M26 Junction 2a Merge and Diverge Assessment Technical Note · separated junctions and interchanges on rural and urban trunk roads and motorways. This guidance is part of the overall

Mott MacDonald 1M26 Junction 2a Merge and Diverge Assessment

389228 Merge and Diverge Assessment_TN_Issued_v4

Project: Tonbridge and Malling Local Plan Assessment

Our reference: 389228 Your reference:

Prepared by: Jannat Alkhanizi Date: 07/02/2019

Approved by: Martina Olley Checked by: Sean Finney

Subject: Merge and Diverge Assessment for the M26 Junction 2a with A20 London Road(Assessment Junction 26)

1 Assessment MethodologyThe Technical Note was prepared following Highways England’s (HE) request to examine the impacts of theLocal Plan development upon the slip road merges and diverges with the main M26 carriageway at itsJunction 2a (Assessment Junction 26 in the Transport Assessment (TA)). This work follows on from previouswork Mott MacDonald delivered in the form of a Transport Assessment (TA) in November 2016, published inMay 2017, and a Transport Assessment Addendum in July 2017. The diagram below illustrates the junctionand identifies two merges and two diverges at the junction which will be assessed within this document.

Figure 1: M26 Junction 2a Slip Road Merges and Diverges

Source: Mott MacDonald

Technical Note



The merge and diverge assessments presented in this Technical Note were carried out in accordance withthe diagrams in TD22/061, which set out the layout and size requirements for new and improved gradeseparated junctions and interchanges on rural and urban trunk roads and motorways. This guidance is partof the overall Design Manual for Roads and Bridges. The assessments compare peak hourly flows for theAM and PM merges/diverges with hourly M26 mainline flows. For the merge assessments, the upstreammainline flows were obtained. For the diverge assessments, the downstream mainline flows were obtained.

The assessments consider three scenarios:

● Base 2017 Scenario – using base (surveyed) flows from the TA for merge / diverge flows and WebTrisdata for mainline flows to benchmark current conditions against current layout

● Do Minimum 2031 Scenario – takes into consideration Do Minimum flows from the TA for merge /diverge flows and WebTris data with background growth added for mainline flows; results will be used toestablish whether a different future junction layout would be required

● Do Something 2031 Scenario – assesses future flows including development traffic on the slip roadmerges and diverges (Do Something flows from the TA) and growthed WebTris data for mainline flows;results will be used to compare the junction layout with the Base Scenario and sets out whether anychanges would be required.

Should any changes to the junction layout form be required, land availability will be checked and commentedupon.

2 Assessment Flows2.1 Upstream and Downstream Mainline FlowsThe mainline assessment flows were obtained from WebTris. Data in each direction (Eastbound (EB) andWestbound (WB)) on the M26 motorway was downloaded using the hourly tabular report for 2017 for pointson the M26 to the east of Junction 2a. Saturdays, Sundays and Bank Holidays were excluded and averagehourly peaks were calculated for the AM and PM. The highest flow (worst hour) was chosen for each peakand direction. This did not align with the junction flow peaks of 08:00-09:00 and 17:00-18:00 in all instancesthat were used in the TA (see Section 2.2 below) but ensures that a worst-case scenario is being tested.

Table 1: Hourly Peaks for Mainline Flows to Account for Worst Case ScenariosDirection AM PMM26 EB Carriageway 08:00-09:00 16:00-17:00

M26 WB Carriageway 06:00-07:00 16:00-17:00Source: Webtris

The upstream and downstream flows within the junction were calculated by deducting the merge or divergeflows from the WebTris mainline flows located to the east of Junction 2a.

2.1.1 Growth Factors

TEMPro Growth factors were calculated using TEMPro Version 7.2 with NTM dataset AF15 for the DoMinimum 2031 and Do Something 2031 future scenarios and were applied to the upstream and downstreammainline flows. The factor was based on Tonbridge and Malling MSOA Area 006 and interrogated rural areatype and motorway road type.

1 TD22/06, Layout for Grade Separated Junctions, Road Geometry Junctions, Design Manual for Roads and Bridges, Volume 6, Section 2.



The following alternative assumptions were applied to the growth factors.

