faq 30179330

Is the speed of a vehicle through a link affected at all by numbers of vehicles?

by Kai Nagel on 2015-06-22 10:39:28

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Proxying for Lin Padgham:

One small question (which I should read the book to try and answer but haven't yet had time), is whether speed of a vehicle through a link is affected at all by numbers of vehicles. I would have expected it to be, but in a couple of different situations now, people have told me that it was affected only by link capacity - as long as # vehicles were below capacity they moved at a constant rate on the link, and once it was over capacity no further vehicles travelled on the link - they chose other routes. Is there a notion of speed (limits) on links and does actual speed get affected by traffic volume?  How does capacity get initialised, and if one wants it to work properly must these be adjusted manually - one of my students said the capacity on all links was the same?

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Comments: 1

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Re: Is the speed of a vehicle through a link affected at all by numbers of vehicles?

by Kai Nagel on 2015-06-22 10:50:42

The assessment is essentially correct: as long as the inflow into a link is less than its outflow, there is essentially no congestion.  (The "essentially" refers to low capacity links and/or scenarios with low flow capacity factor: in such cases, the spacing between exiting vehicles has to be quite large, leading to delays even when the inflow in the average is small enough.)

This corresponds to Newell's zero order theory; it is indeed described in some detail in the "book".

Not correct are the following:

• "once over capacity no further vehicles travelled on the link" – no, this depends on the alternatives.  If indeed the congestion on the first link is large enough, this will cause re-routing.  However, if, say, by this also the alternatives become congested, the routing will eventually come back to the first link and make it even more congested.
• "capacity on all links was the same" – no, definitely not.  Capacity is given by the capacity entry of each link, in the network.xml(.gz) file.  That value, in turn, depends on your input data preparation.  If you used our osm reader, then each link's capacity depends on the link's osm type.  You may have to go through the osm reader and look at how this works, in particular since road types are not standardized across countries.  For example, what is an "Autobahn" in Germany may be a "motorway" or "freeway" or "interstate" in other countries, and even if OSM has agreed on an international naming scheme, the typical capacities may still be different. Also, OSM often does not give the number of lanes, and in the absence of the number of lanes capacities are impossible to guesstimate, and in such cases you really need to do something – preferably add the number of lanes into your local OSM data base.  However, to re-iterate: Input data varies so much from region to region that for the provision of input data it is always necessary to adjust procedures for local conditions.