[LINK] car computers

Richard Chirgwin rchirgwin at ozemail.com.au
Tue Jan 20 11:44:31 AEDT 2009

Scott Howard wrote:
> On Mon, Jan 19, 2009 at 3:47 PM, David Lochrin <dlochrin at d2.net.au> wrote:
>> Re a similar technology, I now always double check <whereis.com.au>
>> directions by reference to the road map (preferably Google Maps :-).  And I
>> don't understand why this application seems so prone to give less than best
>> route - is it the map data or the algorithm?
> Many (all?) of the maps for Australia seem to lack "speed" data for anything
> but major roads, which means that the routing application isn't able to make
> an optimal decision between roads.
It depends on who pays how much for what, I suspect. Roadsnet - the raw
data - includes speed data and direction (ie, for one-way streets), but
analysing it takes processing time and would slow down the results.

The map route algorithms I'm familiar with don't directly look at speed.
Instead, different aspects of roads are assigned different costs ... so,
distance is a cost, low speed is a higher cost than high speed, traffic
lights are a cost, and so on. The routing algorithm says "what is least
cost for A to B?" (note: this is what I'm familiar with, it may not be a
universal truth!).

Where speed data is missing, I would guess someone assigns a single cost
to the road, and it may be either the algorithm or the data at fault.

(BTW, David, I have noted the same problem as you, that the route to
Mittagong is silly.)
> By contrast, the mapping data for the US (or at least around San Francisco)
> includes at least one speed (the speed limit) and frequently two (the
> "expected" speed) for most/all road, which allows for a much more optimal
> routing.  Increasingly they are also starting to include time-of-day based
> speeds (65mph, except during peak hours when it's 35mph) as well as access
> to live traffic data which allows even better routing.
In other words, for the US, there is value in capturing these variables,
but not for Australia yet!

> When I was in Sydney I found that my GPS was only useful for "last mile"
> routing - I very rarely used the route it gave me for getting to a
> particular suburb as I almost always knew a better route, and I would only
> use it to get me to the actual address through local streets which I didn't
> know.
>   Scott
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