Module 1 : Traffic Stream Characteristics

Lecture 01 : Fundamental Parameters of Traffic Flow

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	Distance headway Another related parameter is the distance headway. It is defined as the distance between corresponding points of two successive vehicles at any given time. It involves the measurement from a photograph, the distance from rear bumper of lead vehicle to rear bumper of following vehicle at a point of time. If all the space headways in distance $x$ over which the density has been measured are added, $$\sum_{1}^{n_x}s_i = x$$ (1) But the density ($k$) is the number of vehicles $n_x$ at a distance of $x$, that is $$k = \frac{n_x}{x} = \frac{n_x}{\sum_{1}^{n_x} s_i} = \frac{1}{s_{av}}$$ (2) Where, $s_{av}$ is average distance headway. The average distance headway is the inverse of density and is sometimes called as spacing. Travel time Travel time is defined as the time taken to complete a journey. As the speed increases, travel time required to reach the destination also decreases and vice-versa. Thus travel time is inversely proportional to the speed. However, in practice, the speed of a vehicle fluctuates over time and the travel time represents an average measure.