Highways Agency

• 1 Introduction
• 2 Highway Design and Layout
• 3 Bridge Design and General Arrangement
• 4 Environment and Land
• 5 Scheme Costs
• 6 Traffic and Economics
• 7 Project Appraisal Report
• 8 Conclusions and Recommendations

A19 Dishforth to Tyne Tunnel

Feasibility Study

6 Traffic and Economics

6.1 Background

6.1.1 The traffic and economics work was carried out in order to examine the economicfeasibility of providing a bridge at the Black Swan crossroads, with the existing gapclosure retained to provide 'left-in, left-out' movements at the former crossroadsjunction.

6.1.2 The Scheme was evaluated against two alternative 'Do-Minimum' scenarios:

• The original layout with the junction operating as an at-grade crossroads;
• The current interim layout with the gap closed and the junction providing left-in and left-out movements only.

6.1.3 Two alternative scheme options were examined. These are described at 2.5 above.The essential difference in terms of the traffic and economic assessments are thatOption 2 is more compact than Option 1 and therefore results in a slightly shorterdiversion route for right-turning traffic; and it is less costly to construct.

6.1.4 The traffic and economics work was based upon a spreadsheet model of link flows andturning movements likely to be affected by the Scheme and a COBA model of thejunction and the surrounding road network. A study area was selected aimed atincluding all the links and junctions likely to be affected by traffic displaced from theBlack Swan crossroads by the gap closure. The extent of the study area is shown onthe node-link network diagram contained at Appendix 1.

6.2 Traffic Data

6.2.1 The primary source of traffic flow data were the classified turning movement countsundertaken by Halcrow at the Black Swan and Ingleby Arncliffe junctions on Tuesday15th October 2002 (i.e. before the gap at the Black Swan was closed).

6.2.2 On the basis of these counts, Halcrow made an assessment of the diversion routeslikely to be used by traffic displaced from the Black Swan by the gap closure. Thesediversion routes are all included within the node-link network contained within the COBAmodel illustrated at Appendix 1.

6.2.3 Subsequently, data was made available from a before-and-after count undertaken byNorth Yorkshire County Council (NYCC) in March and November 2004. This new datasupported in general terms the diversion of traffic away from the local roads connectingwith the Black Swan junction but suggested that drivers had adopted slightly differentroutes than had been suggested in the Halcrow report. These routes, which wereadopted for the COBA assessment are described in the table below.

6.2.4 The traffic flows and re-assignments to the adopted diversion routes are outlined inAppendix 2.

6.2.5 Traffic flows were input into COBA as 12-hour weekday flows in October 2002, i.e. nofurther factoring was undertaken in order to convert the flows counted by Halcrow intoCOBA input flows. The underlying levels of use of minor roads away from the A19 wereestimated from the results of the NYCC counts.

6.2.6 A straightforward growth factor approach was taken to modelling the central 'ReferenceCase' and the high and low growth scenarios. At this stage, no competing orcomplementary schemes were considered, nor any planning applications nor local planled development allocations, as none were notified to us by the Project Sponsor.

6.2.7 The default traffic growth profile within COBA was used to represent the central'Reference Case' and high growth and low growth factors from the National RoadTraffic Forecasts (NRTF) for the high and low scenarios respectively. An approachbased upon default national growth rates was considered more appropriate than tryingto construct a localised growth profile based upon TEMPRO, at this stage in theScheme.

6.2.8 No allowance has been made for the effect of induced traffic, as there will be noperceptible saving in travel time for Trunk Road traffic. Any travel time saving for localusers is likely to be purely localised in its effect. Similarly, no allowance has been madefor the potential for land-use developments because none have been notified to thedesign team.

6.2.9 Accident data for the period January 1997 to May 2002 was made available. The wayin which accidents savings have been treated in this assessment is outlined under'Economic Assessment' below.

