Gordon et al. (1989) noted that the
relation between residential density and mean travel times is not
straightforward, because it also depends on whether the city is monocentric or polycentric.
They argue that in a polycentric city model, people have more opportunities to live
closer to their work place, a phenomenon known as ‘co-location’ and that
reduces commuting distances (Gordon and Wong 1985; Handy 1996). The co-location hypothesis builds on the assumption that employees
and jobs follow each other and that labour and housing markets are perfect
(Zhao et al. 2011). Under these conditions residents base their location choice on
a trade-off between housing prices and commuting costs. The assumption is that,
the closer to the job, which according to the monocentric city model is in the
CBD, the higher the house price.

Another factor to take
into account is the effect of polycentricity on congestion. A

polycentric city
structure is expected to lead to less congestion than a monocentric

structure, which will
reduce commuting times (Levinson and Kumar 1994). According to

Gordon et al. (1989), in monocentric
cities, an increase in residential density is generally

expected to decrease
travel times. However, when density becomes too high, the congestion

effect may take over
which will increase travel times.

Based on a review of
existing empirical studies on the relation between polycentricity

and commuting, Tsai (2001) concludes that there
is no uniform relation. Some studies

found that polycentric
structures lead to lower average distances than monocentric structures

(e.g. Gordon et al. 1989; Song 1992; Guiliano and Small 1993; Spence and Frost

1995; Alpkokin et al. 2008) whereas other studies
found opposite results (Baccaini 1997;

Cervero and Wu 1997; Ewing 1997; Schwanen et al. 2003; Aguilera 2005). In their study on 50
metropolitan areas in the U.S., Yang et al. (2012) found that the relation depends on

the threshold of
population density that is used to define polycentricity: high density

increases commuting times whereas moderate high-density polycentricity

commuting times.

average level of polycentricity does not have a significant influence on

and time. This underlines the ambiguous relationship between polycentricity and

Studies on European and US cities have shown both positive and negative

between the two. The fact that we are not able to establish a significant

suggests that both positive and negative relations exist in our sample. The
impact of

may thus depend on additional city-level characteristics (such as jobhousing

that we could not incorporate in our analysis.

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