OPTIMAL MERGE LAYOUT ARRANGEMENTS FOR THE M50: A MICROSIMULATION MODELLING APPROACH

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1 Proceedings O BRIEN and DUNNY: Optimal Merge Layouts OPTIMAL MERGE LAYOUT ARRANGEMENTS FOR THE M50: A MICROSIMULATION MODELLING APPROACH Dr Liam O Brien Senior Consultant, Transportation AECOM Mr Shane Dunny Associate Director, Transportation AECOM Abstract It is well known that merges on motorways are a major source of conflict and potential cause of flow breakdown. Such conflicts often slow down vehicles on the mainline and trigger shockwaves that propagate and dissipate over time and space, or result in localised congestion that could evolve into long-lasting bottlenecks throughout entire peak periods. Research into motorway bottlenecks has shown that driver behaviour at merging sections affects traffic operations and is one major causes of breakdown. A significant proportion of breakdown events are associated with the interaction between the flow on the motorway mainline and the flow on the ramp slip lanes (or acceleration lanes), which compete for the same capacity downstream merging point. Previous research into motorway merge layouts has established that their performance depends on a number of factors including the geometric design characteristics or layout merge, traffic conditions, and interactive behaviour between vehicles on the mainline carriageway and vehicles from the slip roads. This paper sets out the research undertaken to determine the optimal merge layout arrangements for merges on the National Road Network, in particular the M50, under various traffic flow conditions. The aim of this study is to identify the preferred merge layout for junctions on the M50 based on traffic flow patterns, availability of road space and traffic demand. This analysis has been undertaken by the authors on behalf of Transport Infrastructure Ireland (TII) as part of an ongoing task to identify potential short to medium term solutions to ease congestion on National Roads within the Greater Dublin Area and in particular to manage the growing demand and the associated congestion and incident issues on the M50. The performance predominant existing merge layout on the M50 is assessed, under a range of flow volumes on the mainline and slip lanes, by comparing it to two potential alternative layouts via a number of generic microsimulation models developed in VISSIM as part of this study. A number of measures of performance are proposed to compare the effectiveness merge layouts and overall the results are used to demonstrate graphically the flow volumes at which the various merge layouts perform best leading to recommendations for suitable merge layouts on the M50. Finally, this paper concludes by discussing the application of this research to the development and implementation of new merge layouts for the M50/N3 interchange in late Introduction Merges on motorways are a major source of conflict and potential cause of flow breakdown. Such conflicts often slow down vehicles on the mainline and trigger shockwaves that propagate and dissipate over time and space, or result in localised congestion that could evolve into long-lasting bottlenecks throughout entire peak periods. Research into motorway bottlenecks has shown that driver behaviour at merging sections affects traffic operations and is one major causes of breakdown ([1]; [2]; and [3]). Breakdown events are associated with the interaction between the flow on the motorway mainline and the flow on

2 O BRIEN and DUNNY: Optimal Merge Layouts Proceedings the ramp slip lanes (or acceleration lanes), which compete for the same capacity downstream merging point. A recent study, [4] set out the factors upon which the performance of a motorway merge depends. These are as follows: Geometric design characteristics, including the layout merge (e.g. taper length, number of lanes), gradient, curvature, terrain, free-flow speeds; Traffic conditions, such as traffic flow volumes and temporal profiles, composition; and Interactive behaviour between vehicles on the mainline carriageway and from the slip road (e.g., gap-searching and acceptance merging traffic, pre-emptive lanechanging and courtesy yielding by the mainline traffic). In this paper we consider the performance of M50 merges in terms ir layout while considering the traffic flow volumes and interactive behaviour between vehicles on the mainline carriageway and slip lanes. The merging arrangement at a number of junctions along the M50 warrant further investigation to ensure the designs are optimised based on the current and forecast traffic volumes and patterns. In particular, concerns have been raised over locations where DMRB TD22/06 Type F (Option 1) merges exist on the M50. Figure 1 depicts DMRB TD 22/06 Type F Option 1 Layout (Lane gain with ghost island merge). These merges are where two lane slip roads merge with a lane gain. The outside/offside lane slip road merges with the mainline carriageway via a taper merge and the inside/nearside lane continues on to form a new lane on the mainline. The two slip road lanes are separated by a ghost island. Figure 1 Lane Gain with Ghost Island Merge (Type F Option 1) Observations and incident data have identified concerns regarding the operational performance and safety Option 1 merge layout. This paper presents and assesses two alternative layouts as potential solutions to problems arising from the existing Option 1 layout. Through redesign of merge road marking layouts it is hoped to deliver congestion improvements on the M50. This