Abstract
Speed dispersion is an important indicator to portray the quality of traffic flow and is closely related to the road safety operation level. In order to clarify the influence of posted speed limits on the dispersion of traffic flow speed, three sections with speed limits of 80 km/h, 100 km/h and 120 km/h on the same expressway were selected for observation, and traffic volume, speed and other parameters were collected. The characteristic speeds, such as average speed, V15 and V85, were evaluation indicators, where V15 and V85 are the speeds of the 15th and 85th percentiles measured at the feature points of the road when the traffic is in a free-flow state and the weather is good. The relationship between different posted speed limit values and the above indicators was analyzed using the statistical analysis software, SPSS. The results show that the speed limit has a high correlation with the average speed of traffic flow, V15 and V85 in free-flow state, with the coefficient of determination being as high as 0.84, 0.85 and 0.92, respectively. In the restricted flow state, the factors affecting the driver’s driving speed are mainly the decrease in driving freedom caused by the increase of traffic volume rather than the speed limit value. In a free-flow state, when the posted speed limit is increased and the average speed and the V85 also increased by approximately the same magnitude. The posted speed limit values of 80 km/h, 100 km/h and 120 km/h correspond to the 90, 88 and 97 percentile speeds of the traffic flow, respectively. The higher the speed limit is, the larger the speed difference between V15 and V85 becomes. The results of the study are very useful for rationally determining the speed limit scheme under different traffic flows.
Highlights
The discreteness of traffic flow speed can reflect the quality of traffic flow and is an important concept in the field of traffic engineering
From the data collected by Parker [2] for an FHWA (Federal Highway Administration) study, it can be determined that the average speed, the standard deviation of the speed and the 85th percentile speed differ by less than 2 mph (3.2 km/h) before and after the speed limit change
As the posted speed limit increases, the average speed and V85 will increase by about the same magnitude, the increase of V15 speed will be lower than this magnitude
Summary
The discreteness of traffic flow speed can reflect the quality of traffic flow and is an important concept in the field of traffic engineering. Relevant researches in developed countries mainly focus on the relationship between design speed, operating speed and posted speed limit and have revealed some fully demonstrated results and conclusions that are generally accepted. Dixon et al [1] selected multiple fixed observation sites in multi-lane highways, increased the posted speed limit of these locations from 55 mph (88.6 km/h) to 65 mph (104.7 km/h) and collected the velocity data before and after the posted speed limit change. The actual speed limit should be 8~12 mile/h lower than the 85th speed [4] and based on the statistical data, a linear regression equation of the operating speed and the posted speed limit is established [5]. Burritt [6], Dart et al [7] found that as the posted speed limit decreases, the average running speed decreases; Ossiander et al [8], Jernigan et al [9], Freedman and Esterlitz [10], Brow et al [11] and Upchurch [12] found that with a national maximum speed limit from 55 mph to 65 mph, the average speed increased by 2–7 mph
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