Forth Replacement Crossing - Report 1 - Assessment of Transport Network
Jacobs and Faber Maunsell were commissioned by Transport Scotland to undertake the Strategic Transport Projects Review (STPR) study. The STPR commission involves identifying the strengths and weaknesses of the Scottish strategic transport network, identifying gaps between the future demand and capacity of the network, and producing a prioritised list of interventions for the period 2012-22. The commission also covers a study of the Forth Replacement Crossing.
This is the first of the reports, covering the work undertaken under the Forth Work Package ‘Report on Network Performance‘. It examines the condition of the Forth Road Bridge and Forth Bridge and the surrounding transport network. Environmental constraints are defined before base transport information is examined. Performance indicators are then highlighted for possible use later in the Study, before future conditions are examined using information from the Transport Model for Scotland.
The study area and associated transport network is shown in the following figure:
EXISTING CONDITION OF THE FORTH ROAD BRIDGE
Although the Forth Road Bridge has been maintained throughout its lifetime, it is showing signs of deterioration, mainly as a result of the growth and increase in weight of traffic together with the influence of the weather and climate.
Following internal inspection of the main cables, corrosion was discovered and the cables are estimated to be working with a loss of strength of 8 to 10 per cent. The current cable factor of safety is between 2.2 and 2.3. Predictions indicate that, at the present rate of corrosion, the factor of safety could fall below the acceptable value of 2.0 in 2013/2014. It is expected that the installation of a dehumidification system in 2007-08 will arrest the deterioration. It is also recognised that, in the worst case where the dehumidification does not work, steps such as the restriction of Heavy Goods Vehicles (HGVs) would need to be phased in.
In several other aspects, regular disruptive maintenance to preserve the integrity and life of the bridge is required. The condition of the strands in the anchorages is unknown and some form of testing needs to be devised to provide confidence that the strands have not deteriorated through hydrogen embrittlement. A suitable form of testing is being studied but the method and feasibility has not yet been determined.
Strengthening is required to the stiffening truss of the main bridge deck. Although fatigue has not been an important issue to date it is likely that the fatigue sensitive details in the main deck will need major maintenance in the future.
Resurfacing and painting are extremely disruptive operations and both need to be carried out regularly to maintain the integrity and life of the structure. In common with all bridges, the support bearings need to be replaced several times through its lifetime. The existing parapets do not comply with current standards and will need replacing.
The bridge continues to carry an Annual (7 day) Average Daily Traffic flow of 66,000 vehicles in 2006. There are still many years of life in the bridge but with high ongoing maintenance liabilities, provided that the deterioration of the main cable can be arrested. FETA has commissioned a study into the replacement or augmentation of the main cables which will determine the possible work scope and traffic management implications. It is likely that the works involved will be highly disruptive. The study will also investigate methods of determining the condition of the post-tensioned anchorage strands.
Ongoing painting of the bridge is required whatever the traffic loading applied to the bridge. The volume and weight of the traffic loading has a critical effect on the surfacing, deck, main cable, hangers, joints and bearings. In summary, there will be an increased requirement for disruptive maintenance in the foreseeable future, due to increases in traffic levels (particularly HGVs), and remedial measures that require to be undertaken on the bridge structure itself.
SUPPORTING ROAD NETWORK
In the southern Bridgehead the two main roads supporting the Forth Road Bridge are the A90 and the A8000. The A90 provides direct linkage with Edinburgh whilst the A8000 provides connection with the trunk road and motorway network. These two routes in turn connect to the A8/M8, A720 Edinburgh city bypass, M9 and A89.
A major construction project is currently underway which will extend the M9 spur motorway to connect with the A90 which in turn will be widened as far as the Bridge Toll plaza. This work stared in June 2006 and completion is expected by the summer of 2007.
Traffic modelling carried out during the planning stages of this new road found that it is likely to lead to an increase in traffic using the Forth Road Bridge of 3 per cent on opening.
