Transportation modellers have been aware for some time that the modelling of `tours' beginning and ending at home is in principle greatly preferable to the modelling of the component `trips'. Mode choice and trip timing can be made logically consistent, while modelling the dependence of travel on daily activity patterns can be contemplated to relate trip generation to the more fundamental causes of travel. Within the tour framework an improved modelling of non-home-based travel is also possible. Rather than modelling independent trips, account can be taken of the nature of non-home-based trips as `detours' from the simple home-destination-home pattern of the majority of tours. The estimation of models incorporating non-home-based trips into tours in this way has been successfully undertaken in a small number of practical studies. Further problems arise, however, in devising application procedures for models of this type, particularly in large study areas. The interaction of the origin and destination of the non-home-based trip with the traveller's home could cause very large calculations to be necessary. The paper presents a methodology for addressing the issues of modelling and forecasting non-home-based trips within the framework of the tour concept. A simple approximation is presented that retains the essential dependence of choices for non-home-based trips on the accessibilities of modes and destinations while avoiding the excessive calculations that would be necessary to consider all the interactions. Some examples are given from practical demand models in current use. (A)
Abstract