An assessment of TCM
The use of TCM raises many conceptual, analytical and operational issues. Here it has only been possible to give and overview. There are, however, several publications that give useful introductory reviews and explanation of the method. Hanley and Spash (1993), offer an accessible and clear exposition related to its incorporation into CBA in the environmental field. Perman et al. (2003) is a more rigorous approach in which appendices in the relevant chapter deal with the technicalities of the method.
- Multipurpose trips raise the issues as to what value should be assigned to the specific site being examined. Cheshire and Stabler (1976) made the distinction between sole, transit and meandering visitors identified above. For the first, the entry of the visit was attached to the site, while for meanderers information on the number of sites visited on the day was sough and a portion of the total travel costs allocated between them. The issue is whether the value places on each site by respondent should be equal; if not, one solution is to ask them to offer a score on the value attached to each site, so that a weighting scheme can be applied. The transit visitors, who were in a minority, presented a problem and eventually it was decided to remove their responses from the estimation of the value of the site.
- The value of time has been a source of dispute in the literature, not so much as to whether or not it should be included; that has been widely accepted, particularly where visitors incur no monetary travel costs, as Harrison and Stabler (1981) found with regard to informal recreational visit to canal in the UK. The problem is what value should be included for time; currently the issues has not been fully resolved. In the canal study, a sensitivity analysis was applied taking a number of fraction of an hourly average earning rate secured from income information sought from respondents.
- The mathematical function chosen to derive the TCM demand curve is a similar issue to that found with HPM. The need to obtain an intersection with the axes and the position and shape of the curve determined by function will in turn affect the area under the curve and thus the estimate of consumers’ surplus. The truncation to achieve and intersection may be arbitrarily decided on when the curve suggest by the data is asymptotic to the axes. The functional form most frequently employed are linear and semi-log and log-log. The characteristics and appraisal of the functional forms have been examined by Common(1973) and Hanley (1989).
- Quality change can occur where a site is degraded, which reduces the number of visitors, or is improved, for example the provision of new facilities, or enhancement of its natural environment, which increases the number of visitors. Such changes imply that the site should be re-surveyed and the change in demand estimated.
- Competing site can have an influence on the number of visitors, especially if they are in close proximity to the survey site and their quality changes. Also, if the other site give rise to a meandering type of the visitors, necessitating a greater allocation of their travel costs across each site, the estimates of value will be lower.
- The visit of tourists are perceive by some researcher as a distorting factor on the distance travelled and therefore travel costs. This can be resolved by obtaining the location of departure on the day of the visit, treating them as temporary residents. Another recourse is to apply the adjustment for multiple trips and competing site by apportioning their travel cost from their usual residence over all the sites in the location where they are staying. However, overseas visitors present a more acute problem on which there is no consensus as to how to treat their travel costs; one solution is to remove them from the survey.
- Whether to apply marginal or total costs of travel is an issue where visitor arrive by car. The problem is analogous to the tourist’ case above. The majority of researcher have used marginal costs as they are perceive as reflecting the marginal utility theory, in which the marginal cost of consumption is taken into account.
The TCM effectively only measure the consumption values, of visitors at one point in time; it does not capture the option or non-use value, so does not estimate TEV. With respect to the value of time, it is possible that the income-leisure trade-off is not conceptually acceptable, it being argued that the opportunity cost is that relating to another leisure activity.