Collections/routes allow you to obtain results for a subset of links or traffic streams within the network. These can be particularly useful at times, but care needs be taking regarding the interpretation of what the results are telling you.
It is recommended that if you are not familiar/knowledgeable about the output results from collections, that you bring up the TRANSYT Glossary screen (see Main Menu > Help > Glossary Screen). Whenever you select a data item in the data editor the glossary screen will present information about that particular item. Importantly, in the context of collections, it tells you how the value is derived – is a SUM, AVERAGE, WEIGHTED AVERAGE, or ‘SPECIAL’ calculation. This helps you to understand what the collection output represents.
However, even with the help of the glossary, there is still some scope for misunderstanding. For example, one of the commonest misunderstandings is to fail to realise that collection results will be PER LINK.
An example of this is the output value “Journey time per PCU”. Collections simply average or sum the values that you obtain for each link in the collection. This has the unfortunate effect of suggesting that journey times along a route can be obtained in this way. What the “journey time per PCU” actually represents is a “weighted average” (see glossary) of all the journey times on ALL links in that collection. This means that the times relate to journeys PER LINK within this subset of links. It also means it includes ALL movements of ALL traffic – not just those that are taking a particular route through the network. So in this context “journey Time Per PCU” represents… “Journey Time Per PCU per link within the collection and for all traffic movements within this collection”.
All is not lost however – You CAN obtain good accurate journey time results of you use an OD Matrix to allocate traffic flows to your network. As well as making it easier to get flows into TRANSYT, OD Matrices also report journey times. The journey times between each of the LOCATIONS within the OD Matrix is reported. These are genuine journey times for the traffic making that particular journey. Please note that summing multiple OD Matrix journey times to make up longer overall journeys will again result in inaccuracy due to the fact that the overall journeys will include data associated with traffic that only completes part of the journey, which may experience slightly different delays. However, in most cases the inaccuracy is likely to be relatively small compared with the overall journey times.