I found this interesting article that discusses the way GPS units calculate the fastest route. It's a rather interesting read:
Has anyone noticed this happening? I don't get "updates" like this on my Garmin except if there is a traffic event en-route.
I don't look at all the directions before setting out on a journey so don't know if my Garmin changes the route in the background as we get further down the road and there is so spare processor capacity available.
That was an interesting article.
Thanks for the link to it.
How the GPS calculates the difference between the shortest and quickest route has always been a mystery to me.
I know for a fact that several times the unit takes me a way home that I know, for sure, is NOT really the fastest.
I am not sure that this is actually a "flaw" as presented in the article, perhaps a "shortcoming" at best. Saving eight minutes over a 100 minute drive is not really significant.
As for the GPS "updating" the shortest route while I'm driving, there are only so many times I want to here Recalculating...
The half dozen or so GPS units that I've used, all have "quirks" in routing. The fastest is not necessarily the fastest and the shortest is not always the shortest. It would be informative to know what algorithm is used. My 855 seems to be programmed to get onto expressways as soon as possible, and ignores the fact that side roads do not necessarily mean slow roads.
Normally it's not too far out but sometimes there are significant differences, not always due to my own stupidity. It's come that I always check the route, modifying with vias as required.
So far the only route recalculations have been when I miss a turn.
I have no information to know if this is true or not, but something close has to be true for Garmin.
All roads have a ranking. 1 - Interstate, 2 - Toll, 3 State highway, 4 county road, 5 city street. Or something like that (other wise how do you avoid highways or toll roads?).
In routing it always tries to use the highest numbered roads possible. This leads to several problems. In Kansas many state highways are just as good as the interstates. In missouri they are not. So as you travel around the country it may not always be giving you the best route for that area. It will try and send you the way that has the most traffic (interstate) and may not use a state road that can save you 30 min or more because it doesn't have a shoulder, has curves or something.
The only other thing I can think of is that the speed limit associated with some of these other roads is wrong. That state roads are at 55 in the map data when in reality they are 65 or 70. Many times in google maps it will say 130 miles, but 3 hrs 15 min. In actually it will take 2.5 hrs.
One of the reasons I like to plan routes on Mapsource and download to my GPS, is because the route the GPS calculates is not always the best.
There are many times that I can find a better route by playing with Mapsource for a few minutes. The route is frequently faster and shorter. Also, when local knowledge is available, traffic flow and patterns make a lot of difference.
The GPS does a good job for what it is - but a key improvement in the future that I've wanted for several years is better, more robust route calculations. I generally prefer something between the absolute shortest and the absolute fastest route I can find - more of a blend balancing the number of miles with the time trade-off and I prefer less interstates when that is possible and makes sense.
It is pretty understandable that the "fastest" route isn't always the fastest, considering factors like traffic, red lights, school zones, and a drivers own habits with regard to the speed limit. But I'm more disenchanted by the Garmin nuvi's inability to find the shortest distance on it's own maps. With shortest routing selected I routinely see times where it will try to route me a longer and slower way, and when I ignort the directions and turn off the suggested route it will do it's "recalculating" operation and then report a quicker ETA and a shorter distance. No excuse fot it not picking the shorter path in the first place, since it had the shorter roads in the database.
I've also seen cases where, taking the same overall route home, the nuvi will make one of two different choices of which way to cut over to an parallel road. One of the paths is clearly shorter than the other. But which path the nuvi takes seems to be dependent on exactly where it was when I told it to go home, even though both routes start by taking the first road in the same direction.
GPS routing is not perfect, but I am still amazed at how well they do 99% of the time.
We have 2 options:
- Go back to paper maps, and / or,
- don't buy a GPS!
Seriously, I could not go back to the 'old days' since I've had a GPS. Maps are inaccurate too, but we never hear about it, do we?
Of course, if we live in a place, we know all the shortcuts etc., but the units were designed for foreign places we aren't familiar with. Personally, I'm amazed at the power and accuracy these devices offer us for a reasonable price.
I used to drive from Seattle to Boise several times a year and always used a short cut. When I got my first Gps it did not recognize it if told to navigate from either end but if you were somewhere near the shortcut it would take you that way. once My Garmin 360 has a route picked out it will not figure a new one in the Background.
-- snip -- once My Garmin 360 has a route picked out it will not figure a new one in the Background.
Try forcing a recalculation.
[Depending on model, something similar to...]
Menu > Stop > Where To? > Recently Found > your original destination should be at the top of the list -- select it > Go
Garmin Tricks & Tips
I’ve seen other brands (Garmin, Magellan) where I can find a route that even the GPS agrees is faster with a via point, however I haven’t seen them doing any background routing. It is possible they are doing it but don’t alert the user and just change the route without asking.
I know the Magellan will ask if you want to simulate a route if it your route has been re-calculated after it detects a change in your course, or it thinks you have. I have found this to be more of an announce really since it takes away the map and displays a dialogue prompting for a yes/no response. Plus I have never tried to simulate a route, other than to check a few POI's.
If my Garmin units are doing calculations in the background there have yet to be any visible results - note that no 'recalculating' message would be required, because any changes would be done farther down the road, and along the route you were travelling . . . but over the past 8 or so years the arrival time has never spontaneously gotten shorter (even over drives of more than 2,000 miles) so THAT doesn't apply to the Garmin models I've played with.
My understanding is that the unit will try all of the more obvious segments (blocks/roads with endpoints) using a conical projection in the direction you are travelling, looking for those with the shortest travel time.
Since they know the turns, speed limits (for primary & secondary roads) and typical limits for others, it isn't hard to figure out whether using the expressway is quicker than using side streets.
However they do not know about traffic signals - possibly there's a weighting factor to deal with this, and perhaps they weight turns - I know that the system has me go the long way down my street (which is shaped like a 'P') possibly because it thinks that turning left takes more time than going around.
This would explain why it wants you on the freeway as soon as possible for any 'fastest' route with a significant parallel to any expressway.
The reality is that if WE know where we are going, it doesn't make much difference. If we DON'T know where we are gong we probably would never notice . . . and we'd get where we were going eventually in any case.
There are those anomalies that crop up from time to time, perhaps where the unit doesn't properly know that I can make a left, so it has me go straight for a block or two, then negotiate three rights and return to where I was, except facing the other way, turning right where I would have originally turned left . . . but if I am on the ball, I usually catch that type of error. For me it is very rare and is pretty obvious if you don't have any vias in your route.
I may treat my navigator as an oracle in places where I am not familiar with the destinations and the lay of the land; Then again, I also don't blindly drive through barriers blocking access to washed out bridges or try to make U-turns on interstate highways.
They are just machines.
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