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Home » Traffic Congestion » Causes of congestion – road crashes and shockwaves

Causes of congestion – road crashes and shockwaves

October 2015


I noticed that a previous post on common causes of congestion is popular among those who read this blog. The causes mentioned there focused on the lack of discipline among pedestrians and motorists. Their behaviour (e.g., commuters standing in the middle of the road to get a jeepney or UV Express ride, jeepneys and UV Express stopping in the middle o the road to load/unload passengers, etc.) often lead to congestion as they effectively reduce road capacities.

Another major cause of congestion are road crashes. They don’t have to be fatal crashes as the road capacity reduction caused by stopped vehicles is enough to cause congestion along the road including traffic along the opposite direction. The latter phenomenon, if we can categorise it as such, is due to the inquisitive nature (i.e., uses) of people. Traffic approaching the crash site typically slows down as drivers check what happened. Following are some photos I took as we approached a crash site atop the Marcelo Fernan Bridge connecting the Islands of Cebu and Mactan. Note the build-up of vehicles behind the crash involving 3 vehicles.

2015-09-13 17.00.29Road crash involving 3 vehicles in the middle of the Marcelo Fernan Bridge in Cebu

2015-09-13 17.00.41The resulting congestion behind the crash site

2015-09-13 17.00.51Another view of the vehicle buildup upstream of the crash

Such events are typical causes of congestion and can actually be analysed using traffic flow theory involving waves in traffic. A generally normal flow of traffic (before the incident) is disturbed by the crash, which is assumed to be an isolated event. This results in a change in the traffic characteristics (flow, speed and concentration of vehicles) triggering a shockwave. The shockwave moves backwards and manifests through the chain reaction of drivers hitting their brakes in succession to slow down due to the incident. As such, it is possible to determine how long the build-up of vehicles would be upstream of the incident location.

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