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

2 Highway Design and Layout

2.1 Existing Junction

2.1.1 The existing junction at the Black Swan Crossroads with the central reserve gap closedis operating as a left-in/left-out junction onto each carriageway of the A19 Trunk Road.The left-in/left-out carriageway areas of the existing junction have corner radii ofapproximately 15m and are defined by means of road markings, bollards and signing tochannel traffic into and out of the junctions. Acceleration and deceleration splays areprovided on the A19 and appear to have dimensions meeting the criteria in DMRBTD20/82 (now superseded by TD 42/95). This existing layout is similar to that set out inDMRB TD40/94 for compact grade separation, but without kerbs on the splitter islandsand with acceleration and deceleration splays on the A19 carriageways that are longerand wider than in that standard. During the consideration of the type of junction thatmay be suitable, the possibility of retaining these in their present form and incorporatingthem into the proposed junction was kept in mind.

2.2 Junction Options

2.2.1 The traffic flows and turning data for the junction, based on flows from the Halcrowreport Ref.1), indicate that due to relatively low flows on the side roads (1150 AADT in2022, the assumed design year) and an A19 flow of 37500 AADT in 2022 lead to theconclusion that an appropriate form of junction would be Compact Grade Separation inaccordance with TD40. This standard advises that compact grade separation can beeconomically justified down to mainline flows of around 12,500 AADT and is suitable foruse where mainline flows are between approximately 12,500 and 30,000 AADT andside road flows are below 10% of that on the mainline. At the Black Swan, the mainlineflows are currently about 28,200 AADT and are forecast to be about 37,500 AADT in2022 with side road flows forecast at about 1150 AADT. Although in the design yearthe mainline flow is forecast to be greater than 30,000 AADT, the side road flow is onlyabout 3% of the mainline flow. Also a compact grade-separated junction at the BlackSwan is in keeping with the A19 standards that include a mixture of at-grade and gradeseparatedjunctions.

2.2.2 The standard defines various different layouts for compact grade separation. Most ofthese allow for the side road to be continuous with connecting loops between the sideroad and the main line, the loops joining the side road at a simple T-junction. Thedesign speed required for the side road would normally be around 85 km/h.

2.2.3 The alternative is for the side road to be discontinuous so that the section crossing themain line on a bridge forms the loop. This has the disbenefit that through traffic on theside road has to negotiate two T-junctions with their associated delays. However, thestandard does allow for the loop to be designed to a 30 km/h design speed sopermitting vertical radius curves smaller than with a straight through side road. Thesmaller radius vertical curves allow the loop road to be shorter with consequent savingsin cost.

2.2.4 The existing side road originally formed a cross roads with the A19 with no staggeringof the side road connections to the A19. The loop arrangement described in theprevious paragraph readily adds to the current layout to form a compact gradeseparation while retaining most of the existing side road and the acceleration anddeceleration splays on the A19 main line. Being able to keep and incorporate thesesections of existing road into the proposed junction has benefits in less constructioncost; less adverse affect on adjacent property and access; less effect on public utilitiesand less disruption during construction. In view of the existing junction layout and thebenefits of a lower design speed for the loop, this arrangement was selected for takingforward in the feasibility study.

2.3 Junction Location

2.3.1 Having decided on the compact grade separation junction layout type, it was necessaryto consider whether the loop should be positioned on the north or south side of theexisting side road. Matters taken into consideration were topography, drainage, thewatercourse Trenholme Stell/East Rounton Stell, services, field boundaries and landuse.

2.3.2 The topography in the vicinity of the junction is relatively flat. In the north/southdirection the land slopes at a gradient of about 1 in 100 towards the south and in theeast/west direction the land slopes upwards in each direction from the A19 at a gradientof about 1 in 100. To the south of the existing side road and parallel to it there is theStell. As the proposed loop road would have to rise up about seven metres from thelevel of the existing side road to cross over the A19, the slightly higher ground to thenorth would require less earthworks to form the bridge approach embankments thanwould be needed on the south side of the existing road where the ground level is lower.If the loop were to be sited on the south side of the existing road it would be close to theStell. Sub-soil conditions close to the watercourse may be less favourable from anengineering point of view for embankment construction than that on the north side.Also, placing an embankment close to the watercourse could have adverse effects onthe local water regime if the embankment encroached into the flood plain. On the basisof these criteria, placing the loop on the north side is more favourable. A disadvantageof the north side is that the loop would cut through an area of woodland but this appearsfrom inspection to be relatively recent planting and could be mitigated by new plantingas part of the improvement scheme. However, overall it is considered preferable tolocate the loop on the north side and this has been taken forward for the study.

2.4 Services

2.4.1 From information provided by the HA and their agents, from OS mapping and fromobservation on the site visit, the approximate locations of utilities services, highwaydrainage and highway lighting was obtained. Most of the services are in or close to theexisting roads. Those that run alongside the A19 would be affected by the proposedside road embankment and would be likely to need protection or sleeving where theypassed beneath the embankment. This protection or sleeving would be needed tosome extent, regardless of whether the loop is located on the north or south side of theexisting side road. There is an overhead electricity supply line running from northeastto southwest that carries a supply to the Black Swan public house. This overhead lineis affected by the loop on the north side, requiring relocation of one or possibly twopoles. On the basis of these criteria, the advantages of locating the loop to the north orsouth are neutral apart from the overhead line, which would favour the south. However,this is not considered an over-riding matter, as it may be possible to avoid the affectedpole by adjustments to the design at a later stage.

