Projecting Licence Holding Rates in Great Britain Using a Cohort Modelling Approach
Hui Lu, RAND Europe, Andrew Daly, RAND Europe, James Fox, RAND Europe
This paper describes work to develop from NTS data a model to predict changes in licence holding by age band and gender.
As part of work to enhance the National Car Ownership model (NATCOP) for the UK Department for transport a licence cohort model has been developed to predict changes in aggregate licence holding rates over time for 28 age-gender cohorts. This paper presents the development of the licence holding model using National Travel Survey (NTS) data. This study contributes to understanding the changes in licence holding in the Great Britain, predicting changes up to 2051 and identifying and examining the factors that explain these trends as well as reflecting better represent regional variation in licence holding.
Licence holding is a key element for forecasting car ownership and the availability of cars to individuals. Previous studies indicate that relating car ownership to licensed drivers substantially improves car ownership modelling reflecting long-term trends, apart from income changes, that affect car ownership. In the previous NATCOP car ownership models, licence holding was represented as an overall national average rate per year. While this approach successfully picked up the impact of historical increases in licence holding, it fails to predict the impact on specific groups, such as older women, migrant women and the youngest adults, who have lower licence-holding and therefore reduced accessibility. Further it does not provide a basis for forecasting how licence holding by age and gender might evolve in the future.
The revised modelling approach entails grouping individuals into cohorts, defined on the basis of common shared characteristics and then tracing the cohorts over time. As individuals usually acquire driving licences only once in their lifetime and few people give up licences, the basic model assumes that the licence holding for an age-gender cohort is equal to the licence holding for the same cohort in the previous time period with a small adjustment (i.e. net acquisitions or losses) that have occurred over the period. The cohort approach and the modelling of car ownership as dependent on licence holding were originated by Anton van den Broecke in the 1970s (van der Hoorn and Mulder, 1981) and have been successfully applied in a number of large-scale transport models, for instance in the Dutch National Model and the Sydney Strategic Model.
The models are developed using historical data on licence holding from 1975/1976, 1985/1986 and 1989/1991 from published summary tables. More recent NTS continuous surveys contain the data that covers the period from 1995 to 2014. The above two sets of data were combined and used to better understand licence holding trends and for development of the cohort model.
Initial analysis of the NTS data shows an increase in licence holding for people aged 50 and above over time for both men and women. Second, young adults are found to be delaying licence acquisition in recent years. Third, we observe a substantial increase in licence holding rates for women of working age and older which may relate to increased female participation in the workforce.
Acquisition and loss rates for each age–gender cohort per 5-year time period (from 1996 to 2011) are calculated. Previous studies noted that the area type is closely correlated with licence holding (specifically, average licence holding rates are higher in the less populated areas), and in this study Great Britain has been categorised by six different area types: metropolitan, non-metropolitan and by population density. The models for each area type are adjusted to reflect this licence holding rate variation. The model was validated using 2014 NTS data (the latest year available to the research team) to assess its performance.
The cohort model predicts the licence holding rates at five-year intervals from 2011 (base year) to 2051. Interesting findings emerged from this projection. First, for older generations (especially older females) licence holding is predicted to increase as individuals in younger cohorts retain licences into the future. Second, the impact of delayed licence acquisition can be seen for working-age adults, particularly for males. Third, there is increasing convergence between male and female licence-holding levels, with the exception of the oldest two cohorts showing significantly higher male licence holding.
The findings are discussed and compared with the previous evidence. A few factors (for instance, the impact of migration and socio-economic factors) are identified and examined to facilitate understanding of the changes in licence holding rates over time.
Van der Hoorn, A.I.J.M. and Mulder, M.R. (1981) Een gedisaggregeerde registratie van de ontwikkeling van rijbewijs- en autobezit (Disaggregate recording of the development of licence holding and car ownership), Rijkswaterstaat.
Association for European Transport