Module 3 : Microscopic Traffic Flow Modeling
Lecture 12 : Vehicle Arrival Models: Headway
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Classification of traffic state

Generally, traffic state can be divided into three; namely low, medium and high flow conditions. Salient features of each of the flow state is presented below after a brief discussion of the probability distribution.
  1. Low volume flow
    1. Headway follow a random process as there is no interaction between the arrival of two vehicles.
    2. The arrival of one vehicle is independent of the arrival of other vehicle.
    3. The minimum headway is governed by the safety criteria.
    4. A negative exponential distribution can be used to model such flow.
  2. High volume flow
    1. The flow is very high and is near to the capacity.
    2. There is very high interaction between the vehicle.
    3. This is characterized by near constant headway.
    4. The mean and variance of the headway is very low.
    5. A normal distribution can used to model such flow.
  3. Intermediate flow
    1. Some vehicle travel independently and some vehicle has interaction with other vehicles.
    2. More difficult to analyze, however, has more application in the field.
    3. Pearson Type III Distribution can be used which is a very general case of negative exponential distribution.