Introduction Case Study 3: Theory-testing research: testing a deterministic relation

and deaths resulting from traffic accidents, loss of greenfield sites and open space, and damage to infrastructure and historical buildings from heavy vehicles Browne and Allen, 1999; Banister et al., 2000. Currently, these negative effects have the upper hand in residents’ and policy makers’ perceptions of urban freight transport and form the motivation for policies aimed at reducing it. One of the most popular urban freight transport policy measures aiming at improving social sustainability in urban areas, especially in Europe, is the use of time access windows OECD 2003. A time access window forces all distribution activities to be carried out within the time window period at the time window area. The objective of time windows is to improve the quality of the city centres, by reducing the perceived negative impacts caused by large vehicles in shopping centres, as well as to separate the freight carriers from the shopping pub- lic that uses cars to visit the shopping areas Allen et al., 2004; Munuzuri et al., 2005. The use of time windows has increased in the Netherlands over recent years. In 1998, 41 per cent of the 278 largest Dutch munici- palities used time windows. This increased to 53 per cent in 2002. The larger the municipality, the more likely it is that it uses time windows: of the Dutch top 100 municipalities, 71 per cent used time windows in 2002 and all municipalities in the top 20 did so. In 2002, the average time win- dow length was about 4.5 hours PSD 2002. Many carriers and large retail chains consider time windows one of their biggest problems in delivering to their shops in urban areas Crum and Vossen, 2000. Groothedde and Uil 2004 estimate that the current cost caused by time window restrictions for Dutch retail is about €270 million annually.

6.2.2 Theory

6.2.2.1 Object of study

The object of study is the distribution activities by retailers from a retailers’ distribution centre to the shops during one week.

6.2.2.2 Concepts

The concepts of interest are: ■ Time access window pressure. This pressure consists of a the number of windows number of areas in which time access win- dows are present and b the length of these time windows. ■ Distribution costs. Four dimensions of distribution costs are dis- tinguished: number of roundtrips, number of vehicles, total travel distance, and total time.

6.2.2.3 Proposition

Our theory states that distribution costs increase in all four dimensions if time window pressure increases, for the following reasons: first, because shops cannot always be reached in a vehicle roundtrip at times that are convenient for the retailer, the number of roundtrips from a dis- tribution centre will increase. Second, because most of this higher number of roundtrips will take place at the same time, more vehicles will be needed. Third, we expect that the total travel distance and total time spent on roundtrips will increase because, due to the time windows, it will not be possible to always make roundtrips, which are the most effi- cient in terms of distance and time. Based on this theory, we formu- lated the following deterministic proposition: Proposition: Each realistic increase in time access window pressure causes an increase in all four dimensions of distribution costs. The contention of this proposition is that there are no retailers that do not feel the consequences of a higher time window pressure in all four dimensions of distribution costs. With “realistic increase” we mean levels of increase that could be realized in practice not just an increase in minutes but at least a quarter of an hour and realistic duration e.g. maximum a few hours, not up to 24 hours per day. The absence of a qual- ifier such as “often”, or “likely”, makes the proposition a deterministic one.

6.2.2.4 Domain

The theory applies to the typical Western European distribution con- text and within this context to all large retailers that distribute goods from a distribution centre to shops that are at least partly located in shopping areas in cities in which time access windows could be installed. Dutch retailers are instances from this domain.

6.2.2.5 Conceptual model

The theory states that higher time access window pressure results in higher distribution costs as specified in the proposition, and as visual- ized in the following conceptual model.