, which together with the additional winter wetness and greater storm intensity may result in increased catchment runoff.

The Managing Agents noted that some landslide events occurred when a period of intense rainfall followed a longer period of general rainfall. It is possible that this is a demonstration of the implications of increasing soil moisture producing greater catchment runoff.

There are a number of methods available to enable the estimation of surface water runoff for the small to medium sized catchments in which the road network typically lies. It is recommended that further work be undertaken to review the assumptions underlying these methods, in the light of predicted changes in variables such as rainfall, soil moisture and snowfall. This review could propose a range of alternative assumptions to be tested in the design process, providing different outputs for the surface water runoff and hence alternative proposals for drainage provision and watercourse structures. The implications of greater or lesser degrees of provision could then be assessed on a cost/benefit basis, taking account of the increased level of confidence that would be associated with a greater level of provision.

5.4 SNOW

5.4.1 Introduction

Snow has two significant effects on the road network. The contribution of snowmelt to catchment runoff has been discussed previously. The other effect, discussed here, is the impact of winter weather conditions on the operation of the road network. This includes the implications of predicted climate change trends on both snowfall and ice formation.

Figure 5.7 - A90, South of Aberdeen, 2004. Photograph courtesy of Performance Audit Group.

5.4.2 Winter Conditions

Maintaining availability, reliability and safety of the road network is a key objective of winter maintenance. Snow and ice on the road causes hazardous driving conditions and can result in damage to the fabric of the road pavement. Therefore, effective winter maintenance makes important contributions towards road safety and the minimisation of whole life costs.

While the predicted climate change trends suggest a general reduction in snowfall, it is also noted that individual years may have greater than average snowfall as a result of natural variability. In addition to generally reducing snowfall, the predicted climate change trends also suggest that there will be fewer days where the temperature is below 0¡C and therefore less ice formation may also be expected. Therefore, it may be anticipated that future winter maintenance requirements will be less onerous over a period of years than at present. However, as a result of natural climate variability it is possible that individual years may have winter maintenance requirements similar to the modelled present day climate.

Consultations with the Managing Agents suggest that milder winters, with reduced snowfall, are being observed at present. However, they have also suggested that there is a change in some other aspects of the weather behaviour. This includes the more frequent occurrence of snow/ice thawing and then refreezing. As a consequence of these conditions, the practice of gritting to prevent freezing becomes less effective, with grit being washed off the road surface prior to the next freezing event. This requires a heavier spread of grit in order to prevent wash-off, which increases maintenance costs.

Reducing winter maintenance burdens may result in lower costs of winter maintenance services. However, the risk of significant individual events will mean a continuing need for services to be available with short mobilisation periods in order to achieve the desired road availability. It is recommended that at an appropriate time future winter maintenance arrangements for both trunk and local road networks consider this likely pattern of change, in order to make cost-effective use of resources. It is also recommended that further research be undertaken on freeze-thaw patterns relating to night-time and day-time temperatures, to provide guidance on whether changes to current winter maintenance practices are required.

The predicted reduction in winter maintenance requirements may result in road users being less likely to delay or cancel travel plans in the light of individual winter weather events, having become used to a road network that is generally available for use. It may be that additional road user education programmes are required, which could usefully be extended to cover road user behaviour in all severe weather events. This is discussed more fully in a later part of this section, under the heading 'Road User Information and Behaviour'.

5.5 WIND

5.5.1 Introduction

Wind has two main impacts on the road network, namely:

• Effect on Structural Elements/Roadside Furniture
• Effects on Operation of the Road Network

Figure 5.8 - Wind Damage to Road Sign, Edinburgh, 2005. Photograph © The Scotsman Publications Ltd.

5.5.2 Effect on Structural Elements/Roadside Furniture

Extreme wind events are used in the design of many elements of the road network. The achievement of satisfactory operational performance is dependent on the results of this evaluation. Hence, extreme winds may affect the built environment, for example traffic signs. In addition, they may also affect landscaping adjacent to the road network and significant disruption may result as a consequence of damage to large elements, such as trees.