section paper sets out a number of solutions which may deliver congestion improvements on the M50 through redesign of road marking layouts. One possible low cost improvement proposed in this study is to change the Option 1 merge layout to DMRB TD 22/06 Type F Option 2 as shown in Figure 2. Similar to Option 1, these merges are where two lane slip roads merge with a lane gain. In this case the outside/offside lane forms the lane gain while the inside/nearside lane merges with the additional lane mainline further downstream via a taper merge. Again, the two slip lanes are separated by a ghost island. Figure 2 Lane Gain with Ghost Island Merge (Type F Option 2) Paragraph 2.30 of DMRB TD 22/06 states that for layout F Option 1 is the preferred option due to the likely usage of Lane 1 connector road by the majority of large and/or slow vehicles and Lane 2 predominantly by light vehicles. Option 2 has been retained for use in

3 Proceedings O BRIEN and DUNNY: Optimal Merge Layouts circumstances where it is appropriate. It should be noted that DMRB assumes flows per lane within safe operating capacities, on the M50 traffic flows regularly exceed 1,850 PCU s/lane during peak periods with short term flows (with duration of <5minutes) equivalent to 2,200 PCU/lane experienced in the outside lane during peak periods. In addition, as traffic flows increase and the availability of gaps lessens there are potential safety concerns with the Option 1 layout arrangement due to the length and potential sightline issues associated with the taper layout. It should be noted that due to the limitations in road space available at various merge locations it may not always be possible to achieve an Option 2 layout. Therefore, another feasible solution may be to switch to a layout based on a modified/reconfigured version Option 1 layout, termed here as Option 1A, and shown in Figure 3. Figure 3 Option 1A Merge Layout with Lane Gain The proposed Option 1A Merge Layout with Lane Gain is based on retaining the existing nearside slip lane to form the lane gain (as in Option 1) and reconfiguring the offside slip lane to merge with the mainline via an auxiliary lane and merge taper to the mainline. This is achieved primarily by revising the design existing ghost island in line with urban motorway design principles, as set out in DMRB TD 22/06, to provide space for the auxiliary lane. This paper will compare the performance prevailing double lane merge layout (Option 1) to the proposed potential alternative merge layouts i.e. Option 2 and Option 1A, by developing generic VISSIM models of all three layouts and demonstrating how these layouts perform under a number of performance measures for a range of different flow volume scenarios. These models will be used to identify the appropriate layout (Option 1, Option 2 or Option 1A) for the relevant M50 merges based on previous, current, and future flow volumes on the M50. Furthermore, the results will be utilised to demonstrate graphically the flow volumes at which Option 1, Option 2 or Option 1A merge layouts perform best. The overall aim of this research is to identify the preferred merge layout for junctions on the M50 based on traffic flow patterns, availability of road space and traffic demand. This analysis has been undertaken following a request from TII as part ongoing task to identify potential short to medium term solutions to ease congestion on National Roads within the Greater Dublin Area and in particular to manage the growing demand and the associated congestion and incident issues on the M50. This paper is structured as follows: Section 2 of this paper describes the existing situation on the M50 by summarising the existing merge layout types and the relative safety se merges layouts by reference to previous collision data. Section 3 describes the methodology used to develop VISSIM models to test the impact of potential solutions against the existing layouts. Section 4 paper provides in-depth analysis and discussion modelling results and demonstrates which merge layout performs best under various traffic flow volume scenarios. Section 5 briefly describes the implementation of new merge layouts for the M50/N3 interchange following the outcome of this research. Finally, Section 6 of this paper sums up the conclusions of this research and offers some future research directions. 2. Existing Situation In this paper, the first step in the analysis was to identify the existing junction layouts and merge characteristics on the M50 with reference to DMRB. As-built drawings for the M50 were referred to in order to identify which merge locations corresponded to DMRB TD 22/06 Type F Lane Gain with Ghost Island Merge Option 1 and DMRB TD 22/06 Type F Lane Gain with Ghost Island Merge Option 2. Hereafter, these are generally referred to as Option 1 and Option 2. These locations are listed in full in [5]. No examples Option 1A layout exist on the M50 however the layout has been used in other locations in Ireland e.g. N40 in

