TomTom alternative route algorithm

TonyD · Frequent Visitor Joined: May 08, 2005 Posts: 412

I'm a bit curious about the algorithm TomTom uses when I do not take it's advice and take an alternative route. I regularly drive from Oxfordshire to Leeds and there are 3 virtually identical routes. Quite often I pick one of the other routes from the one TT has picked. As I sail past the appointed junction TT replans and invariably adds 10-15 minutes to the ETA. About 5 minutes later it comes to its senses and settles back on the original ETA. I suspect what it is doing is trying to take me back onto the original route, but why doesn't it immediately replan properly.
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Tomo · Posted: Tue Jan 27, 2009 11:50 pm Post subject:

Hi Tony,

I can only assume that TT still sees the old route as the fastest and will try and put you on that. It will continue doing this until you pass a point (or there are no places it sees that it can turn you around) that it sees as been faster than the original.

Sometimes, a route may be faster even though TT thinks its longer, due to different roads or the influence of IQ routes.

Steve

TonyD · Frequent Visitor Joined: May 08, 2005 Posts: 412

MrT · Posted: Wed Jan 28, 2009 4:25 pm Post subject:

I have read before, probably on this forum, that TT actually calculates the route backwards from the destination to where you are rather than from where you are to the destination. If this is true it could explain why this happens.
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TonyD · Frequent Visitor Joined: May 08, 2005 Posts: 412

GerryC · Posted: Wed Jan 28, 2009 4:43 pm Post subject:

You will get a better route using "fastest" route rather than "shortest" which often sends you down the dirt roads in my experience.

As you already know the other routes, I wouldn't worry too much about what the TT is saying - especially if you ignore it's route for your better knowledge one. In the end, it's done it's job right if it gets you there. It's main role is if you DON'T know where you are going or how to get there.

Also, remember that IQ routes may be changing the road data and shows the better ETA once you've passed what it thinks is a slow section of road.
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TonyD · Frequent Visitor Joined: May 08, 2005 Posts: 412

Daggers · Posted: Wed Jan 28, 2009 11:33 pm Post subject:

I did a journey yesterday where I knew the route I wanted to take, but TomTom (520) kept trying to send me in another direction. On at least two occasions, once it had worked out that it needed to recalclate my route, my version was quicker than the "fastest" version that TomTom had calculated!

This may have been due to changing traffic situations, though I'm bound to do the same journey again in the coming weeks, so I'll check again next time!

sgould · Frequent Visitor Joined: Oct 05, 2004 Posts: 1320

There is (was?) a problem with the TT map on the M40. I travel from NW London to Leamington fairly frequently. Going north I get sent up the M40, coming south I get sent to the A45/M45/M1. If I ignore this and drive down the M40, the TT tries to send me across to the M1 by increasingly more lengthy routes, untilI get down toward Bicester when it suddenly chooses the logical route along the M40. I think there's a break in the M40 on the map somewhere.
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TonyD · Frequent Visitor Joined: May 08, 2005 Posts: 412

DennisN · Posted: Thu Jan 29, 2009 12:06 am Post subject:

Daggers · Posted: Thu Jan 29, 2009 12:27 am Post subject:

I think that particular M40 anomaly is now cured, Dennis.

However, one of the incidents I experienced yesterday was also on the M40, though travelling northbound. It was trying to persuade me to leave the motorway at J12, even though I wanted to go all the way to the M42.

But, there may have been an accident which was affecting the traffic situation, so as I said earlier, I'll check again next time I go that way.

cecilrhode · Regular Visitor Joined: Aug 01, 2007 Posts: 66

could it be that TT has a road speed which is incorrect ie less than it actually is? as a result, it calculates a longer travel time, which is then superseded as you make better progress on that road?

TonyD · Frequent Visitor Joined: May 08, 2005 Posts: 412

MrT · Posted: Thu Jan 29, 2009 10:17 am Post subject:

Are you sure when it first recalculates it is not first taking a new 4th route which could be very similar but just that bit different then after a while it recalculates to one of your other two routes?

I must admit I have never seen this behaviour unless there have been incidents on the alternate routes.
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