Table 2: Growth Factors for Future ScenariosTime Periods Do Minimum 2031 Scenario

Growth FactorDo Something 2031 Scenario Growth Factor

AM Peak 1.063 1.116

PM Peak 1.062 1.119

Alternative Assumptions Do Minimum = no developments withinthe area

Base Households (HH) = Future HH

Base Jobs = Future Jobs

Do Something - 50% of developments from within thearea only will be on the mainline motorway at this pointFuture HH = apply 50% on increase of total number ofHHFuture Jobs = apply 50% on increase of total numberof Jobs

Source: TEMPro 7.2

2.2 Merge and Diverge FlowsThe slip road merge and diverge flows were extracted from the Transport Assessment (TA) published in May2018. The base flows in the TA are based on a survey conducted in 2017 and were used for the Base 2017Scenario in this report. The future flows for 2031 in the TA are based on the survey flows growthed using aTEMPro factor; these flows were used for the Do Minimum 2031 Scenario. The development flows for thejunction were calculated in the TA using TRICS, distributed using NOMIS 2011 data (journey to work data)and added to the base 2031 flows; these flows were used for the Do Something 2031 Scenario. The TA onlycontains one hour for the AM and PM peak respectively (08:00-09:00 and 17:00-18:00). The assessmentcarried out in this report therefore had to be carried out with inconsistent hours of flows for mainline andmerges / diverges. However, it does represent a worst case as referred to in Section 2.1.

2.3 Final FlowsLegend

Upstream/Downstream FlowsWebtris Flows

Merge Flows

Diverge Flows



Figure 2: Base 2017 Scenario

Source: Survey 2017 and WebTris



Figure 3: Base 2017 Scenario - PM




Figure 4: Do Minimum 2031 Scenario - AM




Figure 5: Do Minimum 2031 Scenario - PM




Figure 6: Do Something 2031 Scenario - AM




Figure 7: Do Something 2031 Scenario – PM


3 Assessment ResultsThe junction comprises of two merges (M26 WB on-slip and M26 EB on-slip) and two diverges (M26 WB off-slip and M26 EB off-slip). The assessment results are shown below by arm for all three scenarios, with AMand PM diagrams shown.

Note that the current layout for all M26 slip road merges and diverges align with layout type A as shown inthe diagrams below.



Figure 8: Screenshot of M26 EB on-slip mergeand M26 WB off-slip diverge

Figure 9: Screenshot of M26 WB on-slip mergeand M26 EB off-slip diverge

Source: TMBC Source: TMBC

Figure 10: Type A – Taper Merge Figure 11: Type A – Taper Diverge

Source: TD22/06 Source: TD22/06

3.1 M26 EB on-slip – Slip Road Merge

3.1.1 Base 2017 Scenario

The merge assessment shows that the current merge layout type should be E in the AM peak (shown inFigure 12) which requires a lane gain (shown in Figure 13) or a type A/D in the PM peak (shown in Figure14). The current junction layout (type A) as well as type D are shown in Figure 15. Type D is a 2 lane urbanmerge which would not be appropriate for this rural location. The current type A aligns with the requirementfor the PM peak. The junction does not align with the requirements for type E which stipulates only one lanefor the mainline flows plus a joining merge lane. The M26 being a motorway contains two lanes. Instead ofadding one merge lane to a single lane on the mainline, the situation presents itself as two lanes on themainline with the merge lane merging with one of the mainline lanes. The mainline flows are consideredsufficiently low to accommodate the merge flows with two downstream lanes being more than sufficient tocater for both mainline and merge flows.



Figure 12: M26 EB on-slip Base 2017 Scenario Merge Assessment – AM peak

Source: TD22/06 and Mott MacDonald

Figure 13: Type E – Lane Gain Merge

Source: TD22/06



Figure 14: M26 EB on-slip Base 2017 Scenario Merge Assessment – PM peak


Figure 15: Type A – Taper Merge / Type D – 2 Lane Urban Merge



Source: TD22/06

3.1.2 Do Minimum 2031 Scenario

The merge assessment shows that the merge layout type should be E to align with flows in the AM peak andtype A/D according to the flows in the PM peak. The results of the assessment show that the junction typewould not change compared to the Base 2017 Scenario results.

Figure 16: M26 EB on-slip DM 2031 Scenario Merge Assessment – AM peak


Figure 17: Type E – Lane Gain Merge

Source: TD22/06



Figure 18: M26 EB on-slip DM 2031 Scenario Merge Assessment – PM peak


Figure 19: Type A – Taper Merge

Source: TD22/06



3.1.3 Do Something 2031 Scenario

The merge assessment shows that the merge layout should be type E to align with flows in the AM peak. Inthe PM peak, the resulting flow combination borders between merge layout type A and type B. It isrecommended that the current type A layout is maintained.

Figure 20: M26 EB on-slip DS 2031 Scenario Merge Assessment – AM peak




Figure 21: Type E – Lane Gain

Source: TD22/06

Figure 22: M26 EB on-slip DS 2031 Scenario Merge Assessment – PM peak




Figure 23: Type A – Parallel Merge

Source: TD22/06

3.2 M26 WB on-slip – Slip Road Merge


The merge assessment shows that the current merge layout type should be E for both peaks whichstipulates only one lane for the mainline flows plus a joining merge lane. The current junction layout is A. TheM26 being a motorway contains two lanes. Instead of adding one merge lane to a single lane on themainline, the situation presents itself as two lanes on the mainline with the merge lane merging with one ofthe mainline lanes. The mainline flows are considered sufficiently low to accommodate the merge flows withtwo downstream lanes being more than sufficient to cater for both mainline and merge flows.



