Garmin routing

Over the weekend I used the Nuvi 350 to take me to a few destinations. It
left me wondering how they do the routing. There are many ways to get to a
destination, of course, and we can easily pick and choose looking at a map.
Computers use a different logic that left me wondering.

Starting point was the same in both cases. On Saturday, we went from point
A to point B. It took us up route 1 and made an exit and some turns to get
to point B. The next day, we went from A to C. To get to C, we went right
by B. The routing, however, that took us by B was different than used the
day before. In this case, it was a bit easier (less turns) but I did not
record actual distance. No settings were changed.

On Mon, 15 Jun 2009 09:35:38 -0400, Ed Pawlowski wrote:


I've found that routes are decided (quickest or shortest) down to
whatever level of measurement the device uses.

If a route is 1 second faster than another, that will be selected. If a
route is 10ft (or whatever) shorter, that will be selected.

if two routes are almost identical in distance or time, the difference
between which way your car is facing when your program the satnav, or
even if your parked a few feet further in one direction could might cause
a different route to be chosen.

My purely subjective view is that it can appear that the time of day may
make a difference.

I do an ~90 mile route most days. On my way back (I use the satnav for
its ETA) travelling through York, it normally directs me around the inner
ring road to the A19 if I leave before 3pm. If I leave after 3pm it
directs me the same way up to a point, then directs me to the outer
ringroad and around that to my normal exit further up the A19. The rest
of the route is identical.

Given that a computer program ought to produce the same output for a
given set of input data, then something else must be varying. I wonder
if the average speed for a given road is recorded in the database more
than once, eg normal traffic and rush hour traffic.

--
You cannot simply assume someone is honest
just because they are not an MP.

I forgot to mention, along the way it also stated "a better route is
available" That makes some sense at times, but this was about 300' from the
destination that was visible to me at the time. I have to wonder what could
have been better.