The effect of an increase in wind speed is dependent on the pressure of the wind, which is proportional to its velocity squared. For example, an increase in wind speed of 10% has the effect of increasing the structural loading on traffic signs by 20%. While this should have little effect on the vast majority of well built structures, damage may occur as a result of the increased loading where elements have not been correctly designed, fabricated or installed.

While the predicted climate change trends suggest that there will be slight changes in extreme winds, it is also noted that this is an area where significant uncertainty exists. The annual average is shown to reduce in the Fair Isle, Faeroes and Hebrides shipping areas, with small increases of up to 4% along coastlines and in the North Sea. However, wind direction and duration must also be considered, as changes in the extreme and prevailing directions, or relative changes in their speeds, may be significant. Currently little is known on these issues.

While high winds are considered by the Managing Agents to be an area of concern, it is not considered that a greater level of failures of structural elements or roadside furniture is being observed than was previously the case. In most instances failures of elements such as road signs following high winds are considered to be due to inadequate foundations, rather than necessarily being the effects of extreme high winds.

In light of the uncertainty that exists in relation to the predicted climate change trends in wind, it is recommended that further research be carried out on this subject, to enable more definitive guidance to be provided.

5.5.3 Effects on Operation of the Road Network

Extreme winds can disrupt operation of the road network through impacts on high sided vehicles. As their speed increases, high sided vehicles become increasingly unstable in gusts of over 20 m/s (45mph). Therefore, to maintain stability drivers need to slow down when experiencing high winds. At some sites, such as major bridges, closure of the road to high sided vehicles is required to prevent their exposure to these winds. This usually results in such traffic, generally heavy goods vehicles, being diverted from major roads to less suitable local roads. This can cause disruption on, and potential damage to, these roads.

Figure 5.9 - High sided vehicle blown off the road. Photograph © EMPICS/ PA/ AP.

As noted above, while the predicted climate change trends suggest that there will be slight changes in extreme winds, this is an area where significant uncertainty exists.

The Managing Agents noted that where vehicles are blown off the carriageway, it is usually necessary to temporarily close some or all lanes to recover the vehicles. This creates additional disruption to road users. It was also noted that the decision on the closure of roads to high sided vehicles is normally taken by the Police and/or a Toll Operating Company. In making this decision the Toll Operating Companies follow defined procedures, which include assessing the implications of both wind speed and direction. At present closures on other roads do not usually follow any defined protocols. Recent events have identified this as an area that requires clarification of roles, responsibilities and the decision making process. In addition, the Managing Agents expressed concerns that insufficient measures exist for advance signing and the provision of parking/turning areas for high-sided vehicles when sections of the road network are closed.

One method of avoiding the need for such closures would be the inclusion of wind barriers at exposed sites. It is recommended that sites which are regularly closed to high sided vehicles are reviewed to determine whether they have the potential to be fitted with wind barriers. This assessment should include a cost/benefit analysis. It is acknowledged that it may not be technically feasible or economically justifiable for many such sites to be fitted with wind barriers. It is also recommended that all new road schemes which include sites likely to be exposed to high winds be reviewed at the design stage. This would enable an early decision to be taken on the inclusion or otherwise of wind barriers, at a stage when the economic implications of inclusion are at a minimum.

Figure 5.10 - Wind barriers at the Second Severn Crossing. Photograph © Severn River Crossing PLC ( SRC), used with permission of SRC and Highways Agency.

It is noted that a High Winds Strategy is currently in development for the trunk road network. This will address the procedures to be followed, including diversion requirements, in the event of closure of sections of the trunk road due to high winds. It is recommended that this strategy take account of any future information on predicted climate change trends in wind, should such become available.

It is also recommended that consideration be given to the physical measures necessary to accommodate parking/turning of traffic affected by areas of the road network that are regularly closed due to high winds. Recommendations in respect of road user behaviour in high winds are discussed in a later part of this section, under the heading 'Road User Behaviour'.

5.6 FOG

Fog impacts upon the safe operation of the road network through reducing visibility and thus creating a road safety hazard.

Figure 5.11 - Forth Road Bridge, Edinburgh. Photograph © The Scotsman Publications Ltd.

