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How to use short Links in Paramics

UPDATED: April 21, 2017


In microsimulation models, networks are traditionally broken up into discrete elements (links and nodes) so that the network can be represented in a defined structure and so that vehicle behaviour can be correlated to the attributes of the link it is currently on.
Paramics is a link based microsimulation model and as such vehicles between links i.e. between the exit stopline and entry stopline are not represented and reported in the same manner as those on a link (between entry and exit stoplines).

It is common practice to use nodes to represent any point where the roadway characteristics change (see Node Positioning). However there are aspects of the software architecture that only consider upstream and downstream links and some aspects that do not favour short links.

A short link is defined as a link shorter than 10m (33ft) although the effect of a ‘short’ link depends on the speed and traffic conditions of the link. For example a 10m link may not display any symptomatic problems listed below in urban situations, however it may do so in a freeway situation where vehicle are traveling faster.

The effects of a short link can be numerous, depending on the individual situation however some are listed below:
- Vehicles can ‘see’ a vehicle on a downstream link for purposes of the car flowing model, however it cannot ‘see’ a vehicle two links downstream. In some cases this could be as little as 15m away.
- A vehicles length only applies to its current link, therefore if a vehicle is longer than the link the simulation engines cannot take into account parts of the vehicle on upstream/downstream links
- Spaces between exit and entry stoplines can allow vehicles to ‘bunch’ 
- Queuing may not be seen by upstream traffic further than the immediate upstream link, which could be 10m away if the intervening link is short.
- Reporting data on short links can show errors as the link length is considered from node to node but vehicle data can only be recorded between entry and exit stoplines. If short links are used the relative difference between the two distances is greater.

As a result of these symptoms of short links it is recommended that the length of links is considered carefully when building a network so that the detail and the expected vehicle behaviour is representative of the scope of the study.

Environment Details

Products affected: Paramics®

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