2.5 Design Standards

2.5.1 Highway design standards are in accordance with the Highways Agency's DesignManual for Roads and Bridges (DMRB) (Ref 2) and in particular DMRB Volume 6Section 2 Part 5, TD 40/94 Layout of Compact Grade Separated Junctions. Twooptions for the junction layout have been designed.

2.5.2 Option 1, shown on the scheme layout drawing Figure 2, uses the desirable minimumstandards and includes 5% gradients on the bridge approaches. The requirements ofTD40/94 in terms of gradient are that the needs of cyclists and the risk of icy conditionsshould be considered. For cyclists the standard says that gradients should be limited to5%. The Halcrow report (Ref 1) indicates that some cyclists would use the bridge andat the time of the survey in July 2002 the number was 13 in the 12 hour study period.With a bridge in place the number of cyclists that would use it to get from one side ofthe A19 to the other may well increase firstly due to the grade separation being a saferroute compared with the at-grade crossing and secondly due to the trend towardsgreater use of cycles. Cycling and the provision of cycling facilities are also encouragedby government policy.

2.5.3 The need for the use of gradients of 5% would need to be assessed in more detailshould the scheme proceed. Surveys and investigation of potential cyclist use of thescheme and local authority policy relating to cycling would be needed. The localclimatic conditions would need to be investigated to determine frequency and severity ofroad icing.

2.5.4 Option 2, shown on scheme layout drawing Figure 3, makes use of the relaxations ofstandards permitted by TD40/94, relating to horizontal and vertical alignment radii. Thisoption also incorporates bridge approach gradients of 8%, which is to desirableminimum standard, subject to considerations of cyclists' usage and road icing asdescribed above. The benefits of applying relaxations of standards are mainly relatedto the length of the loop being shorter than with Option 1, which results in lessconstruction cost and a better result in the economic assessment. There would be asmall reduction in level of service with Option 2 compared with Option 1 due to thesmaller radius curves and the steeper gradient. These same factors could also havesome effects on safety, which would be considered during safety audits and dealt within subsequent design.

2.5.5 In both options it has been assumed that the existing junction layout within the trunkroad can be incorporated into the compact grade separated junction with nomodifications to carriageway pavement. The existing left-in/left-out junctions betweenthe side roads and the A19 and the acceleration and deceleration splays would beretained along with the recent works to close the central reserve gap. Allowance hasbeen made in the cost estimates for modifications to road markings and signing toprovide for side road traffic's safe entry to and exit from the A19. This aspect of thedesign would need to be reviewed should the scheme proceed to ensure that the layouteither complies fully with TD40/94 or that departure from standards is obtained. In anyevent, this part of the layout should be considered in the safety audits.

2.5.6 The lane width for the loop, or compact connector road, selected from TD 40/94 Table6/4 is 3 metres, giving a carriageway width of 6.6 metres including central hatching butwith widening on horizontal curves in accordance with Table 6/3. Road widths onsections of existing road that are incorporated into the scheme remain unaltered.

2.6 Drainage

2.6.1 Both options would produce an increase in surface water run-off due to the increase inpaved area in the junction. Run-off retention and pollution control is likely to be neededprior to the existing highway drainage system or other outfall. Allowance has beenmade for this in the cost estimate. The details of the run-off retention and pollutioncontrol arrangements would need to be agreed with the environment Agency and localdrainage authority.

2.7 Safety fencing and Street Lighting

2.7.1 New safety fencing would be provided on the A19 in front of the bridge abutments andon the side road embankment approaches to the bridge. The safety fencing is designedin accordance with the Highways Agency's Interim requirement for Road RestraintSystems (IRRRS) (Ref 3).

2.7.2 There is street lighting on the A19 but not on the side roads. Lighting columns on theA19 may need to be relocated on Option 1 but not on Option 2. However, dependingon the method of bridge construction, it may be necessary to temporarily remove someor all of the columns near the bridge.

2.8 Non-Motorised Users

2.8.1 Pedestrians, cyclists and equestrians are not specifically catered for in that there are nopaths or tracks for these users. It is assumed that because of the low numbers of theseusers, according to the traffic surveys and the traffic flows are also relatively light, thesenon-motorised users would walk or ride along the carriageway or verge. Option 1 wouldcater for cyclists in respect of gradient on the approaches to the bridge.

2.9 Utilities

2.9.1 Although the study did not include contact with utilities companies, the location of anumber of services was established during the site visit on 5 January 2005. With bothoptions, telecom cables and water mains may need to be sleeved and/or protected andan electricity pole may need resiting with Option 1.

2.10 Other Options

2.10.1 There is an existing junction scheme on the A1 trunk road at Rainton, which provides asimilar compact grade-separation to that proposed at the Black Swan Junction. Thestudy team visited the Rainton junction to gather ideas that might be included in thescheme put forward in the study. The main difference between Rainton and BlackSwan Options is that Rainton has a single lane bridge with traffic signal control. Duringthe site visit the traffic signals were observed to cause little delay to traffic using thebridge, although at the time of the visit traffic flow was very light. It was considered thatat the Black Swan junction it would be preferable in operational and maintenance termsto provide a two-way road throughout and avoid the use of signals. However, a singlelane bridge design and cost has been produced and the COBA network arranged tofacilitate the inclusion of signals if required. This was done to allow for the possibilitythat neither Option 1 nor Option 2 proved to be economic in COBA terms and a fall backposition of an option with a single lane bridge and signals could be tested.