[OSM-talk] [Talk-us] Am I mapping this wrong, or should the router be fixed for this?

Richard ricoz.osm at gmail.com
Thu Jul 30 20:27:03 UTC 2015


On Thu, Jul 30, 2015 at 09:24:12PM +0200, Colin Smale wrote:
> I assume you are talking about typical speeds, and not a practical maximum. A max speed will almost never be achieved, by definition actually as the vehicle speeds will have a certain distribution. The highest recorded speed will be the de facto practical maximum, assuming the driver survived.

quite clearly the key (maxspeed:practical) has been misnamed whenever 
it was invented.

Sometimes a posted maxspeed is indeed a realistic travelling speed - consider 
the freeway through Nevada - and sometimes there is a huge gap between posted 
(or not even existent) speed limit and practically achievable speed.

> Routers could take account of hundreds of variables in their calculation of predicted journey time from A to B, but in practice their calculations make assumptions for most of them. For example, most of them assume the vehicle is a car, that it is technically not limited to any particular speed, that the weather is perfect, that it is daytime, that the driver is not "inexperienced". And then there are the other volatile variables like traffic density, road works, oversize loads getting in the way etc.
> Routers cannot take everything into account (this would preclude a lot of preprocessing to simplify the real-time calculations), so they use heuristics which work most often.

right, and sometimes they simply need help.

> So how would you define the concept of "typical speed"?

>From the wiki page

<<The name of the key is somewhat misleading - "maxspeed:practical" should be interpreted as "realistic average speed". >>

and

<<To be used especially in places where other tags are not sufficient to describe what kind of traveling speed could be reasonably expected. Many mountain or rural roads as well as desert tracks do not have posted speed limits and the realistic traveling speed may be severely limited by many factors difficult to describe and difficult to use for calculation by routing software.

Practical does not equal "what is physically possible", which varies by vehicle, but roughly a median speed.>>

Urban routing problems like driving through supermarket service roads to save 
a few meters of freeway or avoid a traffic signal could be probably added 
to the list.

Richard



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