4 O BRIEN and DUNNY: Optimal Merge Layouts Proceedings Cork. A comprehensive description of junction layouts and merge/diverge characteristics for the M50 is provided in Appendix A of [6]. In summary, there are 11 merge locations which correspond to an Option 1 Layout with Lane Gain and only two locations which correspond to an Option 2 layout with Lane Gain. In addition, there are a further two locations corresponding to an Option 1 layout albeit without lane gain. These locations have not been included here since the purpose of this study is to develop generic VISSIM models representing Option 1, Option 2, and Option 1A layouts with lane gain. Reference [5] also briefly assessed the relative safety various merge locations along the M50 using Road Safety Authority (RSA) collision data for a three year period only ( ). This period was chosen to reflect the collisions occurring on the upgraded M50 junctions (post 2009) up to the most recent year available The analysis showed that there is a higher average collision rate at merge locations with an Option 1 layout (with lane gain) although these results are based on a small sample so no firm conclusion can be arrived at. Due to the fact that only two locations on the M50 currently have a Type 2 layout and the availability of only 3 years data it was not deemed appropriate to come to any conclusion on the relative safety attributes of each layout based on the available data. There are currently no locations on the M50 with a merge layout corresponding to Option 1A and hence no past collision data relating to this merge layout type. Over the 3 year period considered there were a total of 28 collisions (all minor) at locations with an Option 1 merge layout with the highest number of collisions occurring at: Junction 4 Ballymun Southbound Merge, Junction 11 N81 Northbound Merge followed by Junction 6 N3 Northbound Merge. Due to space limitations it is not possible to report these findings in detail here, however, the reader is referred to [5] for the full analysis. 3. Methodology In this section the methodology followed to construct generic VISSIM models for Option 1, Option 2, and Option 1A merge layouts is described. VISSIM is a microscopic, behaviourbased multi-purpose traffic simulation to analyse and optimise traffic flows. It offers a wide variety of urban and highway applications, integrating public and private transportation. Complex traffic conditions are visualised in a high level of detail supported by realistic traffic models. The primary objective was to construct VISSIM models, suitably validated using traffic count data and recorded video footage at the M50 junctions, which modelled or represented as closely as possible the real behaviour of vehicles merging on to the M50. The purpose of this exercise was to test the performance different layouts and to determine which layout is optimal for locations on the M50. The data collection, model development and model assessment phases are described below. In order to identify the cause of merge issues and potential solutions to these issues, video footage data was obtained from the Motorway Traffic Control Centre (MTCC). This footage was extracted and utilised to ascertain vehicle behaviour at merges and traffic splits across slip lanes. Suitable footage from numerous locations was used to estimate the traffic splits on the slip lanes. In addition, vehicles were counted at two locations on the slip lanes in order to provide some insights into the lane changing behaviour of vehicles as they approach the merge area. From these observations and subsequent analysis it was determined that the split between the nearside slip lane (extended auxiliary lane) and outside slip lane (taper merge lane nearest to the mainline) is approximately 63% and 37% respectively for existing merge layouts. These splits were used as inputs to the VISSIM model to constrain the flows on the slip lanes accordingly. Mainline flows along the M50 were determined by extracting data from TII Traffic Monitoring Units (TMUs). This analysis, combined with counts and observations from video footage various merge locations, was used to determine the split of flows on each lane along the mainline and inputted to the VISSIM model. These splits are as follows - 22% on the inside/nearside lane mainline (next to the slip lane); 33% on the middle lane and 45% on the outside/offside lane mainline. As-built drawings for the M50 were obtained from TII and from these it was determined which merge locations correspond to Option 1 and Option 2 merge layouts. In Section 2 it was found that 11 locations have a layout corresponding to Option 1 while only two locations have a layout corresponding to Option 2. Using the as-built drawings, the dimensions of

5 Proceedings O BRIEN and DUNNY: Optimal Merge Layouts each Option 1 layout were determined. The purpose of this was to determine which layout ( 11 locations identified) best represents an Option 1 layout for the purposes of constructing a VISSIM model broadly representative of all locations. The M50/N3 southbound merge was chosen as the location upon which the VISSIM model was constructed. This was due to its dimensions which best represent a broad number of other locations, the availability of video footage at this location and moreover it is one highest priority locations for intervention based on total delays being incurred by users on an annual basis based on current conditions. The VISSIM model for Option 2 was based on reconfiguring the existing Option 1 layout at the M50/N3 southbound merge with reference to the dimensions merge layout at Junction 14 (Sandyford/Leopardstown) northbound merge since this is one of only two locations on the M50 with an Option 2 layout. This location was considered the most appropriate for use as a comparison with Option 1 since total morning flows at this location are also higher than the other location with Option 2 (M50/M11 Southbound Merge). The VISSIM model for Option 1A was developed based on reconfiguring the existing Option 1 layout at the M50/N3 southbound merge. VISSIM models based on these layouts using flow splits on