  The quantitative study of traffic dynamics is crucial to ensure the
efficiency of urban transportation networks. The current work investigates the
spatial properties of congestion, that is, we aim to characterize the city
areas where traffic bottlenecks occur. The analysis of a large amount of real
road networks in previous works showed that congestion points experience
spatial abrupt transitions, namely they shift away from the city center as
larger urban areas are incorporated. The fundamental ingredient behind this
effect is the entanglement of central and arterial roads, embedded in separated
geographical regions. In this paper we extend the analysis of the conditions
yielding abrupt transitions of congestion location. First, we look into the
more realistic situation in which arterial and central roads, rather than lying
on sharply separated regions, present spatial overlap. It results that this
affects the position of bottlenecks and introduces new possible congestion
areas. Secondly, we pay particular attention to the role played by the edge
distribution, proving that it allows to smooth the transitions profile, and so
to control the congestion displacement. Finally, we show that the
aforementioned phenomenology may be recovered also as a consequence of a
discontinuity in the nodes density, in a domain with uniform connectivity. Our
results provide useful insights for the design and optimization of urban road
networks, and the management of the daily traffic.
