Mobility and access are important for the economic well being of the city and the quality of life of the population as a whole. Due to easy accessibility, the city centre has been chosen for the location of specialised operations of both retail distributors and office activities. In most cities, the demand for central spaces has maintained high rent and land values, leading to a sharp fall in employment densities beyond the central business district (CBD), and then to gradual changes according to the inverse relation of the distance from the city centre. This is the typical case of London, whose annular job density, defined as the number of jobs in each two kilometres annular band from the centre, falls from around 600 in the central area to 100 within the first four kilometres (Dasgupta et al. (1990b)). The authors also found some other cities in Britain which have peripheral job locations, but still present low densities when compared with the CBD.

 

Improvements in accessibility to urban centres have played an important role in the increase of commuting to Central London. The process led to the incorporation of urban areas to the extended metropolis, and the link of former remote areas to the centre of economic activities. As a consequence, the overflow population of the metropolis could keep their activities in the central area. Accessibility has been traditionally associated with the spatial economy of the cities as concerning urban rents, densities, and land use. However, more recently emphasis has been put on the effects of changes in accessibility on travel demand, due to urban transport investments. This is the concept that is used in this thesis. Martinez (1991) explained accessibility in terms of spatial distribution of activities and the transport costs involved. Since individuals perceive trips and activities differently according to their taste and/or ‘income, one might expect different accessibility valuations amonst them. This is clearly explained by an example given by the author in which the accessibility level in the city CBD is perceived differently according to the spatial distribution of the activities even if the transport costs are kept the same : … accessibility for the activity “doing business during working hours” is normally the highest in the city while for the activity “attending school” it is usually very poor.

 

The author concluded that accessibility should be represented by a collection of measures, and not by just an overall measure for a particular location. His proposed accessibility measure deals with the interaction between transport and urban rent, and consequently is of little application in this present work. The study by Dalvi and Martin (1976) accounted not only for travel impedance but also considered location attraction as perceived by residents of the area analysed. One of their conclusions was that the accessibility pattern of the private transport in the Inner London area was very sensitive to the choice of the attractor variable considered (total employment, retail employment, households or population). If total employment was the attractor variable, then the central areas were more accessible than any other in Inner London.

 

On the contrary, if household on employment were used, then the accessibility differentials would not be so pronounced. Other measures of accessibility were proposed by Williams (1977), under the concept of composite costs, and by Ben-Akiva and Lerman (1979), as the expected maximum utility. Both concepts are compatible with the underpinning of the methodology considered in this thesis, which is based on random utility theory. Since this theory has been successfully applied in the spatial distribution analysis and modelling, a measure of accessibility given by a generalised cost function ‘is proposed here.

 

In the current literature, decisions concerning job and house location have been frequently described as interdependent rather than simultaneous. Decisions on home and job location are interrelated and, among other factors, dependent on the spatial separation between home and work place. The decision to change residence may be associated with an increase for example in the number of children, or the wish to have a more comfortable house in a better environment. It might be the case that the residential change is not a consequence of any decision concerning job change. This happens when a household wants to keep the same job but decides to move to a bigger house due to life cycle effects, for instance. If the household works and lives in London, the relocation might possibly take place outside the conurbation due to constraints of the housing market in London. Then, as a consequence of housing location, the household may find it easier to change the work place, eg. to work closer to the new residence.

 

Such an attitude would result in a structure of interdependence between job and house location, in which the work place would change consequent upon a change of house. Similar analysis may be drawn for a change in work place. The decision to change job may happen completely independent of housing location, eg. a resident in Kent who works in London may decide to choose another job, either in London or elsewhere, without taking any decision about house location. However, it might also be the case that consequent upon a change in job, the home location also changes.

 

 

This is a typical example applied to those who retire from Central London jobs and decide to change home. Most of them tend to relocate elsewhere in the South-East, where better housing condition might be easily achieved. The above explanation clearly shows the complex links between the structure of location and the patterns of the various activities carried out by the household. The dynamic of this process is also illustrated by those who retire and have their vacancies filled by new entrants to the labour market. In this case, a change in commuting patterns may arise without anyone changing home location. The simultaneous relocation of both job and home places is possible, but it is rather unusual. Changes in the location of both households residence and its worker’s workplaces usually happen close in time to one another. Imperfections in both the housing and labour markets, and time lags in decision-making are pointed out by Weinberg (1979) as factors which lead one to believe that simultaneous moves are not likely to occur in metropolitan areas.

