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ETA calculation for TomTom 920 way off!

hudel 0 Points
edited November -1 in TomTom Car Forum
I have noticed very conservative (i.e. WRONG) estimates for TT ETAs/trip duration.

For example, a recent drive from the Bay Area to LA was predicted to take 7 hours and 31 minutes at the start of the trip, but the drive ended up taking exactly 6 hours, i.e. the initial ETA was off by 1.5 hours.

Initially I thought this had to do with the speed that the TT assumes is allowed/possible on the various segments (even though much older Garmin units use recent speed data and come up with far more reasonable predictions). Turns out this is not the case, there is a MAJOR bug in the ETA routine, this can be verified with the "Prepare route" and "Show route Demo" features by doing this:

Main Menu 2 of 4: Prepare route
Address
City center
Pick "Lone Pine CA"
Address
City center
Pick "Bishop CA"
Fastest route

You will see this calculation/estimation:
1:36 hrs - 61.3M

To get to the bottom of this error, do this:

Main Menu 3 of 4: View route
Show route Demo
500%

Once your simulation gets you out of Lone Pine onto highway 395 the speed is 62 mph. At the point when there are 55 miles left to go (~54 of which are on the 395 with the 62 mph estimated speed), the remaining duration is still 1 hours and 22 minutes, when it should be 53 minutes!?!?!?


So somewhere the TT algorithm really screws up - I am wondering whether it's just my unit or whether others have noticed this, too.

Comments

  • Tim 1480 Points
    The demo does really strange things to routes and doesn't follow a complete route which might be why it does strange things. I wouldn't take that as any evidence of an issue.

    Generally I find (as do most others) that the TomTom devices are much more conservative with time estimates than others. I find that it will only take me about 85% of the time that it says I will. In your example, you are saying it takes about 80% of its estimate so that isn't far off from what I've experienced.

    On the other hand I can never "keep up" with many Garmin devices. I find it takes about 105% - 110% of the time suggested.

    So with the TomTom I'm always early to my destination. The Garmin is much more accurate, but I'm always late.
  • hudel 0 Points
    The demo does really strange things to routes and doesn't follow a complete route which might be why it does strange things. I wouldn't take that as any evidence of an issue.

    Generally I find (as do most others) that the TomTom devices are much more conservative with time estimates than others. I find that it will only take me about 85% of the time that it says I will. In your example, you are saying it takes about 80% of its estimate so that isn't far off from what I've experienced.

    On the other hand I can never "keep up" with many Garmin devices. I find it takes about 105% - 110% of the time suggested.

    So with the TomTom I'm always early to my destination. The Garmin is much more accurate, but I'm always late.
    Your observations are similar to mine, although my very old Garmin GPS V is ususally dead on (down to the minute) for drives as long as four hours (all highway). I gave the demo example only because it mirrors quite well what happens on a real drive - so I think the TT ETA algorithm is seriously flawed (20% errors seems unacceptable to me) - of course it's usually better to be early than to be late, but I'd rather get a reasonably accurate estimate, not one that is padded by 20%.
  • Seems like garmin calculates your speed as well making it close to ETA time. My 920 always has extra time left with it's ETA calculation.
  • infama 0 Points
    Seems like garmin calculates your speed as well making it close to ETA time. My 920 always has extra time left with it's ETA calculation.
    Neither of them calculates your speed!

    The ETA is calculated up front.

    The TT converges to the real arrival time close to the destination. This is because it keeps recalculating the remaining time based on the original algorithm for that stretch of road.

    The real diff is because both manufacturers use different assumptions for drivable speed on the various roads. TT seeems to assign roads into classes and then give a speed to each class. Garmin may do the same and give diff speeds, or they may even be using the actual speed limits in their database...not sure.

    TT seems to be so legally focussed, I think they use conservative speeds, so they never have to face litigation that they encourage speeding!

    BTW Green, did you find the speed limit warning option I pointed out?
  • Seems like garmin calculates your speed as well making it close to ETA time. My 920 always has extra time left with it's ETA calculation.