the slip lanes and mainline as determined above were developed for each Option 1, Option 2 and Option 1A merge layouts. These models were suitably validated by comparing the simulated user behaviour in merging to that observed from the video footage of merging locations obtained from the MTCC. This was to ensure that the models represented as closely as possible the real life merging behaviour on the M50. The VISSIM models were used to determine which layout (Option 1, Option 2 or Option 1A) performs best under different scenarios of flow on both the mainline and slip lanes. The models were simulated five times over a defined interval of time representing 08:00 09:00 on a typical weekday. A total of 36 scenarios representing different combinations of flows on the slip lanes and mainline were used ranging from low flows of 1,000vph per lane on the slip lanes and mainline increasing in increments of 250vph up to 2,250vph per lane. The intention was to test the different layouts under low flow conditions and at the highest flow conditions to compare the performance of Option 1, Option 2 and Option 1A layouts. Detectors were placed in the VISSIM model to measure the total travel times of all vehicles on the slip lanes and M50 mainline with data reported in five minute intervals. Using these outputs, the merge layouts were compared under a number of different measures of performance. The results presented and discussed in detail in the next section will inform the decision making process on the preferred merge layout at junction locations on the M50 and other National Roads in the GDA. Once the preferred layout is established, the detailed design priority locations may be undertaken to allow TII to progress implementation. 4. Analysis and Discussion of Results A number of measures were proposed to compare and assess the performance of Option 1, Option 2 and Option 1A merge layouts in terms of total average travel times. These measures of performance were as follows: Performance Measure No. 1: Total average travel times under different combinations of flow volumes on the slip lanes and the mainline during the peak period of 08:40 08:45. Performance Measure No. 2: Total average travel times over the time period considered in this study (08:00 09:00). Performance Measure No. 3: Comparison of Network Statistics (Average Travel Time/ Speed/Delay per Vehicle). For the purposes of comparing the layouts, under the first two performance measures, two appropriate routes were defined over which the travel times were measured, as follows: Route 1 (Slip Lane to Mainline) the distance travelled by vehicles from a point chosen arbitrarily upstream on the slip lanes to a point downstream on the mainline; Route 2 (Mainline to Mainline) the distance travelled by vehicles from a point upstream merge on the mainline to a point downstream merge on the mainline. Performance Measure 1: Average Travel Times under Various Flow Scenarios: The travel times of all vehicles for each specified time interval in the VISSIM model were calculated for five different simulation runs VISSIM model for Option 1, Option 2 and Option 1A

6 O BRIEN and DUNNY: Optimal Merge Layouts Proceedings merge layouts. From this, the average travel times were calculated for route 1 and route 2 (as defined above). In addition, the standard deviation average travel times and maximum and minimum travel times were also calculated. This procedure was repeated for 36 different scenarios or combinations of slip lane and mainline flows ranging from low flow volumes to highly congested conditions. The purpose of this was to compare the merge layouts under all flow conditions, to ascertain which layout is suitable at lower flow volumes and moreover to investigate which layout performs best when the network is tested at higher flows such as those currently experienced or expected to be experienced in the future on the M50. The results are discussed below. Figure 4 below depicts the average travel times of vehicles travelling on route 2 (mainline to mainline) for the model study period (08:40 08:45). This period was chosen for illustrative purposes since it captures the worst case scenario travel times, for the merge layouts, study period considered in this model. This is appropriate to illustrate the contrasting average travel times resulting from the different merge layouts under a range of flows on the slip lanes and mainline. Later, this paper will also address the performance merge layouts (in terms of average travel times) over the entire study period (08:00 09:00). Figure 4 - Mainline to Mainline (Route 2) Average Travel Times for Various Flow Scenarios under Option 1, Option 2 and Option 1A Merge Layouts As Figure 4 demonstrates, Option 1, Option 2 and Option 1A merge layouts produce quite similar average travel times at lower flow volumes on the slip lanes and mainline with the travel times resulting from the Option 2 layout being marginally lower. However, as flows increase on the mainline, the average travel times of vehicles travelling on the mainline (route 2) increases. This result is expected since the flows per lane on the mainline and slip lanes approach and then exceed their corresponding lane capacities. However, the resulting increase in average travel times is quite different for each layout with Option 1 and Option 1A producing much higher average travel time increases whereas Option 2 produces considerably lower increases in average travel times. The increase in travel times under all layouts is explained by the higher flows, however, Option 2 results