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Vehicle release probability in Paramics

UPDATED: April 5, 2017


Paramics is probabilistic micro-simulation software and as such uses Random Numbers in order to represent variation in human behaviour and the variation witnessed in day-to-day traffic volumes.

Vehicles released a probabilistic manner having a "chance" of being released at each Time Step. This "chance" is compared to a random number and the
  • Trips defined in demands file
  • Number of Time Steps per second
  • % VT associated with demands file
  • Release profile over the period for the OD movement
So for example 1800 per hour
90% cars 10% trucks in demads file
4 Time Steps per second
5% release of traffic released in this 5 minute interval for the OD pair, in a 60 minute period).

1800 x 0.9 = >1620 
1620 / 3600 => 0.45 per second
0.45 / 4 => 0.1125
5% x 12 intervals => 0.6
=> 0.1125 x 0.6 
= 0.0675
= 6.75% chance of a car being release per Time Step for this OD pair in this interval.

In this instance each time a random number associated with this release scenario is 0.0675 or lower then a car is released appropriately.

Once a vehicle has been designated for release an appropriate gap must be available on the link associate with the zone. If a suitable gap is not available on the link due congestion then the vehicle is placed in a stack held outside the network and is later released onto the network when a suitable space is available. A vehicle that is not able to be released in this way is defined as blocked in the Release Counts file; a vehicle can be blocked several times before it is released, each time increasing the blocked count.

It is good coding practice to avoid congestion in zones so that the full demand from a zone is released onto the network, in this way any comparison between scenarios is on a like-for-like basis as vehicles awaiting release do not count in network, vehicle or trip based statistics.

It is also important that the correct zone loading type is selected. If an Area Zone loading approach is taken for high flows (where a Route Zone would be recommended) then vehicles can be released downstream of previously released vehicles. This has the effect that, if the link associated with the zone is congested then new vehicles can be released into gaps in the congested flow. This has the effect of trapping some vehicles at the upstream end of the link, creating unrealistically long journey times when they finally reach their destination.

Note that the loading type can be set by making the node either normal or a Zone Connector (set in the Editor Palette, Junctions). See the Modeller manual for more details on how the Zone Connector typebehaves.

The base Release Probabilities can be altered using the API or the Matrix Tuning functions.

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