Economic, Environmental and Social Impacts of Changes in Maintenance Spend on the Scottish Trunk Road Network
12 Impact on carbon footprint
12.1 Overview
A key component of the total carbon emissions in the UK is the emission from vehicle exhausts. It is known that pavement condition can affect this by changes in the rate of fuel consumption of vehicles but the direct correlation is unclear because of the wide range of related variables which also have an effect. Existing research on changes in fuel consumption has been described in Section 7. There has also been research into quantifying the impacts of roadworks in terms of congestion and delays to travel times as noted in Section 8.
In the extreme cases of increased closures on the network and reduced accessibility due to potentially higher risks being taken managing the assets (e.g. bridges requiring closure), there would be a severe impact on journeys from trips not made as well as longer alternative routes. This may lead to both reduction and increases in emissions.
This Section describes the investigations undertaken into the various sources of emissions.
12.2 Analysis results
Based on a qualitative assessment of the impact on CO2 emissions resulting from cuts in pavement maintenance it has been concluded that:
(i) As long term pavement conditions deteriorate, road roughness increases and predictions based on fuel consumption and therefore CO2 emissions will increase;
(ii) As long term pavement conditions deteriorate, travel speeds will be reduced and fuel consumption will decrease, countering the effect in (i);
(iii) As the number of roadworks interventions reduces with reduced funding, the amount of travel disruption will be reduced and emissions resulting from maintenance activities and due to vehicle idling and queuing will decrease; and
(iv) As risks on the network increase, the chance of unforeseen closures increases and these would lead to increased queuing and CO2 emissions.
A quantitative analysis was carried out to demonstrate the first 3 of these effects. Since data on unplanned (emergency) maintenance works was not available the fourth effect could not be assessed.
The quantitative assessment included consideration of the following contributors to the CO2 produced on the Transport Scotland trunk road network (excluding DBFO roads):
- Embodied carbon in the asphalt used in planned maintenance works;
- Change in carbon emissions from vehicles due to the presence of traffic management required during the planned maintenance on the network; and
- Change in fuel consumption and associated carbon emissions from vehicles using the network due to changes in the pavement surface condition (i.e. increased pavement roughness).
The first two components of the analysis are dependent on the level of maintenance carried out on the network under the different funding scenarios and build upon the analyses carried out to calculate the increase in user delay costs through roadwork sites set out in Section 8. The third component of the analysis builds on the vehicle operating cost analysis reported in Section 7 and uses the outputs from the HDM-4 emissions model and the projected change in network condition to calculate the change in vehicle emissions. Each component of the analysis is described in detail in Sections 12.2.1 and 12.2.2.
12.2.1 Carbon emissions from maintenance works
Using the areas of treatment derived from the scheme information in the Transport Scotland asset database, and used in the assessment of roadworks in Section 8, the total volume of material was estimated for each of the years in the analysis using the notional treatment thicknesses shown in Table 8.2.
Based on work carried out by TRL, as part of the development of the asPECT lifecycle carbon assessment tool (British Standards Institution, 2008a), (British Standards Institution, 2008b), a generic figure for the embodied CO2 equivalents in a tonne of asphalt, using an asphalt density of 2.3t/m3, the mass of embodied CO2 per cubic metre of asphalt is 104kg CO2/m3. This figure was used to convert the volumes of asphalt material to weight of CO2 and the cost of the carbon monetised using the non-traded carbon costs given in STAG (Transport Scotland, 2011b) (Transport Scotland, 2011b) and webTAG (Department for Transport, 2011) shown in Table 12.1.
During maintenance, traffic management and temporary speed restrictions delay vehicles travelling through the maintenance site. The QUeues And Delays at ROadworks (QUADRO) model (Highways Agency, 2009) generates costs for the difference in vehicle carbon emissions due to the delayed vehicles compared with normal flow conditions. Using the delay methodology from Section 8 the carbon costs for the traffic management arrangements assumed for the maintenance works were aggregated over the analysis period. QUADRO uses a cost base of 2002 for all costs so the carbon costs were updated using RPI to 2010 values.
The carbon costs from the maintenance works and the vehicle emissions due to delays at roadworks sites were combined and the undiscounted costs from the analysis are shown in Table 12.2 and Figure 12.1.
The reduction in maintenance works due to the lower maintenance budgets delivers a small saving in CO2 costs. It should be noted that closures on Motorways and dual APTRs can actually reduce carbon emissions since the reduction in vehicle speed increases fuel efficiency.
12.2.2 Carbon costs from vehicle CO2 emissions
Using the projected IRI condition from Section 7.3 and the outputs from the HDM-4 emissions model using the same notional network lengths and default vehicles as used in the vehicle operating cost analysis, the total mass of carbon was calculated from the mass of CO2 produced and monetised using the factors in Table 12.1. The undiscounted costs from the analysis are shown in Figure 12.2.
The results demonstrate that the carbon costs decrease as budgets are reduced and the network condition deteriorates. The effect is relatively small, but larger in magnitude than the savings in carbon costs from the reduced maintenance works programmes. Carbon costs reduce as the network deteriorates because HDM-4 predicts reductions in vehicle speeds as the network becomes rougher.
Figure 12.1 Costs of carbon from maintenance works
Figure 12.2 Carbon costs due to pavement condition for each Scenario