in more consistent travel times across the range se flow combinations on the slip lane and mainline compared to Option 1 and Option 1A which produce significantly higher travel times which generally increase as the flows become heavier on the mainline. It is also worth noting that flow increases on the mainline have a greater influence on increased travel times than slip lane flow increases for both layouts as Figure 4 demonstrates (discussed in more detail later). Overall it can be observed that the Option 2 layout performs better than the Option 1 layout in terms differences between the travel times two options at higher flows and also for the consistency travel times produced by the Option 2 layout. Option 1A results in equal or improved travel times over Option 1 although this trend only occurs up to the point at which flows approach lane capacity. Option 2 produces better average travel times than Option 1A and as a result is the preferred solution. Nonetheless, Option 1A can be considered an improvement on Option 1 and is worthwhile to consider should road space

7 Proceedings O BRIEN and DUNNY: Optimal Merge Layouts constraints dictate that Option 2 is not possible to pursue. It is also worth noting that at lower flows, Option 1 performs almost as well as Option 2/Option 1A, underlining its appropriateness as a choice of layout for lower flow volumes such as those experienced on the M50 in the past. Figure 5 below depicts the average travel times of vehicles for route 1 (slip lane to mainline) for the same time interval and conditions of flow shown above for route 2 (mainline to mainline). The trends observed are broadly similar to the mainline to mainline travel time measurements. Again, in terms of average travel times, Option 2 and Option 1A perform as well as Option 1 under low flow conditions on the mainline/slip lanes and better than Option 1 under higher flows on the mainline/slip lanes. Furthermore, the Option 2/Option 1A layouts result in more consistent travel times with the average travel times produced ranging from 64 seconds to 108 seconds versus Option 1 (64 seconds to 202 seconds). Figure 5 - Slip Lane to Mainline (Route 1) Average Travel Times for Various Flow Scenarios under Option 1, Option 2, and Option 1A Merge Layouts One other trend is also worth observing. In general, under the Option 1 layout, as mainline flows increase (for different scenarios of slip lane flow) the average travel times increase as the mainline flow reaches 2000vph and then decrease thereafter. This occurs in most scenarios under Option 1 and to a lesser extent under Option 2/Option 1A with Option 2/Option 1A it occurs when the flows on the slip lanes are 2000vph or greater (over capacity). This phenomenon is most likely a result criteria used in the VISSIM model to remove/extract blocking vehicles from the model that are unable to merge or complete their journey due to the heavy flow on the mainline. In reality, this issue occurs where vehicles are unable to merge in a reasonable time frame due to the heavy flows on the mainline. In the model, this occurs to a much greater extent under Option 1 meaning that for high flows on the mainline, vehicles under Option 1 find it difficult/impossible to merge. Overall, the Option 2/Option 1A merge layout results in more consistent and lower average travel times than Option 1 for vehicles travelling from the slip lane to the mainline. In this case, Option 1A results in the lowest average travel times, however, the difference between it and Option 2 are negligible. Moreover, Option 2 produces the better result in the critical mainline to mainline travel time measurements meaning it has less impact on a greater number of users and therefore provides greater travel time savings overall. For completeness, these results have also been repeated for the maximum/minimum travel times and standard deviations average travel times. Similar trends and conclusions can be drawn from these results which are not shown here due to space constraints. As noted above, flow increases on the mainline have a greater influence on increased travel times for each merge layout. Therefore, it is also worthwhile to explicitly consider the performance of each merge layouts in terms of average travel times for both routes when the flows on the mainline are increasing under constant (high) flows on the slip lanes. Figure 6 and Figure 7 show, respectively, the effect on average travel times of increased flows on the mainline for a constant high flow of 1500vph per lane on the slip lanes for route 1 (slip lane to mainline) and route 2 (mainline to mainline). Figure 6 clearly shows how Option 1A and Option 2 perform similarly and better than Option 1 in terms slip lane to

