Economic, Environmental and Social Impacts of Changes in Maintenance Spend on the Scottish Trunk Road Network
4 Predicted network conditions
4.1 Pavement analysis
4.1.1 Overview
In recent years, WDM Ltd has provided estimated projections of pavement condition based on specified budget levels, using the Transport Scotland network level pavement model. It was agreed that results from this model should form the basis of the investigations in this study into changes in road conditions based on reduced budgets.
The model describes the network condition in terms of a Road Condition Index (RCI [4]) derived from the survey data for the network. Future condition is estimated by transforming the RCI data into estimated remaining lives and a combination of the treatments applied and the annual shift (reduction) in remaining life.
Remaining life in the model is derived from the RCI using a polynomial relationship developed from engineering experience as illustrated in Figure 4.1 and Equation 1.
RCI = 0.048 RL2 - 4.3383 RL + 98.384 (1)
Where RL = Remaining Life (years)
Figure 4.1 Relationship between RCI and pavement remaining life
WDM provided outputs from the model runs for use in this study based on the funding levels described by the three scenarios given in Section 1.
Scenario 1 represents continuation of the 2010 maintenance funding for all of the 20 years analysis period, Scenarios 2 and 3 represent budget reductions for 10 years followed by restoration of the budgets to the 2010 level over 5 years in equal annual steps. For these 2 scenarios there is an annual 2.5 percent increase in the budgets for the last 5 years of the period. These are illustrated in Figure 4.2.
Figure 4.2 Scenario budgets
The effect of Scenario 3 is to suffer a bigger cut in maintenance budget for 10 years and therefore, in the following 5 years, to have a higher rate of funding increase than Scenario 2, so the budget is restored to the 2010 level in year 15 for both Scenarios 2 and 3. The annual budgets for Scenarios 2 and 3 in the last 5 years of the analysis period are the same but the network condition is different because of the pattern of funding in the first 10 years.
Over the first 10 years the effect was more deterioration in network condition with Scenario 3 than with Scenario 2. Although the rate of increase in budget was higher in the next 5 years, the budget was still lower with Scenario 3 than with the other scenarios and network condition remains worse for Scenario 3 than Scenario 1 or 2 through the analysis period. Table 4.1 shows the percentage of the network in bands of remaining life for each of the scenarios at key points in the analysis period.
The results from the analyses included the length of network by remaining life, in a range from 0 to 50 years for 2010, 2013, 2017, 2020, 2025 and 2030. The output results from the model, as provided by WDM Ltd, are given in Appendix C.
The results were broken down into three road types: Motorway, dual carriageway APTRs and single carriageway APTRs.
Figure 4.3 shows the condition of the network in 2010, by network length and road type. This is the starting condition for the three funding Scenarios. In addition to the pavement lengths shown in Figure 4.3, there is approximately 2000 lane-km of the network with a remaining life of 50 years (i.e. in good condition). This represents around a third of the total network length and will not require maintenance during the 20 years considered in this study.
Figure 4.3 Network length by remaining life (0-40 years) - Starting Condition
The total network length used for the analysis (provided from the WDM Ltd analysis) is shown in Table 4.2 (note these lengths do not represent the full network, see Table 3.1). These lengths do not include roads operated as DBFO concessions or roundabouts, but do include slip roads. The proportion of each road type in different condition bands was used as the basis for all the economic analyses and calculations presented in this report unless noted otherwise.
|
Road Type |
Route |
Carriageway (km) |
Lane 1 |
|---|---|---|---|
|
Motorway |
268 |
537 |
537 |
|
Dual APTR |
492 |
983 |
983 |
|
Single APTR |
2,247 |
2,247 |
4,494 |
|
TOTAL |
3,007 |
3,766 |
6,013 |
4.1.2 Scenario 1: Maintain current spend
Figure 4.4 shows the percentage of the network by remaining life assuming the current spending level continues. For presentational clarity, pavements with a remaining life of longer than 25 years have been removed. These pavements include a large proportion of the network with a remaining life of at least 50 years as noted earlier.
Figure 4.4 Network length by remaining life for Scenario 1
4.1.3 Scenario 2: 20% reduction in budget
Figure 4.5 shows the percentage of the network by remaining life for each year assuming the funding levels described for Scenario 2. The results confirm that more of the network is in poor condition after 10 years (i.e. in 2020) than if the current spending level is maintained.
