Stages in the TCM process
- Conduct a survey on site of visitors, or in the home if investigating potential visits to giving sites. An initial survey as a pre-test is desirable as it inform stage two, particularly the identification of the catchment area as a basis for distance travelled, which also contributes to the derivation of the trip generation function.
- Establishing the catchment area of the site is important, as it is the basis for establishing the furthest distance visitors are likely to travel to it; an issue that can arise is the point where outlier observation are omitted from the analysis, as they have an impact on the estimations of the demand curve considered below. Outliers also are factor in the calculation of visitor rate for a given population in the zonal TCM, explained below.
- Derive a trip generation function (TGF). Its purpose is to estimate how to many visits will occur, but it also is the stage when likely explanatory variable can be considered, in addition to those identified in the introductory paragraph above, such as mode of transport, size and composition of the visitors’ party; resident or tourist; wealth; income; occupation education level ; age ; class nature of the visit, i.e. sole purpose, or in transit, or meandering on a pleasure outing. The TGF, where the dependent variable is the number or rate of visits, can takes two form;
- Trips generated by individuals;
- Trips on a zonal basis; as a visit rate for a given population size, e.g. rate per 1,000 population in each zone, covering the catchment area around the size. Zones can be concentric rings ( Cheshire and Stabler, 1976), or postal districts or local authority areas, or rectangular in the case of linear sites, for example a canal (Harrison and Stabler, 1981); a problem with the last is the many access points it has. This is also the case with very large sites, for example national parks.
There is no clear indication that either of the TGFs is superior to the other, nor is there any widely agreed approach; for a review of them see Willis and Garrod (1991)
- Estimate the demand curve for the site. The demand curve derived indicates that as travel costs increase (the vertical axis), the number of individuals, or visit rate in the case of zonal approach, will fall ( horizontal axis). The function form of the demand curve should be such as to ensure an intersection of the curve with the travel cost, visit number or rate axes.
- The final stage is to calculate the total value of the site by measuring the area under the demand curve. This yield the consumers’ surplus for a single visit. The estimate so obtained can be aggregated to provide a total value for a given period. This, however, is problematic as use of a site almost certainly varies over the week, higher visit rates occurring at weekends and seasonally. This suggests that a series of surveys need to be considered to take account of these factors.