Module 1 : Traffic Stream Characteristics

Lecture 01 : Fundamental Parameters of Traffic Flow

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	Derived characteristics From the fundamental traffic flow characteristics like flow, density, and speed, a few other parameters of traffic flow can be derived. Significant among them are the time headway, distance headway and travel time. They are discussed one by one below. Time headway The microscopic character related to volume is the time headway or simply headway. Time headway is defined as the time difference between any two successive vehicles when they cross a given point. Practically, it involves the measurement of time between the passage of one rear bumper and the next past a given point. If all headways $h$ in time period, $t$, over which flow has been measured are added then, $$\sum_{1}^{n_t}h_i = t$$ (1) But the flow is defined as the number of vehicles $n_t$ measured in time interval $t$, that is, $$q = \frac{n_t}{t} = \frac{n_t}{\sum_{1}^{n_t}h_i} = \frac{1}{h_{av}}$$ (2) where, $h_{av}$ is the average headway. Thus average headway is the inverse of flow. Time headway is often referred to as simply the headway.