8 O BRIEN and DUNNY: Optimal Merge Layouts Proceedings mainline travel time measurement. Figure 7 shows how Option 2 performs the best of all three layouts for the mainline to mainline travel time measurements with Option 1A producing a performance somewhere between Option 1 and Option 2. Overall, Option 2 appears to be a better layout since it (a) performs better than Option 1 for both routes; (b) performs as well as Option 1A for route 1; and (c) performs better than Option 1A for route 2 travel time measurements which affects the greater number of users (since the number of lanes and vehicles on the mainline is higher). Comparing Figure 6 and Figure 7 we can observe that increasing flow on the mainline has a greater impact on the slip lane to mainline travel times than the mainline to mainline travel time measurements for the Option 1 layout. This underlines how higher flows on the mainline increase the length of time it takes for vehicles to merge under this layout. Figure 6: Effect of Increased Mainline Flow on Slip Lane to Mainline Travel Time Measurements for Constant Slip Lane Flow (1500vph per lane) Figure 7: Effect of Increased Mainline Flow on Mainline to Mainline Travel Time Measurements for Constant Slip Lane Flow (1500vph per lane) Performance Measure No. 2: Comparison of Total Average Travel Times over the Study Period: Figure 8 below compares the average travel times resulting from Option 1, Option 2 and Option 1A merge layouts under two different scenarios of flow (light and heavy) on the slip lanes and mainline for 5 minute intervals over the study period considered in the VISSIM models (08:00 09:00). At low flow levels on the mainline and slip lanes, the average travel times under all merge layouts are almost identical and consistent across the entire study period. However, at higher flow levels on the slip lanes and mainline, the travel times for route 2 (mainline to mainline) are consistently higher for Option 1/Option 1A (on average Option 1 is 55 seconds higher or 70% greater than Option 2). This result shows how an Option 2 layout performs consistently better than Option 1 and Option 1A at high flow levels and equally as well as Option 1 and Option 1A at low flow levels, over the entire period. Similarly, Figure 9 shows the average travel times for route 1 (slip lane to mainline) for the merge layouts under high and low flow conditions on the slip lanes and the mainline. Figure 8: Average Travel Times (Mainline to Mainline) for Option 1, Option 2, and Option 1A Merge Layouts under Light and Heavy Flow Conditions for the Study Period Figure 9: Average Travel Times (Slip lane to Mainline) for Option 1, Option 2, and Option 1A Merge Layouts under Light and Heavy Flow Conditions for the Study Period

9 Proceedings O BRIEN and DUNNY: Optimal Merge Layouts Again, as with the mainline to mainline travel time measurements, Option 1, Option 2 and Option 1A layouts perform almost identically at lower levels of flow on the slip lanes and the mainline with average travel times under each layout being approximately similar. At higher levels of flow the average travel time increases under all merge layouts, as expected. However, under Option 2 and Option 1A, the increase in average travel times from the slip lane to the mainline are more consistent across the entire study period and far less than the increases experienced under Option 1 which results in increased travel times of over 1.5 minutes at certain time intervals in the study period. Performance Measure No. 3: Comparison of Network Statistics (Average Travel Time per Vehicle per Hour; Average Speed per Vehicle; and Average Delay per Vehicle): In the studies above we examined the average travel times of users for two routes those travelling from the slip lane to the mainline and those travelling upstream merge on the mainline to a point downstream on the mainline. It is also worthwhile to consider the overall average travel time per vehicle for each merge layout based on the total distances travelled by all vehicles in the entire study network. This measure accounts for the fact that overall mainline flows across all lanes are significantly higher than on the slip lanes. Figure 10 shows how the average travel time per vehicle increases as mainline flow increases under each layout for a constant slip lane flow volume of 1750vph per lane (chosen for illustrative purposes). Figure 10: Average Travel Time per Vehicle in the Network for Option 1, Option 2 and Option 1A Merge Layouts These results are expected as the mainline flow volumes approach and then exceed lane capacity. However, the magnitude increase in average travel time per vehicle varies under each merge layout. Figure 10 demonstrates that at low flow volumes on the mainline, the average travel times per vehicle in the study network are the same under each merge layout. As the mainline flows begin to approach lane capacity, Option 1 results in higher average travel times per vehicle while Option 2 and Option 1A produce lower/similar values. Under heavily congested conditions Option 1 results in the highest average travel time per vehicle while Option 2 results in the lowest average travel time per vehicle with the performance of Option 1A lying somewhere between Option 1 and Option 2. It is also useful to compare the average speed per vehicle in the network under each merge layout. Figure 11 depicts the average speed per vehicle in the network under each merge layout as mainline flow increases for a constant slip lane flow of 1750 vph per lane (chosen here for illustrative purposes). The overall trend under all layouts is for the average speed per vehicle to decrease as mainline flow increases, as expected. Similar to the results for average travel time per vehicle, at low flow volumes, the performance of Option 1, Option 2 and Option 1A layouts are similar in terms average speed per vehicle. As mainline flow increases the average speed per vehicle decreases under each layout but the decrease is greatest under Option 1. Option 2 produces the best performance of all layouts with average speeds decreasing by 25km/h versus 36km/h (Option 1A) and 40km/h (Option 1) as the mainline flow increases from low flow volumes of 1000 vph per lane up to heavily congested conditions of 2250 vph per lane. This underlines the benefits in terms of improved speeds (in addition to improved travel times) Option 2 layout.

