Modelling
Modelling
Why we have doubts about the modelling
The traffic modelling
for the Putney Link Road scheme was carried out using the Leicester and Leicestershire Integrated Transport Model (LLITM). We have looked carefully at this modelling and we set out below, as simply as possible, the concerns which arise from it.
Traffic modelling is essentially based on traffic counts at a number of fixed points from which the overall pattern of traffic can then be computed. Before a model can be used it has to be verified as fit for purpose, meaning it does not produce misleading or biased results. There are two stages to this verification – calibration and validation. Calibration
is a check of the accuracy of the vehicle count and traffic flow at each counting point – it compares the observed (actual) count with the modelled count generated by the traffic modelling. Validation
is a check of observed (actual) journey times between counting points and compares them with computed journey times.
To be considered satisfactory a model needs to meet certain specified standards of calibration and validation. The tests are not described here, but the acceptable level, or ‘pass’ rate, is 85% or higher.
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