Economic, Environmental and Social Impacts of Changes in Maintenance Spend on Local Roads in Scotland
Appendix G Increase in Travel Time Costs
G.1 Methodology
This analysis is based on the observation that vehicles travel faster on pavements in good (recently maintained) condition. The analysis assumes therefore that as pavement surface condition deteriorates vehicle speeds reduce. The analysis is based on the findings by (Cooper, Jordan, & Young, 1980) who observed the increases in vehicle speeds shown in Table G.1 on a newly maintained trunk road compared with the observed speeds prior to maintenance.
| Vehicle Type | Speed increase (km/h) | Base Speed (km/h) |
|---|---|---|
| Car | 2 | 72 |
| LGV | 2.3 | 72 |
| HGV | 2.6 | 65 |
| PSV | 2 | 65 |
Since the conclusions in (Cooper, Jordan, & Young, 1980) are based on observations on trunk roads the travel time analysis for the Local Authority network was based on vehicles travelling over A class roads only.
The methodology for proportioning vehicles by carriageway surface condition detailed in Section F.1 was used, but instead of proportioning the network by IRI, 3mLPV was used ranging from 3mLPV 0.5 to 5 mm2 in 0.5 mm2 increments.
To calculate the change in speed it is assumed that vehicles travelling over lengths of the network with a 3mLPV of 0.5 mm2 travel at the base speed shown in Table G.1. Vehicles travelling on lengths with and IRI of 5 or greater have a reduced vehicle speed as per the Speed Change values shown in Table G.1 and the change in speed is assumed to vary linearly between these two condition extremes.
Using this methodology the total increase in travel time for each vehicle type was calculated for each of the sample Authorities, in each model year and under each of the 3 budget scenarios. The increase in travel time was then monetised using the appropriate values for the value of time taken from TAG unit 3.5.6 (Department for Transport, 2011b). Since the condition of the network in 2011 is not in perfect condtion (all roads having a 3mLPV of 0.5mm2 or less) there is an initial starting value for the increase in travel time in 2011.
The Travel time costs for each of the modelled years under the 20% and 40% budget cut scenarios were then linearly interpolated and extrapolated to represent a 35% and 69% budget cut to account for the fact that the WDM model runs were carried out using a 20% and 40% budget cut, but the subjective budget analysis identified that these would be equivalent to a 35% and 69% budget cut in carriageway maintenance respectively.
To calculate the travel time costs for the intermediate years, between the modelled years, the results of the analysis were linearly interpolated.
G.2 Results
The results of the journey time analysis are shown in Figure G.1 to Figure G.8. Fife and Edinburgh demonstrate a larger difference in travel time costs for the different scenarios compared to the other sample Authorities. The data for Edinburgh and Fife also show changes in the gradient of the Scenario 3 line at 2020 and 2025, commensurate with the increase in budget over these periods and the corresponding improvement in carriageway deterioration.
Figure G.1 Aberdeenshire increase in travel time costs
(2002 prices undiscounted)
Figure G.2 Dumfries and Galloway increase in travel time costs
(2002 prices undiscounted)
Figure G.3 City of Edinburgh increase in travel time costs
(2002 prices undiscounted)
Figure G.4 Fife increase in travel time costs
(2002 prices undiscounted)
Figure G.5 Glasgow City increase in travel time costs
(2002 prices undiscounted)
Figure G.6 Highland increase in travel time costs
(2002 prices undiscounted)
Figure G.7 North Lanarkshire increase in travel time costs
(2002 prices undiscounted)
Figure G.8 South Ayrshire increase in travel time costs
(2002 prices undiscounted)
G.3 Effect of discounting
The Travel Time Costs were discounted by 3.5% per year the effect of this discounting can be seen in Figure G.9 for Fife. The discounted values show the Net Present Value of the VOCs decreasing with time. This behaviour was common to all 8 sample Authorities.
Figure G.9 Fife increase in travel time costs
(2002 prices discounted)
