In transport planning and forecasting there are many variables to be taken into account. Obtaining accurate base data and robust projections of key factors such as economic growth, household incomes and fuel prices can be problematic. However, in the UK we tend to assume that population forecasts are one of the more accurate inputs to the process. Recent evidence suggests that population data and forecasts should be challenged as much as any other input to our models, particularly in large urban areas.
I have written before about the changes in government forecasts of population growth which have taken place over the last twenty years. Setting the population forecasts used in the 1997 National Road Traffic Forecasts against the latest government projections shows the importance of this issue.
An interesting piece of research has recently been carried out by Gillian Harper and Les Mayhew at the Cass Business School. Their contention is that the number of households in six ‘Olympic’ London Boroughs in East London may have been underestimated by as much as 6.6% in official statistics.
Drawing on a range of administrative data from local government bodies and health authorities the authors used a method which is different from that currently applied by the Department for Communities and Local Government (DCLG) to ‘count’ and categorise the number households. In Hackney and Newham the differences were particularly large (over 10%). Interestingly the numbers appear to agree reasonably well with estimates produced by the Greater London Authority.
The population estimates produced by Harper and Mayhew were even more divergent from those of the DCLG. In fact the population estimate for the six boroughs was 11% higher than the ONS estimates. The authors concede that there are problems in keeping-up with change in areas with high levels of regeneration and in-migration. Obviously there are important implications for those planning transport in our larger towns and cities if we have to build these additional uncertainties into our future scenarios. Food for thought !