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When is the fastest route not the fastest?


When the GPS didn’t try hard enough. :) And no, we’re not just talking about agreeing with the GPS about what is the fastest route. This is all about the GPS agreeing with itself. When you calculate a route on a GPS, the feeble little processor analyzes thousands of road segments, but since people want fast routing it doesn’t try all possible combinations of roads– that would take too long. So what does this look like?

tomtom-fastest-route-notThink of it like this. You ask your GPS for a route from point A to point B. It says the fastest route is one hour and forty minutes long. But that is just the fastest combination of roads that it checked. Then you insert a via point into the route so now your route goes from A to viapoint 1, and finally to B. The GPS builds the route and guess what… it agrees that your route is faster by about eight minutes! You can see this scenario with in the image to the right. The original route was 1:40 but when I forced a via the route becomes 1:32. This screenshot comes from the TomTom iPhone App, but if you look hard enough you can see the same thing happen on a variety of GPS devices and navigation software.

So there is a trade-off…. do you want faster routing or do you want slower routing but with more exhaustive checks to make sure the GPS really is checking all possible route combinations? Granted, this issue is very rare.

tomtom-found-faster-routeThere is a way some GPS devices overcome this flaw and try to obtain both fast routing as well as more exhaustive searches. The GPS will calculate a pretty fast route, get you on your way, and then periodically check more and more road combinations in the background. Along the way you might come across a screen like the one shown to the right. While driving around the GPS by itself found the shortcut I had demonstrated with the via point above. The GPS then says “Hey, I’ve been doing some more crunching of numbers over here and I’ve found a route that is faster, want it?”.

You can see that the new route is eight minutes faster, just like I illustrated with the route above. The first time I saw this message I assumed it was simply looking at live traffic data– then I realized the GPS I was using (TomTom XL 340S) didn’t have a live traffic feed. I also considered it might have had something to do with IQ Routes but ruled that out as I could duplicate the scenario across multiple times of day, and with the same starting time.

This is such a rare occurrence that it is difficult to say which devices have such a background routing feature and which ones don’t. The TomTom iPhone app doesn’t appear to do it right now, while the XL-340s obviously does. 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.

3 Responses

  1. Once already en route, my Garmin’s will sometimes show a yellow road icon on the screen and say, “A better route is available.” Is this the same thing?

    Ron - December 15th, 2009
    • Maybe. It depends which device you have as it could be a traffic report. Does it consistently give that at the same point in the same route?

      Tim - December 16th, 2009
      • Mine have no traffic. I have a Nuvi 200 and 350. I don’t know if it does it at the same point in the route, as I never back tracked to try to duplicate it! They will say, “A better route is available,” and display either a yellow squiggly road diamond icon in the lower right, or it might be a yellow road diamond icon with an exclamation mark in the middle, I forget which. It happens very seldom. IIRC, the “better” route is sometimes better, sometimes, um … interesting. It does seem to act like your description, in that it apparently avoided the logical route the first time, but then “reconsidered.” I say this because once I was in a parking lot and set a route which would have me enter the freeway. Once routed, it wanted to immediately take me around the block in a circuitous clockwise path to get me on the freeway, but then within seconds gave me the “better route is available” message. When I tapped the icon, it had me just hang a quick left and onto the freeway that way.

        Ron - December 18th, 2009

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