Item 11 - Appendix 2
West Dunbartonshire Council · 21 May 2025
Back to Infrastructure, Regeneration & Economic Development Committee — 21 May 2025 and its agenda
Text extracted from the council's PDF. Layout and reading order may differ from the original.
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Technical Note West Dunbartonshire 20mph Scoring System Our reference: WDC/241000/TN01 – Rev00 1. Each major locality within the West Dunbartonshire council area – Alexandria, Clydebank and Dumbarton was initially divided into smaller zones for 20mph implementation. This division was based on the previous RAG assessment, as well as physical boundaries such as major roads, railways, natural barriers, and changes in area character. This process resulted in 26 zones spread across the localities with the detail of the approach set out within the standalone 20mph appraisal report. 2. To determine the order of priority for implementing 20mph zones, a scoring system was applied to each zone. The criteria included collision history, road geometry and engineering, existing traffic speed limits, the composition of road users, and active travel routes. Each criterion was scored individually on a scale from -3 to +3 (to align with the principles of Scottish Transport Appraisal Guidance). The scores were then averaged across each zone to calculate an overall score, which was subsequently used to establish the priority order. History of Collisions 3. Personal Injury Collisions (PIC) data from the 5-year period 2018-2022 was used to examine collision statistics. Accident data within the zones was analysed and categorised into three types: slight, severe, and fatal collisions. A severity weight was assigned to each category: slight collisions were weighted at 1, severe collisions at 12 and fatal collisions at 100, reflecting the substantial cost differences in Scotland in 2023, with fatal collisions costing £2,597,781 compared to £321,719 for severe and £32,945 for slight collisions. These weights were used to calculate a weighted average for each zone, where a higher score indicates areas with a higher number of collisions and/or more severe collisions. The weighted scores ranged from 0 to 1 and were divided into brackets of 0.17 to assign a numerical score between -3 and +3. Road Geometry and Engineering 4. RAG scores for each road, which reflect road geometry and the engineering required to implement a 20mph speed limit on each road, were used to analyse the percentage of ‘amber’ roads in each zone. These percentages were then translated into a score. For example, zones with 0-10% amber coverage received a score of +3, as they are simpler to implement as 20mph zones, requiring little or no additional infrastructure over and above appropriate signage. Zones with 40-60% amber coverage were assigned a score of -3 as these would almost certainly require some additional physical measures to support the 20mph roll out. Traffic Speed Limits 5. Roads in each zone were assessed to identify speed limits of 15mph, 20mph, ‘20’s plenty’, ’20 when lights flash’, and 30mph. As all zones included roads with 30mph, these were excluded from the scoring system. The remaining roads were weighted, with 15mph and 20mph roads assigned a higher weight of 40, while ‘20’s Plenty’ and ’20 When Lights Flash’ roads were ITEM 11 - APPENDIX 2
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241000 / West Dunbartonshire 20mph, Scoring System Technical Note weighted at 10, as road which already have limits, be they advisory or statutory, of 20mph or below are likely to be more straightforward to introduce the 20mph limit. These weights were used to calculate a weighted average, producing scores ranging from 0 to 0.6. The scores were then divided into 0.15 segments to assign an overall score between -3 and +3. Composition of Road Users 6. This primarily sought to examine the type of road users and specifically pedestrian likely to be present in an area. To achieve this, each zone was evaluated for the presence of various amenities, including community centres, spots and leisure centres, hospitals, healthcare centres, care homes, churches, primary schools, and secondary schools. 7. All amenities were assigned a weight of 10, except for primary schools, which were weighted at 30 to account for the higher concentration of younger pedestrians in these area. These weights were used to calculate weighted averages for the number of amenities in each zone. 8. The resulting weighted averages ranged from 0 to 2.1. These averages were then translated into a scoring system where -3 indicated a lack of amenities (suggesting lower pedestrian activity), and +3 reflected a higher number of amenities and/or a greater presence of primary schools, signifying a higher priority for traffic calming measures. Active Travel Routes 9. Since all zones had adequate pedestrian walkways, active travel routes were assessed based on the presence of formal cycle networks. 10. Zones containing a cycle route were assigned a score of +1, while those without one received a score of -1, as areas with more active travel routes are considered a higher priority for implementing 20mph traffic calming measures. Scoring System 11. This system produced a score ranging from -3 to +3 for each criterion for each zone. These scores were then averaged to generate an overall score for each zone. 12. Figure 0-1 presents a radar diagram illustrating each criterion and its corresponding score for the Renton zone.
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241000 / West Dunbartonshire 20mph, Scoring System Technical Note Figure 0-1 Renton Criteria Scores 13. The overall scores were used to rank the zones in a priority list, with higher scores indicating higher priority. The priority order for each major locality is as follows: Alexandria: 1. Renton (1) 2. Alexandria Central (0.4) 3. Alexandria East (0.2) 4. Bonhill (-0.4) 5. Alexandria North (-0.6) 6. Redburn (-1.2) 7. Balloch (-1.6) Clydebank: 1. Kilbowie (2) 2. Whitecrook (1.4) 3. Drumry (1) 4. Clydebank Waterfront (0.8) 5. Old Kilpatrick (0.2) -3 3 22 1 History of Collisions Road Geometry and Engineering Traffic Speed LimitsComposition of Road Users Active Travel Routes Renton Renton
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241000 / West Dunbartonshire 20mph, Scoring System Technical Note 6. Duntocher (-0.2) 7. Faifley (-0.2) 8. Goldenhill (-0.2) 9. Dalmuir (-0.4) 10. Linnvale (-1) 11. Mountblow (-1.4) Dumbarton 1. Silverton (0.2) 2. Castlehill (0) 3. Townend (0) 4. Dumbarton Central (-0.2) 5. Bellsmyre (-0.4) 6. Highmains (-1.2) 7. Kirktonhill (-1.4) 8. Milton (-1.4) 14. Figure 0-2, Figure 0-3 and Figure 0-4 show the spatial distribution of the priority areas, where the dark blue zones indicate higher priority areas. Figure 0-2 Priority Rank Map - Alexandria
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241000 / West Dunbartonshire 20mph, Scoring System Technical Note Figure 0-3 Priority Rank Map - Clydebank Figure 0-4 Priority Rank Map - Dumbarton