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Traffic Flow

Conventionally, traffic flow is considered to be the movement of motor vehicles along roads. The theory and principles developed for this can, however, be used for the movement of any discrete autonomous units along a road that are not subject to external time-tabling or control (for example, walking between and within terminals at an airport, cyclists on bikeways, etc.). For the purpose of this article, only motor vehicles on roads are considered. The impact of different kinds of vehicles on different roads is also discussed.

Analogies

In certain circumstances, traffic flow has similar characteristics to fluid dynamics. This is especially true for freeways, where the majority of vehicles are passenger cars with similar dynamic characteristics. Freeways are also designed to provide the smooth access and exit of motor vehicles from traffic streams. On freeways, there can be streamlined flows when traffic volumes are under road capacity and traffic is moving at nearly constant speed. As in fluid dynamics of liquids in a pipe, the traffic nearest the side flows more slowly than the traffic in the center of the road.

Turbulent flows occur when traffic volumes approach road capacity. This leads to shock waves caused by minor disturbances, such as a vehicle stopping on the road edge. At traffic volumes close to road capacity, such disturbances cause traffic to come to a halt randomly; it then starts again flowing freely, until it comes to another halt.

For all other roads, with mixed traffic mixed use (for example, roadside parking, bus stops, and others), traffic flow is more like Brownian motion—random and difficult to model, except at a macro scale. Nevertheless, traffic flows on such roads are important because they lead to and away from major roads like freeways. So, there is little point in having a freeway if its feeder and distributor roads are at a standstill. This will cause traffic to back up the freeway and bring it to a halt, with a zero flow.

Measurement and Impacts of Vehicle Types

Historically, traffic flows were measured manually and recorded on paper charts. Two main methods were developed: static roadside and “moving car,” the latter also determining the average traffic flow speed. Today, there are many remote or telemetric methods to capture real-time traffic flows. Some of these methods include magnetic loops in the road surface, which can also be used to determine vehicle speeds; closed-circuit television (CCTV) with vehicle-recognition software; and radar, among others. When coupled with software, not only can traffic volumes be recorded, but also traffic problems predicted, which can be used to trigger automatic control systems or alert traffic control staff.

The biggest impact on traffic flow is the mixture of large- and private-car-sized vehicles. Determining this can help planners and engineers get a better understanding of the mechanics of traffic flow. This has led to adoption of a standard unit to represent traffic flows, the passenger car unit (pcu). This models the impact on traffic of a single passenger motor car. All other road vehicles can be represented in pcus. So, a rigid bus is 2.5 pcu, an articulated truck 4 pcu, a motor cycle 0.5 pcu, a pedal cycle 0.25 pcu. By this means, a heterogeneous traffic stream can be represented and measured in standardized (pcu) units.

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