Ttb-speed: Key Link of the Urban Space-time Dynamic

Ttb-speed: Key Link of the Urban Space-time Dynamic


I Joly, Laboratoire d'Economie des Transports, FR



The car-ownership generalisation has increased the space-time prism of accessibilities of
a large part of the urban population. As a consequence, individuals have adapted their
use of the city opportunities to the new accessibilities. The observation of the last
decades reveals to us an increase of the daily mobility, particularly in the daily travelled
distance, which results from the new activity patterns. Hence, transport behaviour has
been influenced by this new organisation of the urban system.
In the long term, urban structure is modified and re-organised around the new
interaction places and transport possibilities, which have risen with the new transport
behaviours. The new localisation choices are altered by the new accessibilities

The paper aims at enlightened this co-production relationship between land-use and
transport. This dynamic have to be studied with respect of the three interacting
dimensions: transport behaviour, urban structure and transport system. In order to
investigate the co-production link with respect of these dimensions, we propose then to
analyse the Travel Time Budget-speed relationship as a key of the three interacting

In the first part, we will discuss the place of TTB in the mobility analyses. TTB has
often been used as key variable to understand travel behaviour, it introduced a second
constraint on the individual transport choices. Despite, a trade-off between the monetary
and temporal resources could be expected, the TTB seems to be stable since the 50?s.
This stability constitutes the Zahavi?s hypothesis, which stems that TTB are stable in
space and time. Then, after a survey of researches accepting the TTB stability, the
second step will be to discuss the relationship between TTB, speed and the way it
models the urban structure. Linked to the urban speed, the TTB obtains a spatial
dimension to be used in the analysis of interrelation between transport and urban
structure. The speed is then a way to structure mobility choices, land-use and transport
supply, around the TTB. At a macroscopic level, the urban area is historically sized to
the available speeds. And the distribution of villages and cities in space depends on travel
times. At a microscopic level, the TTB constraint leads to densify the uses of space and
to organise streets and paths in order to optimise accessibilities. The recent increase in
available speeds has hardly disrupt the ?traditional? localisation choices. The TTB-speed
link appears then as key variable in the co-production relation between transport choices
and urban structure.
The TTB-speed link will permit us to explore the urban structure formation and then to
question the urban and transport policies, which are particularly challenged when
considering the Zahavi?s hypothesis. Under TTB stability hypothesis, travel time-savings
are totally reinvested in transport. This reinvestment mechanism could then explain the
city sprawl, and give to the increased speeds all the responsibilities of the ?urban
transport diseases?.

In the third part, attention will be paid on the TTB levels. Referencing aggregate mobility
data of 100 cities for 1995, the UITP database gives us opportunities to test the TTB
stability hypothesis. The analysis supports the Zahavi?s hypothesis defined at a world
aggregated level, but it shows distinct tendencies for TTB between North-America and
Occidental Europe. Continental stability will be questioned and distinct regressions of
TTB will be proposed for both continents. Furthermore, two opposites urban models
appear: an extensive urban model with development based on extensive consumptions of
space and time resources, and an intensive urban model restricting its spatial and
temporal extension. This opposition is also based on speeds contrast and questions then
the complementarity between spatial extension and temporal extension of the extensive
cities. Will the European dynamic converges on the extensive model?

Finally, transport policies that aim at reducing the daily distances are discussed and
politic answers to the potential travel time expansion will be proposed. On the one hand,
the speed appears responsible of the distance explosion and their reduction will appear as
the way to contains the urban spatial extension. On the other hand, speed fails to reduce
TTB and, it seems on the contrary that speed increase it. The new accessible
opportunities seem to balance more and more of the disutilities of the travelled time.
Then a speed reduction policy appears unrealistic and inefficient if TTB are only flexible
upwards. The reinvestment of travel time-savings shows a part of mechanisms acting in
the mobility choices. But the answers are to be found not only in the TTB-speed link
but in the numerous dimensions interacting in the city dynamics.


Association for European Transport