The Northern Bridgehead road network is primarily the A90/M90 corridor together with its principal feeder routes, the A92, A921, A985 and A823 (M). Dunfermline, the major town to the north west of the Forth Road Bridge, can be accessed from the M90 through two roads: the A823 from the south and the A907 from the east.
The A985 runs along the north side of the Firth of Forth linking the Kincardine Bridge and the Forth Road Bridge. The A985 is heavily trafficked with a high proportion of HGVs. The existing alignment does not comply with current trunk road design standards and exhibits few overtaking opportunities. A Route Action Plan Study of the A985 is currently being undertaken by Transport Scotland to look at ways of providing additional overtaking opportunities and safety schemes along its length.
The Upper Forth area provides further options for crossing the Firth, via the Kincardine Bridge and the A91. The Kincardine Bridge crosses the Forth 15 miles upstream of the Forth Road Bridge. It carries one lane in each direction, and is the diversionary route for traffic should any closure (partial or full) of the Forth Road Bridge be required.
A new bridge crossing of the Forth at Kincardine is currently under construction and is due to open towards the end of 2008. It is located to the west of the existing bridge and will form a connection from the upgraded A876 to the A977 south of Gartarry Roundabout. As part of the overall strategy for Kincardine, the ‘Eastern Link Road’ was built to provide relief from traffic using the A985.
Anecdotal evidence suggests that the diversion of traffic from the Forth Road Bridge due to maintenance, or high winds, causes very significant congestion on and around the bridge, and at Kincardine. Traffic modelling to examine the effect of closing the Forth Road Bridge to HGVs on the surrounding road network will be undertaken later in the study and reported in Report 3.
The Forth Bridge and operations of the rail network in the corridor have been examined under the following headings:
- route capability;
- bridge maintenance;
- current route improvements being undertaken by Network Rail; and
- areas to be explored to improve performance and/or increase train paths.
Structurally the Forth Bridge has an expected remaining design life of over 100 years.
The environmental constraints on the Study have been examined under the following headings:
- Ecology and Conservation;
- International Protection of Sites;
- International Protection of Species;
- National Protection of Sites;
- National Protection of Species;
- Other Ecological Designations;
- Nationally Protected Sites;
- Locally Protected Sites;
- Archaeology and Cultural Heritage;
- Scheduled Ancient Monuments;
- Listed Buildings;
- Conservation Areas / Heritage Conservation; and
- Countryside Policy Areas.
The major constraints are the Special Protected Areas, which will heavily influence the corridor selection for any Forth Replacement Crossing option.
Existing Transport Data
Analysis of the most current traffic flow data has proven the continuing demand for vehicle crossings of the Forth. This is demonstrated in the continued profile of traffic crossing the bridge in both peaks, with southbound flows highest in the morning and northbound flows highest in the evening. The highest combined peak is in the evening, where the combination of northbound and southbound flow (which is nearly as high as the morning peak) creates the highest daily peak.
The profiles of the combined morning and evening peaks show that both have a build-up period followed by a two hour peak which is followed by a post peak "tailing off" hour.
The availability of data from the A90 northbound counter has demonstrated that over the last eight years traffic flow has increased, which in turn has led to a noticeable occurrence of peak spreading around the peak hours. This may be as a result of regular drivers starting their journeys slightly earlier or later to avoid the peak periods and may also be a consequence of the wider use by companies of flexible working hours.
The following Performance Indicators have been selected for use in subsequent analysis:
- total vehicle traffic across the Forth Road Bridge and any new crossing. This can be presented by direction and time of day, and split by mode, particularly HGVs;
- road congestion indicators (number of vehicle-hours in excess of that experienced at free-flow speed);
- journey times of selected routes for bus and car;
- average speeds by geographic area, and on a link by link basis;
- mode share of public transport;
- select link analysis, to show the origins and destinations of journeys using a particular link;
- • Carbon Dioxide (CO2) emissions;
- • other transport related pollutant emissions;
- public transport flows;
- public transport mode share, as a proportion of total trips;
- bus journey times; and
- crowding levels on rail services crossing the Forth Bridge southbound in the AM Peak.
