WebTAG update sees raft of changes


The Government's WebTAG transport analysis guidance sees a batch of significant updates this month, including changes covering rail demand forecasting, the wider economic impacts of transport schemes and the economic impact of air pollution.

Units A5.3, M4 and the data book see updates to the parameters and variables used in rail demand forecasting, following the recent release of the Passenger Demand Forecasting Handbook version 6, while the data book will also include forecasting parameters from the Rail Demand Forecasting Estimation study.


The study reviewed methods for forecasting rail demand to reflect the fact that actual growth had outstripped predictions. It suggested a new forecasting methodology and set of recommended forecasting parameters.

A number of units and documents are updated to reflect changes announced in December 2017 to guidance for appraising the wider economic impacts of a scheme.

Forthcoming changes have also been announced to unit A3 and the environmental worksheets reflecting  interim guidance on valuing changes in emissions of oxides of nitrogen and concentrations of nitrogen dioxide (NOx).

Based on interim advice from the Committee on the Medical Effects of Air Pollution, the new damage costs now capture the direct human health impacts of exposure to nitrogen dioxide (NO2), rather than just the indirect health impacts of NOx emissions via their contribution to the formation of particulate matter.

Department for Transport officials said the guidance is expected to be more fully revised ‘in due course’ following the publication of definitive guidance from environment department Defra, reflecting significant uncertainties around the quantitative link between exposure to NO2 and health impacts.

A number of units and documents are also updated in respect of bus and cycle diversion factors, i.e. calculations of the number of people who will be diverted from other modes, including unit 5.1, which gives guidance on how to estimate and report impacts on active modes such as walking and cycling.


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