Figure 24: M26 WB on-slip Base 2017 Scenario Merge Assessment – AM peak



Source: TD22/06



Figure 26: M26 WB on-slip Base 2017 Scenario Merge Assessment – PM peak



The merge assessment shows that the merge layout does not require amendments and the junction typewould stay the same as for the Base 2017 Scenario.



Figure 27: M26 WB on-slip DM 2031 Scenario Merge Assessment – AM peak


Figure 28: Type E – Taper Merge

Source: TD22/06



Figure 29: M26 WB on-slip DM 2031 Scenario Merge Assessment – PM peak



Source: TD22/06




The merge assessment shows that the merge layout type does not need to be amended and that thejunction type would stay the same as for the Base 2017 and Do Minimum Scenarios.

Figure 31: M26 WB on-slip DS 2031 Scenario Merge Assessment – AM peak




Figure 32: Type E – Taper Merge

Source: TD22/06

Figure 33: M26 WB on-slip DS 2031 Scenario Merge Assessment – PMpeak





Source: TD22/06

3.3 M26 EB off-slip – Slip Road Diverge


The diverge assessment shows that the current diverge layout type should be A to cater for the flows in thePM peak. The current junction layout is A and therefore aligns with the requirements. For the AM peak, thelevel of flow indicates that a one-lane downstream section would be sufficient with the diverge diagram notspecifying a junction type for this flow pattern. The current layout with two lanes downstream is thereforeappropriate.



Figure 35: M26 EB off-slip Base 2017 Scenario Diverge Assessment – AM peak




Figure 36: M26 EB off-slip Base 2017 Scenario Diverge Assessment – PM peak

Source: TD22/06

Figure 37: Type A – Taper Diverge





The diverge assessment shows that the diverge layout type should be A to align with flows in the PM peakwith the diverge diagram not specifying a junction type for the AM flow pattern. The results of theassessment show that the junction type should stay the same as for the Base 2017 Scenario.

Figure 38: M26 EB off-slip Base 2017 Scenario Diverge Assessment – AM peak




Figure 39: M26 EB off-slip Base 2017 Scenario Diverge Assessment – PM peak

Source: TD22/06






The diverge assessment shows that the diverge layout type should be A to align with flows in the PM peakwith the diverge diagram not specifying a junction type for the AM flow pattern. The results of theassessment show that the junction type should stay the same as for the Base 2017 and Do Minimum 2031Scenario results.

Figure 41: M26 EB off-slip Base 2017 Scenario Diverge Assessment –AM peak




Figure 42: M26 EB off-slip Base 2017 Scenario Diverge Assessment –PM peak



Source: TD22/06



3.4 M26 WB off-slip – Slip Road Diverge


The diverge assessment shows that for both the AM and PM flow pattern, the diverge diagram does notspecify a junction type. The current junction layout is A and is considered to align with the minimumrequirements.

Figure 44: M26 WB off-slip Base 2017 Scenario Diverge Assessment – AM peak




Figure 45: M26 WB off-slip Base 2017 Scenario Diverge Assessment –PM peak



The diverge assessment shows that the diverge layout type does not require amendment according to theAM and PM flows. The results of the assessment show that the junction type should stay the same as for theBase 2017 Scenario, the current junction layout type A.



Figure 46: M26 WB off-slip Base 2017 Scenario Diverge Assessment –AM peak




Figure 47: M26 WB off-slip Base 2017 Scenario Diverge Assessment –PM peak



The diverge assessment shows that the diverge layout type should be A to align with flows in the AM peakwith the diverge diagram not specifying a junction type for the PM flow pattern. The results of theassessment show that the junction type should stay the same as for the Base 2017 and Do MinimumScenarios, the current junction layout type A.



Figure 48: M26 WB off-slip Base 2017 Scenario Diverge Assessment –AM peak



Source: TD22/06



Figure 50: M26 WB off-slip Base 2017 Scenario DivergeAssessment – PM peak




4 Conclusions● The M26 EB merge (on-slip) is currently a type A layout.● Although the PM peak flow combination borders between merge layout type A and type B, it is

recommended that the current type A layout is maintained.

● The M26 WB merge (on-slip) is currently a type A layout.● No changes are required to this merge layout for both future scenarios.

● The M26 EB diverge (off-slip) is currently a type A layout.● No changes are required to this merge layout for both future scenarios.

● The M26 WB diverge (off-slip) is currently a type A layout.● No changes are required to this merge layout for both future scenarios

Documents

M26 Junction 2a Merge and Diverge Assessment Technical Note · separated junctions and interchanges on rural and urban trunk roads and motorways. This guidance is part of the overall