Traffic bottleneck
Encyclopedia
A traffic bottleneck is a localized disruption of vehicular traffic on a street, road or highway. As opposed to a traffic jam, a bottleneck is a result of a specific, often temporary, physical condition.
Bottlenecks are also caused by terrain, such as uphill sections or very sharp curves. A poorly timed traffic light is another example. Slow vehicles also disrupt traffic flow upstream. This is known as a "moving bottleneck."
Furthermore, bottlenecks can be psychological in nature. Vehicles safely pulled to the shoulder by a police car, for example, often induce passing drivers to slow down to "get a better look" at the situation. See rubbernecking
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theory can be used to model and represent bottlenecks.
Before capacity is reached, traffic may flow at A vehicles per hour, or a higher B vehicles per hour. In either case, the speed of vehicles is vf, or "free flow," because the roadway is under capacity.
Now, suppose that at a certain location x0, the highway narrows to one lane. The maximum capacity is now limited to D', or half of Q, since only lane of the two is available. D shares the same flowrate as state D', but its vehicular density is higher.
Using a time-space diagram, we may model the bottleneck event. Suppose that at time 0, traffic begins to flow at rate B and speed vf. After time t1, vehicles arrive at the lighter flowrate A.
Before the first vehicles reach location x0, the traffic flow is unimpeded. However, downstream of x0, the roadway narrows, reducing the capacity by half - and to below that of state B. Due to this, vehicles will begin queuing upstream of x0. This is represented by high-density state D. The vehicle speed in this state is the slower vd, as taken from the fundamental diagram. Downstream of the bottleneck, vehicles transition to state D', where they again travel at free-flow speed vf.
Once vehicles arrive at rate A starting at t1, the queue will begin to clear and eventually dissipate. State A has a flowrate below the one-lane capacity of states D and D'.
On the time-space diagram, a sample vehicle trajectory is represented with a dotted arrow line. The diagram can readily represent vehicular delay and queue length. It's a simple matter of taking horizontal and vertical measurements within the region of state D.
State A represents normal approaching traffic flow, again at speed vf. State U, with flowrate qu, corresponds to the queuing upstream of the truck. On the fundamental diagram, vehicle speed vu is slower than vf. But once drivers have navigated around the truck, they can again speed up and transition to downstream state D. While this state travels at free flow, the vehicle density is less because fewer vehicles get around the bottleneck.
Suppose that, at time t, the truck slows from free-flow to v. A queue builds behind the truck, represented by state U. Within the region of state U, vehicles drive slower as indicated by the sample trajectory. Because state U limits to a smaller flow than state A, the queue will back up behind the truck and eventually crowd out the entire highway (slope s is negative). If state U had the higher flow, there would still be a growing queue. However, it would not back up because the slope s would be positive.
For more information, see .
Causes
Traffic bottlenecks can be caused by a wide variety of things. A construction zone, where one or more existing lanes may become unavailable, is pictured on the right. An accident site can also temporarily close lanes. Even in the absence of construction, a highway permanently narrowing by a lane or more can be a bottleneck because the capacity falls.Bottlenecks are also caused by terrain, such as uphill sections or very sharp curves. A poorly timed traffic light is another example. Slow vehicles also disrupt traffic flow upstream. This is known as a "moving bottleneck."
Furthermore, bottlenecks can be psychological in nature. Vehicles safely pulled to the shoulder by a police car, for example, often induce passing drivers to slow down to "get a better look" at the situation. See rubbernecking
Rubbernecking
Rubbernecking describes the act of gawking at something of interest. It is often used to refer to drivers trying to view the carnage resulting from a traffic accident. The term refers to the craning of a person's neck in order to get a better view....
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Graphical & theoretical representation
Traffic flowTraffic flow
Traffic flow, in mathematics and civil engineering, is the study of interactions between vehicles, drivers, and infrastructure , with the aim of understanding and developing an optimal road network with efficient movement of traffic and minimal traffic congestion problems.-History:Attempts to...
theory can be used to model and represent bottlenecks.
Stationary bottleneck
Consider a stretch of highway with two lanes in one direction. Suppose that the fundamental diagram is modeled as shown here. The highway has a peak capacity of Q vehicles per hour, corresponding to a density of kc vehicles per mile. The highway normally becomes jammed at kj vehicles per mile.Before capacity is reached, traffic may flow at A vehicles per hour, or a higher B vehicles per hour. In either case, the speed of vehicles is vf, or "free flow," because the roadway is under capacity.
Now, suppose that at a certain location x0, the highway narrows to one lane. The maximum capacity is now limited to D', or half of Q, since only lane of the two is available. D shares the same flowrate as state D', but its vehicular density is higher.
Using a time-space diagram, we may model the bottleneck event. Suppose that at time 0, traffic begins to flow at rate B and speed vf. After time t1, vehicles arrive at the lighter flowrate A.
Before the first vehicles reach location x0, the traffic flow is unimpeded. However, downstream of x0, the roadway narrows, reducing the capacity by half - and to below that of state B. Due to this, vehicles will begin queuing upstream of x0. This is represented by high-density state D. The vehicle speed in this state is the slower vd, as taken from the fundamental diagram. Downstream of the bottleneck, vehicles transition to state D', where they again travel at free-flow speed vf.
Once vehicles arrive at rate A starting at t1, the queue will begin to clear and eventually dissipate. State A has a flowrate below the one-lane capacity of states D and D'.
On the time-space diagram, a sample vehicle trajectory is represented with a dotted arrow line. The diagram can readily represent vehicular delay and queue length. It's a simple matter of taking horizontal and vertical measurements within the region of state D.
Moving bottleneck
For this example, consider three lanes of traffic in one direction. Assume that a truck starts traveling at speed v, slower than the free flow speed vf. As shown on the fundamental diagram below, qu represents the reduced capacity (2/3 of Q, or 2 of 3 lanes available) around the truck.State A represents normal approaching traffic flow, again at speed vf. State U, with flowrate qu, corresponds to the queuing upstream of the truck. On the fundamental diagram, vehicle speed vu is slower than vf. But once drivers have navigated around the truck, they can again speed up and transition to downstream state D. While this state travels at free flow, the vehicle density is less because fewer vehicles get around the bottleneck.
Suppose that, at time t, the truck slows from free-flow to v. A queue builds behind the truck, represented by state U. Within the region of state U, vehicles drive slower as indicated by the sample trajectory. Because state U limits to a smaller flow than state A, the queue will back up behind the truck and eventually crowd out the entire highway (slope s is negative). If state U had the higher flow, there would still be a growing queue. However, it would not back up because the slope s would be positive.
For more information, see .
See also
- Three-phase traffic theory
- Traffic congestion: Reconstruction with Kerner’s three-phase theoryTraffic congestion: Reconstruction with Kerner’s three-phase theoryVehicular traffic can be either free or congested. Traffic occurs in time and space, i.e., it is a spatiotemporal process. However, usually traffic can be measured only at some road locations...