10 O BRIEN and DUNNY: Optimal Merge Layouts Proceedings Figure 12 shows the average delay per vehicle in the network, for each merge layout, for a constant slip lane flow of 1750vph per lane under increasing mainline flow. The observations are similar to the results for the average travel time and average speed per vehicle in the network and are included here for completeness. Again, at the highest levels of flow on the mainline the average delay experienced by vehicles under the Option 1 merge layout is almost twice that Option 2 layout with the Option 1A layout resulting in an average delay per vehicle approximately between that produced under Option 1 and Option 2. Figure 11: Average Speed per Vehicle in the Network for Option 1, Option 2 and Option 1A Merge Layouts Figure 12: Average Delay per Vehicle in the Network for Option 1, Option 2 and Option 1A Merge Layouts In conclusion, Option 2 provides the most suitable alternative to Option 1 since it provides the same performance at low levels of flow, performs better for the slip lane to mainline travel time measurements, better for mainline to mainline travel time measurements, and better for network wide travel time, speed and delay measurements. Combining results, Option 1A could be said to provide a performance somewhere between Option 1 and Option 2 and is therefore also a viable alternative to Option 1. The results suggest that Option 2 is the preferred alternative but in the absence of road space to reconfigure the existing Option 1 layouts to Option 2 layouts then Option 1A is also worthwhile to consider since it also provides a benefit. 5. Implementation Option 2 Layout A number of issues impacting the Level of Service on the M50 southbound and northbound carriageways during the morning and evening peak were previously identified in [6]. In particular, five junctions along the M50 (at the N3, N4, N7, Ballymount and N81) were identified as key congestion areas based on the 2015 data obtained from the M50 Traffic Model and other sources. One main factors leading to the congestion on the M50 mainline at these locations was identified as the merge layouts. Specifically, M50 Junction 6 (N3 interchange) was identified as one highest priority locations requiring intervention having an Option 1 Type Layout at both its northbound and southbound merges. Whilst research undertaken in this paper has shown that revising the merge layouts from Option 1 to Option 2 has significant benefits for all users in terms of travel time and safety, it was felt prudent to undertake junction specific analysis at this location to ensure no unforeseen impacts occurred. Therefore, as part Junction 6 M50/N3 Preliminary Design Study undertaken by the authors, a detailed VISSIM model was constructed to ensure that site specific conditions were captured. For the purposes N3 Preliminary Design study, the modelled area of Junction 6 (N3 Interchange) in VISSIM extended approximately 2.5km in the northbound and southbound directions M50 mainline. This was considered an appropriate area of influence to consider as it captures the delays to the M50 mainline arising from merging/diverging movements at this junction in addition to existing M50 mainline delay arising from vehicles already on the mainline. This model was suitably calibrated and validated and future demand matrices were developed based on TII National Transport Model growth factors. A more detailed description of this study is outside the scope of this paper. Overall, the N3 preliminary design assessment confirmed the findings in this paper - that revising merge layouts from Option 1 to Option 2 has significant travel time benefits for users

11 Proceedings O BRIEN and DUNNY: Optimal Merge Layouts in addition to safety benefits. Specifically, the N3 preliminary design assessment showed that it is worthwhile in terms of user travel times to implement a Type 2 merge layout for both the northbound and southbound merges M50/N3 interchange. The study also noted that the interventions proposed by the N3 preliminary design study are short term in nature and whilst the merges will continue to operate better than the existing merge layout well into the future the junction delays (in terms of travel times) will return to current levels in 3 5 years. At that stage, a longer term intervention, whether it be some form of demand management or merge upgrade, would be required. Due to the short term nature scheme, assessments were undertaken which assumed a 2 and 5 year economic appraisal period. These tests returned positive Benefit Cost Ratios (BCRs) further highlighting the benefit of delivering the scheme, even in the short term. Following these recommendations, TII implemented the new Option 2 layouts for both the northbound and southbound merges at this location in the latter part of Figure 13 shows the old Option 1 layout and the new Option 2 layout as constructed at M50 Junction 6 (M50/N3) Northbound merge. The southbound merge was reconfigured in a similar manner but is not shown here due to space limitations. Anecdotal evidence suggests that the new layouts are operating well to date; however, it is the authors intention, as part of a future research study, to assess the performance new merge layouts at this location. It should also be noted that in addition to the M50/N3 interchange a new Option 2 layout was also implemented on the M1 Junction 2 (Airport Interchange) Southbound Merge. Lane Gain Old Layout (Option 1) New Layout (Option 2) Lane Gain Figure 13: M50 Junction 6 (N3) Northbound Merge Old and New Layout Configurations 5. Conclusions and Recommendations This study developed VISSIM models to analyse the performance of double lane merge layouts on the M50. The first layout, DMRB TD22/06 Type F Option 1, represents the existing situation at 11 locations on the M50. This study has highlighted potential concerns around the use of this layout arrangement both in terms of its performance under congested conditions and safety. This study showed that there is a higher average collision rate at merge locations with an Option 1 layout (with lane gain) although these results are based on a small sample