6.3 Economic Assessment

6.3.1 The method adopted was based on the use of COBA to evaluate on a common basissavings in accidents, journey time and fuel costs, and the cost of constructing theScheme. The version of COBA used was version 11, release 6 (COBA 11.6), releasedin August 2004, which embodies the current values of these items. It was determined inadvance that a QUADRO run would not be required, since queues and delays duringthe construction of the scheme are unlikely to be significant.

6.3.2 The following basic assumptions were embodied in the COBA runs:

• COBA was run for a 60 year assessment period in accordance with latest 'Green Book' guidance;
• The scheme was assumed to open in 2007, with the construction costs incurred in 2006-7 and preparation costs in 2005-6.
• The default values of discounting rate were used, i.e. 3.5% until 2033, thereafter 3.0%.
• The network was classified as 'Trunk Non-built-up' (TNB);
• The default values of seasonality index, E- and M-factor applicable to a TNB classification were used;
• National default growth factors for the Central, Low and High growth scenarios were used;
• National default vehicle category proportions were used (although classified counts were available, it was decided that because the majority of the traffic on the COBA network was A19 through traffic, the national defaults would provide a more robust approach).

6.3.3 Each of these assumptions could be reviewed and the default values replaced by locallyderived data as the scheme progresses if it is considered that further precision isrequired.

6.3.4 The extent of the COBA network was defined by the need to encompass all thediversion routes identified. A network diagram is included at Appendix 1. The junctionsat the Black Swan itself were modelled in COBA, together with the Ingleby Arncliffestaggered crossroads. Other junctions on the network were not modelled as such.

6.3.5 Link lengths and minor junctions per km were measured digitally from the O.S.mapping. Carriageway widths were measured on site. Values of other data items, suchas hilliness, bendiness, verge width etc were based upon default values in COBAadjusted to reflect local conditions where appropriate.

6.3.6 Local accident data was made available for the period January 1997 to May 2002. Itwas determined in advance that local accident data would be confined to the BlackSwan crossroads itself, since this was the primary location at which accident benefitswould be likely to be significant.

6.3.7 Having examined the history of the site, it was found that a local safety scheme wasimplemented at the Black Swan crossroads in September 2002 in advance of theclosure of the gap itself in July 2004. It was predicted by Halcrow in December 2002that improvements in the accident record were likely to result from the local safetyscheme. It would have to be accepted therefore, that even if the Black Swan gap wereto be re-opened with no further enhancements, the accident rate would not necessarilyrevert to that which applied before September 2002. Therefore, this earlier data couldnot be used. It is acknowledged that shortly after the safety scheme was put in placethere was a fatal accident at the junction. However, the time period between theimplementation of the safety scheme and the gap closure is too short to provide anaccident rate for input to COBA.`

6.3.8 The use of the accident data for the period after the local safety scheme but with thegap still open was also considered. Unfortunately, a period of 21 months is notnormally regarded as sufficient to allow local accident data to be used in COBA as it isnot regarded as statistically significant. It was intended to sensitivity-test the impact ofthis accident data on the COBA results, however, at the time of writing this report, theadditional data had not been received and this therefore proved not to be possible.

6.3.9 Scheme and land costs were input into COBA as 4th quarter 2003 estimates, asprepared by the design team. In addition, a further cost was input in the year 2047 torepresent a major reconstruction of the carriageway to acknowledge that a 60-yearassessment period demands an allowance for more than the routine maintenance andresurfacing that it is implicit in COBA. The bridge itself was assumed to have a designlife of 120 years and no further bridgework costs were assumed.

6.3.10 Validation of the COBA results was undertaken. In addition to checking the accuracy ofthe input data, the results were scrutinised to determine that link speeds, junctiondelays, overcapacity reports and the size and location of link, junction and accidentbenefits were within intuitively sensible ranges.

6.3.11 A detailed record of the results of the COBA runs are contained in Appendix 3 andbriefly summarised below.

Notes:

Costs and benefits are expressed in £ Million at 2002 market prices, discounted to 2002 within COBA.
Accident savings are total number of accidents saved over the 60-year assessment period.
FYRR = benefits arising in the first year of operation, expressed as a % of the scheme cost.