Expanding bottlenecks reveals hidden bottlenecks and leads to more congested city centers

Urban traffic congestion is a perennial and critical issue. During the morning and evening rush hours, a large number of people flood into the urban road network within a short period of time, resulting in severe traffic congestion and significant inconvenience to daily life. Bottlenecks are weak links of the road network and congestion frequently occurs at these points. Many researchers postulate that unblocking bottlenecks can enhance the traffic environment. However, this perspective overlooks the propagation characteristics of congestion. To mitigate traffic congestion, our study focuses on the traffic flow propagation process post-expansion of bottlenecks in urban road networks. Our findings indicate that merely unblocking bottlenecks may lead to an increased influx of suburban residents into city centers for work within a short period of time. It leads to a surge in vehicle density in the central area, which can cause severe traffic congestion problems and make significant increase in the length of commuting time. Additionally, we identify numerous hidden bottlenecks within the road network that become active and transform into new bottlenecks once existing bottlenecks are unblocked. We simulate the commuting behavior of residents during morning rush hours on a simplified road network to quantify the adverse effects of bottleneck expansion. A large-scale simulation of the Beijing road network supports our finding that simply unblocking bottlenecks just only greatly increases residents’ commuting time. This research can provide an in-depth understanding of traffic congestion problems’ mechanisms. It is very beneficial for optimizing the construction of cities.