Experimental Features and Characteristics of Speed Dispersion in Urban Freeway Traffic

Abstract

Characteristics of speed dispersion in urban freeway traffic are presented. Two definitions of speed dispersion are proposed: the standard deviation of the individual speeds and the average speed difference of two neighboring vehicles. On the basis of the definitions, traffic data obtained from two urban freeways in China are studied, and different characteristics of speed dispersion are found in four substates of traffic flow, which correspond to four regions in the empirical fundamental diagram. In the bunching state of congested traffic, the flow rate decreases with an increase in speed dispersion at a given mean speed. In the bunching state of fluid traffic, the speed dispersion of traffic flow is small, and in the free state of fluid traffic, speed dispersion is distributed in a disorderly manner in a wide range. Such phenomena are more remarkable under the definition of the average speed difference for speed dispersion. In addition, some possible explanations are presented for the characteristics of speed dispersion in each traffic substate. These speed dispersion studies provide a new approach for microscopic modeling and understanding of traffic flow characteristics.

Characteristics of speed dispersion and its relationship to fundamental traffic flow parameters

“…First, CVS ranged from 10% to 35% northbound and 10% to 55% southbound. The CVS range of the northbound is close to the observation by Wang et al (2007) from 7% to 32%, but that of the southbound is broader than the proposal by May (1990) from 0 to 1 ffiffi S p , which implies that time mean speed is within 1 mph greater than space mean speed. In fact, Rakha and Zhang (2005) indicated that large differences between these two mean speeds, from 10% to 30%, are not uncommon when traffic is congested.…”

“…Another 'complicated' form between speed and SDS may be expected, as shown in Equation (7) derived from Equation (5). Finally, although Wang et al (2007) proposed flow as an exponential form of SDS, we do not have similar findings but expect, through the relationship between flow and CVS, that flow and speed can jointly explain SDS to a certain degree.…”

“…The exponential forms can better explain the relationship, albeit the unlisted quadratic is slightly superior to the exponential for Lane 1 (the HOV lane). As a contrast, Wang et al (2007) suggested density be an exponential form of CVS based upon around 40 observations and with R 2 of 0.34. It is the reverse of what was found here -CVS is an exponential form of occupancy (as well as density attainable from occupancy via a multiplier).…”

“…Del Castillo and Benítez (1995) set CVS 15% or less, as a rule of thumb, to filter off the nonstationary regime, but did not mention the relationship between CVS and the fundamental parameters. Based upon observations of urban Chinese highways, Wang et al (2007) proposed flow as an exponential regression equation of SDS with the coefficient of correlation (R 2 ) between 0.26 and 0.74. They also identified density as an exponential equation of CVS that distributes from 7% to 32% as a result of R 2 of 0.34.…”

A comparison of outlier detection algorithms for ITS data

“…The traffic flow on freeways is described traditionally in term of three parameters: the mean speed, the traffic flow rate, and the traffic density, and the relationship between flow rate and traffic density can form a curve called the fundamental diagram for traffic flow. In order to study the relationships between the three traffic parameters and speed dispersion, traffic data were obtained by video-tapping from Lukou airport freeway, Nanjing, Jiangsu province, Wang et al (2007). The site data collected from is a 5 km segment between two ramps in Lukou airport freeway, instrumented with video cameras every two kilometers each side, and the traffic data collected from station 3 during the period from 14:34 to17:37 on July 3, from 07:40 to 10:45 on July 6, and from 16:32 to 17:50 on July 11, 2007 were utilized in our study.…”