From the "better late than never" department:
I just discovered a description of how Inrix work with speeds to assist
with predicted journey times.
Inrix are a leading supplier of this kind of information to
organisations such as ITIS (in the UK) and TomTom, so I think they have
probably thought it through...
http://euscorecard.inrix.com/scorecard_eu/UK/methodology.asp
In short they have a "reference speed" for each road segment, which
represents the speed in the absence of traffic (capped to the legal
maximum). Beyond that they build a database of average speeds for each
hour of each day of the week, giving 1+7*24=169 values for each segment.
They can then do some mathematical wizardry to produce statistics to
show the effects of congestion.
The "reference speed" is definitely something that could have a place in
OSM and as it is based on actual journey times it would surely be
beneficial to routing decisions - better anyway than blindly taking the
legal maximum speed or attempting to predict the driving speed based on
attributes of the way in OSM.
If we could agree a tag for this it would at least give a place to put
the "reference speed". Routers etc can then use this instead of maxspeed
where it is available.
Colin
On 04/10/2010 11:57, Woll Newall wrote:
I'm talking about speeds that can be consistently measured, whether
because of consistent rush hour conditions or other factors.
I wouldn't use 'average speed' for the tag, because it implies
something else, but that's what the OP chose for this thread. 'traffic
speed' or something like that would be better.
I'm not talking about "the fastest speed you can drive down this
curving country road", which I agree is going to vary depending on the
driver etc.
If I go to the nearest main road to my house in between the hours of
16:00 and 18:00 every weekday, I can measure the speed at which the
traffic is moving. It will be consistent every day. It will be
significantly lower than the 'maxspeed' (something like 1/5th
maxspeed, like 8mph). All of the motor traffic will be travelling at
this speed (it's so slow that 'slow' drivers will not be left
behind!). For the rest of the weekday daytime the speed is more
variable but is also significantly slower than the maxspeed. Only late
at night/early morning is it physically possible for the traffic to
get up to the maxspeed.
In UK cities, I expect many roads are like this. As you may imagine,
using the raw maxspeed for routing (as all the routing systems and
online routers I have ever used seem to do) is useless in this
situation. Not only is the 'time to destination' totally incorrect,
the route chosen is often wrong as well, because choosing a slightly
different route would make journey times much quicker.
Routing programs can't use heuristics to work out these speeds, they
are too dependent on local micro-conditions. But we can measure them.
Woll
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