Object

Local Plan 2040 Draft Plan - Strategy options and draft policies consultation

Representation ID: 4681

Received: 01/09/2021

Respondent: Mr Joshua Zwetsloot

Representation Summary:

BBC have concluded that Twinwoods creates such a traffic problem at the Clapham Road/Manton Lane junction in Bedford that it is not a suitable development site as the developer has not proposed a solution to the congestion at that junction. Staploe Parish Council believe that conclusion is incorrect.
• The transport model starts with the 2018 base year model previously developed and then adds to that the growth identified by Bedford Borough Council in the adopted local plan to 2030 and includes the mitigations (traffic schemes) that are planned to happen by 2030. This then creates the reference case at year 2030, against which the different development scenarios to 2040 and to 2050 are compared.
• One of the measures of the impact the developments will have on the highway network, is the Junction-Volume Capacity Ratio. This shows how congested the junctions will be – so a figure of 100% means that the junction is fully utilised, i.e. congested, leading to delays at that junction.
• Tables 3.15 (for 2040) and 3.16 (for 2050, but incorrectly titled for 2040) show the Junction-Volume Capacity Ratios for the PM peak Hour in 2040 and 2050 for the unmitigated and mitigated Twinwoods scenarios. The 4 lines for the Clapham Road/Manton Lane junction as follows: (Table 3.15 attached here)
You will see that the black boxes denote 100% or more Ratio – i.e. the junction is completely full (on 3 of the 4 legs) and congested.
• The report at paras 3.5.21 and 3.5.22 states:
• 3.5.21 Table 3.9 to Table 3.16 highlight that there is forecast to be significant delays and congestion issues at the Clapham Road / Manton Lane Junction, particularly in the PM Peak hour. These forecast delays and high volume-capacity ratios are present within the 2030 reference case, with volume-capacity ratios forecast to be above 100% on three out of the four arms at this junction. These high forecast volume1capacity ratios are maintained or are forecast to worsen in the 2040 and 2050 forecasts including the proposed Twinwoods and / or Colworth developments.
• 3.5.22 As part of the forecasting assumptions for the reference case (see Table 2.2), an improvement scheme at this location, which provides a dedicated left-turn lane for Clapham Road southbound traffic to Manton Lane and the part-signalisation of the roundabout, is included. Further mitigation at this location was considered as part of this study to alleviate the forecast congestion, but due to the constraints at this location (such as existing buildings and Bedford Modern School) no further mitigation has been proposed as part of this study.
• The junction is already completely full at 2030 before any new developments or the addition of an east west rail station in Bedford Town Centre. Therefore this is a Bedford Borough Council problem that will already happen due to the growth in the adopted Local plan 2030 (1500 new homes all north of Bedford which will impact the A6 by 2030). Bedford Borough Council will have to solve this – not the developers of Twinwoods but a development at Twinwoods might contribute some infrastructure funding to help mitigate the problems on the A6
• It should be noted that when Twinwoods is included the Junction-Volume Capacity Ratios increase (on 3 of the 4 legs) from 100% to between 101% and 105% which is negligible in the overall scale of the issue.
• Para 4.2.9 concludes :
• 4.2.9 Significant forecast delays and capacity constraints are forecast at the Clapham Road / Manton Lane roundabout in northern Bedford, primarily in the PM Peak hour, both in the reference case and with the inclusion of the proposed developments and mitigation measures. The reference case includes improvements to this junction; however, further mitigation at this junction to alleviate the forecast congestion has not been proposed as part of this study due to the physical constraints at this location. Without further capacity improvements or measures to reduce the forecast traffic at this location, the forecast congestion at this junction is likely to be a constraint on the delivery of growth along the A6 corridor to the north of the borough.
• This demonstrates that this is not a new development issue, but an existing issue that Bedford Borough Council will have to solve before 2030 – particularly in view of the fact that all traffic wanting to access the new East West Rail Station from the north of the Borough will need to use the A6 to reach the station in central Bedford.
• The Transport Modelling undertaken by AECOM in support of the strategic options in the Draft Local Plan 2040 is fundamentally flawed for the following reasons.
• No validation or calibration of the traffic model has been undertaken which is not in keeping with Department for Transport WebTAG guidance.
• It is assumed that the Bedford Borough Transport Model was undertaken using Saturn, which was the model of choice for the Bedford Town Centre modelling project in 2015. In this case, Saturn cannot directly Model Public Transport therefore it is assumed that the engineers have made some significant assumptions with respect to public transport and trips being used, which have not been validated or calibrated.
• TAG unit M1.2 introduces the National Trip End Model (NTEM). It includes forecasts of population, households, workforce and jobs over 30 years which are used in a series of models that forecast population, employment, car ownership, trip ends and traffic growth by Middle Layer Super Output Area (MSOA). The NTEM data set can be viewed using the TEMPro (Trip End Model Presentation Program) software. TEMPro estimates of trip ends at any level below aggregate regions (e.g. MSOA, district, or county level) are subject to uncertainty and should not be used as constraints in matrix development process without verification and possible adjustments. No uncertainty log was prepared which is a recommendation of WebTAG modelling guidance.
• For direct use in matrix development, trip rate information estimated from household survey data should be considered instead to underpin trip end estimates at zone level. There is a risk that model may not be realistic or sensible due to the error around the model parameters used, or limitations in the extent to which the model can represent human behaviour. Therefore, before using any mathematical model, it is essential to check that it produces credible outputs consistent with observed behaviour. This is usually done by running the model for the base year (either the current year or a recent year), and:
• comparing its outputs with independent data (validation);
checking that its response to changes in inputs is realistic, based on results from independent evidence (realism testing); and
• checking that the model responds appropriately to all its main inputs (sensitivity testing).
• Five types of data can be collected and used to inform most models:
• data on the transport network, including the physical layout, number of lanes, signal timings, public transport frequencies and capacities;
• counts of vehicles or persons on transport services, links or at junctions;
• journey times;
• queue lengths at busy junctions;
• interview surveys, in which transport users are asked to describe trips either through household travel diaries or intercept surveys (e.g. roadside interviews, public transport onboard interview surveys.
• These types of checks have not been undertaken to validate / calibrate the model.
• AECOM have derived trip ends using CTripEnds for a number of journey purposes. Expanding synthetic trip ends produced by CTripEnd to the local zoning system is considered to be subject to significant discrepancies from observed especially if validation and the calibration exercise has not been undertaken.
• It is also important to note that strategic models are not designed for use in a scheme specific assessment. For such an assessment it is recommended a revised forecast model would be produced from a recalibrated base year model using additional and more recent data and targeted to reflect a more specific geographical focus of resources and modelling effort.