 

An additional complexity in location matters concerns transitional relationships between home and job places. This case applies when a person has to start a job in a different place but can not buy a house before starting that job. This implies a conditional relationship that the home location might be expected to change as a consequence of a change in job. In the interim, for example, a lengthy, disequilibrium commute may be required. A definitional question arises with the relationship of a temporary ‘home base’ which is arranged during the working week (eg. a guest house or pied a terre). The work journey would be the movement to/from the main residence, if the beginning and end of the working week is considered. During the working week, journeys to/from the temporary home would then be undertaken. Given the different cycle of regularity between journeys involving permanent (weekly or monthly) and temporary (daily) homes, they are usually modelled separately (see Kirby (1979)). Another complication may arise when more than one of the above home-work relationships apply to the members of a household. This is exactly the case of a family member who takes a new job in a far away location, changing consequently the residence. If the other member(s) also had a job in the old

place, then change(s) in employment location could happen as a consequence of the change in house. Studies on migration and mobility analyses generally consider the relocation process divided into household’s aspirations, decision-making and the actual search stages. The first two stages involve the decision to move from the previous residence or job, whereas the search stage comprises the screening of a particular search area and the selection of availabilities in the market, usually announced by the media and estate agents. The understanding of the patterns of mobility requires the identification of factors which affect the complex interaction of job and residential movements, and definition of the relative importance of these variables as regards location decisions.

 

A general view of the influence of some variables on mobility behaviour was previously presented in sections 3.2 and 3.3 of this chapter. Although most studies do not make a clear distinction between factors influencing the search area from those influencing home/job location, this section attempts to give emphasis to the search process. Raji (1987) concluded that accessibility to work and t ranspo rt- related variables are the most important factors in search-area decisions. His finding is supported by several studies on the role of transport and accessibility in residential location decisions. For the earlier stages of the relocation decision, ie. household’s aspirations and decision-making, factors usually related to the decision to move are’. (i) dissatisfaction with the standard of living (housing conditions, neighbourhood quality, costs of living); and (ii) life cycle (retirement, family size, composition and age). Alternatively, the most frequent reasons associated to the decision to change job are redundancy and the decision to quit: the latter due to dissatisfaction ¬†with the job duties and activities, office environment, wage, and desire of a better position.

 

Miron (1978) quotes and questions Sjaastad’s (1962) argument that migration tends to occur whenever there are differences in the standard of living between regions that are sufficient to exceed the costs of migration. According to Miron (1978), such a consideration would require perfect information by the potential migrant about the disequilibrium among regions. Therefore, this is a condition hardly to be found in practice.

 

Lyon and Wood (1977) observed that real-life decisions are subject to psychological constraints not present in laboratories or hypothetical situations. The authors found that house owners are not very systematic in their searching and evaluating process of choosing a house. Their finding was corroborated by Barrett (1976), who concluded that search behaviour in residential relocation is not a thorough process. Barrett (1976) attempted to quantify search areas by studying the distances between the vacancies actually considered by a group of movers. The author developed three indices to measure search behaviour, which showed that spatially, the intensity of search was a random distribution; and that the areal extent of search was limited.

 

The evidence found in his study indicated that the behaviour of most people is to buy a house after a very short search which covers only a few houses in a small area. Actually, only about two per cent of the trial group of movers considered had search clusters of more than five miles. Kirby and Raji 0 992) argue with these results on the basis that “the vacancies were of those actually inspected, which might therefore define a smaller (and possibly biased) area compared with that defined by the vacancies that one could inspect (ie. those in the ‘search space’of Moore and Brown)”. Moreover, the authors quote Davey’s (1978) research in which 35 percent of the respondents considered areas beyond five miles from their chosen house. According to Kirby and Raji (1992), all the areas considered by the respondents were taken into account in the latter study.

 

The limited extent of the area of search is also supported by Brown and Moore (1970), who supposed that the migrant’s knowledge of the possibilities was the principal constraint on the search pattern. Their study indicated that the density of information per unit area of the housing market tends to decrease with the household’s location as the centre of the density surface. Moreover, the space of direct contact, provided by the day-to day activities undertaken by the household, was oriented along the major transport arteries to the city centre. For transport planning, and in particular for the formulation of a destination choice model, Kirby and Raji (1992) suggest a choice process, which basically involves the range of house prices that an individual is willing to consider, and the geographic extent of the area of search. The authors agreed that the definition of a search space for a home or for a job is the main factor affecting the pattern of journeys between home and work; moreover, that the individual’s choice of home (of a given type) within this area is random distributed. Kirby (1979) considered different shapes of the search area assuming single and multi foci points. Given that most migrants will be unfamiliar with the area, for inter urban or labour migrants he suggested a monocentric area whose reference is the site of work. The case of more than one focus (eg. actual home and job) was considered for intra-urban migrants, who would probably have a more detailed knowledge of the search area.