so a non-firm conclusion can be arrived at. An alternative layout - DMRB TD22/06 Type F Option 2 was proposed in this study as a potential solution to ease congestion on the M50. At the time this study was conducted this layout was already in place in only two locations on the M50, the Sandyford and the M11 junctions. This research also proposed a second alternative layout known as Option 1A. The existing layout (Option 1) and potential alternatives (Option 2 and Option 1A) were tested using the VISSIM models, developed as part of this study, under a range of flow volumes on the slip lanes and the mainline. Two measures of performance based on average travel times of all vehicles travelling on two routes (slip lane to mainline and mainline to mainline) were used to compare the effectiveness merge layouts. A third measure of performance based on network statistics extracted from the VISSIM models was used to compare the average travel time/speed/delay per vehicle for the entire model network. The first measure, the total average travel time under different combinations of flow volumes on the slip lanes and the mainline, showed the range of flow volumes under which the Option 1, Option 2 or Option 1A layout works best. In general, the results indicate that, at low flow volumes, Option 1 performs almost as well as the proposed alternatives, Option 2 and Option 1A. At higher flows, Option 2 produces the best performance in terms mainline travel time measurement while Option 2/Option 1A produce a similar performance to each other and an improved performance over Option 1 for the slip lane to mainline travel time measurements. It is clear that overall Option 2 performs much better under more heavily congested conditions such as those currently experienced and forecasted for the M50. Thus,

12 O BRIEN and DUNNY: Optimal Merge Layouts Proceedings from a network operation point of view, rather than safety, Option 1 may still be a viable or appropriate choice of layout at locations where mainline/slip lane flows are low however this layout does not provide for future traffic increases. Overall, the graphs produced under this approach provide us with a framework to determine which layout is appropriate once the existing/projected flows on the mainline and slip lanes are known. The second measure, the average travel times merge layouts over the entire study period, shows the performance existing and proposed alternative merge layout over the entire time period study for two scenarios of flow volumes low flows on the slip lanes and mainline and high flows on the slip lanes and mainline. The results further underline how Option 2 always outperforms Option 1 across the study period and not just for the worst case scenario time interval as shown above. Option 1A also provides a better performance than Option 1 albeit to a lesser extent particularly in terms mainline travel time measurements. The third measure of performance considered in this study is based on comparing the network wide average travel time/delay/speed per vehicle for Option 1, Option 2 and Option 1A merge layouts. These results, included for completeness, and to account for the fact that overall mainline flows across all lanes are significantly higher than on the slip lanes, clearly demonstrate that the Option 2 layout produces the best performance in terms of average travel time/speed/delay per vehicle of all three merge layouts. Option 1A also provides a superior performance to Option 1 but to a lesser extent. Therefore, it can be concluded that Option 2 merge layouts should be considered as the preferred alternative layout to Option 1 where possible and where not possible (due to road space constraints or geometry) Option 1A should be considered since it also provides some benefits/improvements over Option 1. The results from these models and in particular the graphs in Figure 4 and Figure 5 provide a basis upon which decisions can be made regarding the appropriate merge layout to choose. In any event, where road space exists, switching from an existing Option 1 to an Option 2 layout could be regarded as a relatively straightforward/inexpensive line marking exercise with the potential to produce lower average travel times at higher flows. If geometry or road space constraints do not permit an Option 2 layout at a specific merge location then this study has shown that there are still benefits to be realised by pursuing the other proposed alternative layout Option 1A. In general, the choice of appropriate layout at each specific merge location will also be dictated by site-specific conditions (e.g. road space constraints and geometry) and it may also be worthwhile to carry out specific VISSIM modelling (as carried out for the N3 merge) for each location (e.g. N4/N7 merge). Nonetheless, the generic models developed here provide a good guide on which layout to use but junction or merge specific models may also be required to gain a greater understanding at complex junctions. Finally, this study concluded by describing the implementation merge layout recommended by this study (Option 2) at the M50 Junction 6 (N3 Interchange). One appropriate future research direction may be to assess the performance of this junction and other locations where an Option 2 layout has been implemented. References [1] L. Elefteriadou, R.P. Roess, and W.R. McShane, The probabilistic nature of 42 breakdown at freeway-merge junctions, Transportation Research Record: Journal Transportation Research Board, Vol. 1484, pp , [2] B.S. Kerner and H. Rehborn, Experimental properties of phase transitions in traffic flow, Physical Review Letters, Vol. 79, Iss. 20, pp , [3] H. Yi and T.E. Mulinazzi, Urban freeway on-ramp invasive influences on 3 mainline operations, Proceedings 86 th Annual Meeting Transportation Research Board (TRB), Washington D.C., [4] R. Liu and G. Hyman, Modelling motorway merge: The current practice in the UK and towards establishing general principles, Transport Policy, Vol. 24, pp , [5] Transport Infrastructure Ireland, M50 Merge Layouts Research Findings Report, [6] Transport Infrastructure Ireland, GDA Congestion Study Findings Report, 2015.