4.1.4 Scenario 3: 40% reduction in budget
Figure 4.6 shows the percentage of the network by remaining life for each year assuming the funding levels described for Scenario 3. The results confirm that more of the network is in poor condition after 10 years (i.e. in 2020) compared to the condition shown for Scenario 1 or Scenario 2.
Figure 4.5 Network length by remaining life for Scenario 2
4.1.5 Issues from the pavement condition analyses
Based on a review of the analyses outputs and the assumptions used in the model, the following points have been identified as having a significant impact on the use of the outputs for this study:
- Figure 4.3 shows the distribution of network condition by remaining life (in years). It shows that there is considerable variation with life. The model uses this information as its basis for projecting the annual change in condition (each year, the remaining life on a length of road decreases by one year). Outputs showing the projected condition will continue to reflect this level of variation in future years;
- The largest part of the network (33% of network in 2010) comes from roads in excellent condition (remaining life equal to 50 years or more). The second largest part of the network (11% of network in 2010) comes from roads at the other extreme - in the worst condition (remaining life of 0 years or less). These two proportions of the network have a significant influence on the network conditions predicted by the model over time;
- If deteriorating pavement conditions do not impact on economic outcomes until they reach some threshold between the above two extremes of condition, then significant economic changes will not be projected to occur until those roads currently in excellent condition reach this threshold. At such a time, the projected economic impact may show a sudden step change and then continue to increase;
Figure 4.6 Network length by remaining life for Scenario 3
- In the Pavement Network Model, the mix of different treatments applied by the model is based on the proportion of different treatments required in year 1 of the analysis. This is because there is data available for year 1 at the individual defect level (e.g. cracking, rutting) which allows the different treatment requirements to be modelled. In future years, because the only parameter projected into the future is the remaining life (and the corresponding RCI value), there is no ability to prescribe treatments based on individual treatment types and so the modelled approach is to continue to apply future treatments in the proportion of the treatment types to meet the requirements for year 1. In reality, a Road Administration might choose to limit the application of more expensive treatments under reduced funding scenarios but the model does not attempt to address this refinement.
- At the current level of pavements budget, the model predicts that the condition of Motorways will improve but this is offset by deterioration in single carriageway roads. Even with a 20% reduction in pavement budgets, the condition of the Motorway network is projected to improve during the first 10 years of the analysis period. For years 10 to 20 of the analysis, the higher priority for maintenance funding for Motorways was removed and budget allocated simply on the basis of the condition of parts of the network.
The implication of these points is considered in the relevant Sections in this report.
In an assessment of the effects of changes in funding for structures maintenance, Transport Scotland assumed three budget options and compared these with the current budget level (£26m for 2010/11):
- Desirable (£51m per year). Needed to clear the backlog of defects over 10 years, maintains asset value and ensures optimal safety and minimal disruption;
- Essential (£38m per year). Clears the backlog (requires more than 10 years), maintains asset value, and delivers reasonable safety and levels of disruption; and
- Minimum (£22m per year). Backlog increases, asset value reduces and risks of safety (accidents) and disruption are increased.
The Transport Scotland assessment included no quantification of the customer or economic impacts caused by the different budget levels. For this study, which considered budgets less than the current spending levels, the minimum budget level option in the Transport Scotland assessment was the only relevant case.
Table 4.3 summarises the results for that budget level. In Table 4.4 the values in Table 4.3 are expressed as percentage changes compared with the current activity spends.
Unlike carriageway maintenance, structures maintenance and renewal shows more pronounced potential variation in budget from year to year due to the discrete packages of work which are either included or excluded, and the substantial investments often associated with those packages. Hence the budgets for Replacement, Strengthening and Major Schemes show significant year to year variation between 2011/12 and 2015/16.
The key result from the Transport Scotland report is the expected increase in routine maintenance required to maintain the structures stock. The increase is more than offset by the very significant reduction in capital (major schemes) investment on the network. Routine maintenance is carried out as a lump sum item by the Operating Companies (OCs) while the renewals maintenance is undertaken as specifically funded works.