The public transport data available to the study is not as robust as that available for the road network. It is intended to update this data as and when it becomes available from other sources.
Base and Forecast Modelled Transport Conditions
The Transport Model for Scotland was used to compare the network conditions in 2005 with forecast conditions in 2012, 2017, and 2022 for scenarios with only those infrastructure projects that are likely to be in place by those dates. It should be noted that these predicted conditions do not take account of many of the proposals put forward by the SEStran Regional Transport Strategy as these are not committed. However, these proposals will be assessed in later reports.
It must be emphasised that this report does not define aims or objectives, or deal with solutions to perceived problems. These are dealt with in later reports. The conclusions reached are as follows:
- there is very little growth in traffic on the Forth Road Bridge in the peak directions, indicating that it is already very close to capacity;
- the inter-peak period experiences the highest rates of growth, as the peaks are already constrained by capacity;
- forecasts of congestion indicators show that congestion is predicted to worsen significantly;
- road journey times in the peak to Edinburgh significantly increase in duration, though this is largely due to congestion in Edinburgh itself;
- road journey times in the peak across the Forth Road Bridge to non-Edinburgh locations do not worsen significantly, indicating that it is already operating at or near capacity;
- average road speeds across the SEStran region are forecast to decrease, with Edinburgh experiencing a drop of over 20 per cent by 2022;
- there is forecast to be little change in the pattern of origins and destinations of those using the Forth Road Bridge;
- C02 emissions from transport in the SEStran region is forecast to increase by 23 per cent by 2022;
- growth in rail patronage is forecast to be modest;
- bus patronage is forecast to decline; and
- bus journey times will lengthen as road congestion worsens.
A series of sensitivity tests were undertaken to test the responses of the Transport Model for Scotland to changes in travel costs and land use. Scenarios that varied the cost of private road vehicle fuel, public transport fares, the Forth Road Bridge Toll, and economic growth were undertaken.
The model has been shown to behave in a predictable and logical fashion:
- changing the Forth Road Bridge toll has the greatest impact on the use of the bridge;
- changing the public transport fares has the greatest impact on modal split;
- changing private road vehicle fuel prices has the greatest impact on vehicle kilometres and vehicle hours, and, hence, on transport related pollutant emissions.
The model is therefore a suitable tool for predicting and assessing the results of changing transport costs on the aggregate behaviour of users of the transport system.
This report demonstrates that there will be an increased requirement for significant maintenance on the Forth Road Bridge in the future regardless of the problems associated with cable corrosion. This maintenance cannot be undertaken without temporary traffic management measures being put in place which will restrict the capacity of the crossing. Anecdotal evidence from previous occasions when traffic management measures are installed indicates that serious congestion is experienced. Substantial queues develop on all approaches to the bridge.
The forecast increases in daily traffic crossing the Forth will exacerbate the high levels of congestion experienced during restrictions or closures on the Forth Road Bridge. It is also envisaged that due to the type of maintenance works expected to be undertaken on the bridge in future, that it will not be possible to limit these traffic management restrictions to weekends as is currently the case.
It is envisaged that in the future road users will be faced with an increased number of occasions when restrictions are placed on vehicles using the bridge on both week days and weekends. Due to the growth in traffic the delays and queues experienced are likely to be greater than those encountered during maintenance periods.
The Way Forward
This work package has largely been an information gathering exercise, which sets a baseline for the future elements of the study. The future reports of the Forth Replacement Crossing Study will be:
- Report 2: Gaps and Shortfalls;
- Report 3: Option Generation and Sifting;
- Report 4: Appraisal; and
- Report 5: Final Report.
Specific objectives for the Forth Replacement Crossing Study will be identified in Report 2. The appropriate Performance Indicators will be linked to the Objectives in that report, in order to give an initial assessment of how the current and forecast situations compare with the aims of the Study.
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