[Tagging] Tagging average speed [Was: Re: Residential roads]
colin.smale at xs4all.nl
Mon Nov 8 09:36:49 GMT 2010
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...
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.
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.
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