What Is a Flow Curve?

The speed-flow cure refers to a curve showing the correlation between the speed V and the flow Q of a traffic flow. When the speed increases from zero, the flow also increases accordingly; when the flow increases to the maximum flow, and the speed continues to increase, the flow starts to decrease due to the widening of the headway until the final flow drops to zero.

The speed-flow cure refers to the relationship between the speed V and the flow Q of the traffic flow.

Among the three basic parameters of traffic flow, flow and speed are easy to obtain, so research on the relationship between flow and speed is extensive.

The 2000 edition of the US "Road Capacity Manual" summarized the speed and flow graphs from many international research results indicating that the speed will initially remain the same as the flow increases, until the flow approaches 1/2 or 2/3 of the capacity , The speed began to decline to a small extent. In addition, many traffic engineering experts have established different regression models based on data from field surveys, but the forms and coefficients of regression models vary widely.

It can be seen from these models that different survey objects, road conditions, and observation locations will lead to different conclusions. Moreover, most of the data of these traffic flow models come from highways, and there are few studies on urban roads. Due to the influence of intersection signals on urban roads, vehicle arrivals on road sections are not continuous.

In addition, urban roads are also affected by oncoming cars, on-street parking, vehicle lane changes, and pedestrian crossings, which are significantly different from the characteristics of traffic flow on highways. Therefore, it is necessary to study the speed-flow characteristics of urban roads ^[2]

Based on the measured data of traffic flow on typical sections of urban arterial roads, the change law of speed with flow rate is described, and the change law of traffic flow state is studied, which provides a theoretical basis for traffic state discrimination in urban traffic management and control.

When designing the green wave for coordinated signal control at the main intersection, fleet speed is a key factor to be considered. For the convoys departing from the upstream intersection, the speed of the vehicles in them will be different, and there will be a discrete phenomenon before reaching the parking line at the downstream intersection, which will make the green wave effect worse.

The green wave design usually chooses the average speed value with the highest frequency as the design basis. From the normal distribution probability density curve, we can know that the smaller the probability density, the steeper the curve, the greater the probability that the vehicle speed falls near the average speed, and the better the green wave effect; the larger the probability density, the flatter the curve, and the probability that the vehicle speed falls near the average speed. The smaller the green wave effect, the worse.

When designing a coordinated control scheme for arterial intersections, the speed-flow relationship based on the scatter diagram drawn on the measured speed and flow data of typical sections of urban arterials can be used to analyze the fluctuation of speed with flow and find that the speed fluctuation range has become significantly smaller Corresponding flow area and speed average, so as to determine the flow range suitable for the use of trunk coordinated control and the fleet speed of the green wave design ^[3] .