Predicted changes in fog are not routinely output by current climate models, although it is a current area of active research. At present the predictions, which suggest a reduction in the number of fog days, must be seen as provisional until a better understanding can be reached.

Although the Managing Agents noted that fog was an ongoing concern, they had no particular comments on this issue.

The concerns in respect of safe operation of the road network in fog conditions are capable of being addressed through improved road user behaviour, and this is discussed more fully in a later part of this section, under the heading 'Road User Behaviour'.

5.7 COASTAL FLOODING

The predicted trends in climate change for coastal flooding, including the implications of surge heights, suggest that this has the potential to affect low lying roads in coastal areas. This may result in damage to the road, road closure, or the occurrence of road safety hazards, as seen following the severe winter storm of January 2005. It should be noted that the roads at risk are predominantly part of the local, rather than the trunk, road network.

The Managing Agents considered that there are few areas of the trunk road network susceptible to coastal flooding. In addition to flooding, there were concerns expressed that coastal erosion may affect sections of the road network.

Figure 5.12 - Coastal Damage in South Uist, 2005. Photograph courtesy Mr D I Caimbeul.

It is recommended that the road network be reviewed to identify areas of potential risk from coastal flooding, taking account of the cumulative effects of sea-level changes and storm surges. Areas at risk may then be addressed through a combination of warning signage, edge-strengthening or introducing sea-defences. In extreme cases, consideration could be given to whether re-routing is appropriate. It is also recommended that any new projects proposed in low-lying areas should be reviewed with respect to these risk factors, to enable appropriate decisions to be taken at the design stage.

5.8 ROAD USER BEHAVIOUR

In addition to the impact of severe weather on performance of the road network discussed previously, road users are also affected in other ways by severe weather events, such as:

• Heavy rainfall and/or poor surface water drainage, which can result in excessive spray, reducing visibility, and wet pavements providing poorer skidding resistance
• Flooding, whether catchment or coastal in origin, which can create areas of deep ponding that may not be apparent to road users
• Winter conditions, which can result in poorer skidding resistance
• High winds, which can result in unexpected forces being applied to vehicles, affecting driving behaviour
• Fog, which can reduce visibility

While the occurrence of some of these events, such as winter conditions and fog, may reduce in the future, instances of others are expected to increase. Due to natural variability in weather events, it is not possible to eliminate all of these potential effects completely through design. Therefore, there is the potential for road safety hazards to continue to occur. Although effective management of the road network can provide additional information to road users, the avoidance of these hazards is largely dependent on road users modifying their behaviour in response to this information. It is considered that ongoing road user education is an essential component in raising the awareness of the need to modify behaviour during severe weather events. It is also considered that the provision of relevant information to road users in respect of such events would assist in encouraging modified behaviour.

The Scottish Executive already supports a range of road user education programmes, including anti drink-driving and speed reduction campaigns. An example of this in relation to weather is the guidance provided to drivers relating to winter weather conditions. It is recommended that consideration be given to developing a similar approach for all severe weather events where modified driver behaviour would be desirable. This could clearly identify specific messages, such as the need to reduce speed in poor visibility.

To further support this road user education programme, it is recommended that the specific messages for driver behaviour in severe weather conditions be incorporated into the information provided to drivers through the Variable Message Signs ( VMS) operated by NADICS. This would have the benefit of reinforcing the specific messages. It could also encourage improved road user behaviour than is observed at present.

Figure 5.13 - Variable Message Sign on the M8 near Glasgow

In addition, it is recommended that consideration be given to the use of VMS's to convey additional information relating to severe weather events. This would be of local relevance and, for example, could indicate the risk of heavy rain, or the likelihood of fog. These messages, conveyed in terms of probability, would inform road users of changing circumstances. It is understood that expansion of the VMS network is planned and this would afford the ability to convey locally relevant information. It is acknowledged that the systems required to support dissemination of this information would entail additional capital expenditure. However, it is considered that opportunities exist to integrate these with existing weather monitoring and prediction systems, thus minimising the expenditure required.

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