What characterizes a free vehicle in an urban area?

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The concept "free vehicle" is used widely in the transportation research literature, but as yet there is no common standard for a definition. In this article a standard is provided that is based on the assumption that a vehicle can be considered free, when its speed is not influenced by the speed of the vehicle traveling ahead. In order to determine empirically a threshold for free vehicles in urban areas, speed, distance headway, and time headway data of more than 100 000 vehicles were analyzed. Analyses showed that the speeds of two vehicles are linearly dependent on time headway for headways up to 6 s. It is important to distinguish between this value and the drivers' preferred headway, which is in the region of 2 s

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