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Mean Target Headway in Paramics

UPDATED: April 13, 2017


Headway is the Target Mean Headway (MTH) and is based on the time between the leading edges of successive vehicles (or front bumper to front bumper).
 
Vehicles will attempt to drive at the Mean Target Headway; however this will be influenced by driver characteristics and other parameters. MTH is modified by the vehicle characteristics, particularly aggression and on a link or category basis with the Headway Factor.
 
If an observer is standing beside the road and two successive vehicles pass with a Target Mean Headway of 1 second (all other simulation effects disregarded) then the observer will record a lapsed time of 1 second between the front edges of the two vehicles passing a static point (the observer).*

The default value of 1 second has been validated in studies throughout the initial Paramics development, however there are very different traffic flow conditions worldwide and it is recommended that the default value be used as an initial value and then altered as part of the calibration process if required.

The MTH is set at a global level in the Core Network Attributes>>Advanced>>Core>Mean Target Headway and the Editor Palette>>Link>>Behaviour>>Headway Factor. If no link based factor (value of 0) is used then the category based factor is applied from the appropriate category. The MTH applied to an individual vehicle is a product of both factors; the global factor and the link/category factor.

The MTH and MDRT are key components of the car-following simulation engine and have a large influence on dictating the throughput of freeway sections of high speed high flow.

The Target Mean Headway was validated using 1 second which means that lane capacity would be calculated at 3600 vehicle/lane. This is rather high but please note that this is purely theory based. The 'true' capacity should be calculated to include restrictions/impedences.

To date there is no definite method to calculating lane capacity. Given that traffic conditions are never the same by using a Target Mean Headway of 1 second, we are simply providing users a starting point and it is up to the user to make their own adjustments to suit local conditions.

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