    Neither of them calculates your speed!

    The ETA is calculated up front.

    The TT converges to the real arrival time close to the destination. This is because it keeps recalculating the remaining time based on the original algorithm for that stretch of road.

    The real diff is because both manufacturers use different assumptions for drivable speed on the various roads. TT seeems to assign roads into classes and then give a speed to each class. Garmin may do the same and give diff speeds, or they may even be using the actual speed limits in their database...not sure.

    TT seems to be so legally focussed, I think they use conservative speeds, so they never have to face litigation that they encourage speeding!

    BTW Green, did you find the speed limit warning option I pointed out?
    I did a long trip with the Garmin 760 and found that had I not stopped it would have been about 5% too long. I would have arrived 5% early.

    I think Garmin use about 80 - 90 % of the speed limits they have for each type of road. This is where they start. They then, en route, check the time expired to where you are and add a calculation based on the database for the distance left to travel to give an ETA.

    In theory, this should get you there spot on but it depends on when they did the last recalc in relation to your arrival point. Saying all that I would love to actually see the algorithm.

    Regards,
  • Hmmm, I really hope this new map software coming soon will address the TT's "conservative" nature of ETA's. :wink, wink:

    :D
  • infama 0 Points
    Hmmm, I really hope this new map software coming soon will address the TT's "conservative" nature of ETA's. :wink, wink:

    :D
    Hahahahahahahahaha


    :roll:
  • hudel 0 Points
    Seems like garmin calculates your speed as well making it close to ETA time. My 920 always has extra time left with it's ETA calculation.


    Neither of them calculates your speed!

    The ETA is calculated up front.

    The TT converges to the real arrival time close to the destination. This is because it keeps recalculating the remaining time based on the original algorithm for that stretch of road.

    The real diff is because both manufacturers use different assumptions for drivable speed on the various roads. TT seeems to assign roads into classes and then give a speed to each class. Garmin may do the same and give diff speeds, or they may even be using the actual speed limits in their database...not sure.

    TT seems to be so legally focussed, I think they use conservative speeds, so they never have to face litigation that they encourage speeding!

    BTW Green, did you find the speed limit warning option I pointed out?
    Well, as I was showing in my example in the 1st post in this series, TT assumes 62 mph on highway 395 (posted speed limit is 65 mph). So that by itself would not account for the whopping 25% error. That HUGE error occurs somewhere in TT's calculations, when they estimate that one would take 82 minutes for driving 55 miles at that speed (62 mph)! So instead of using the 62 mph they are effectively using 40 mph!

  • BTW Green, did you find the speed limit warning option I pointed out?
    I have check the box but still won't display the speed limit. :cry:
  • rav 0 Points

    BTW Green, did you find the speed limit warning option I pointed out?

    I have check the box but still won't display the speed limit. :cry:
    Don't be sad. Mine doesn't work either in Canada. I guess it only works in Europe. :)
  • Noticed when you stop for a few minutes with the 760 unit on, ETA time is also moved to a later time.
  • Hi Greenapple.
    If it is doing it's job, the ETA will move forward minute for minute while you are stopped. While you are stopped you are getting no closer to your destination so the ETA algorithm will continue to clock up the time and add to it the estimated time for the rest of the journey.
    Regards,
  • infama 0 Points
    Hi Greenapple.
    If it is doing it's job, the ETA will move forward minute for minute while you are stopped. While you are stopped you are getting no closer to your destination so the ETA algorithm will continue to clock up the time and add to it the estimated time for the rest of the journey.
    Regards,
    Totally agree.

    With the 920, if I speed up for a while my ETA goes down and if I slow down in traffic congestions, the ETA goes up.

    I find that in a 45 minute journey of mostly highway, the 920 overestimates the trip by 2-3mins on average. I also tend to push it for most of the journey, so that may partly explain it.
  • Hi Infama,
    The error depends on when it did its last recalc and the driving condition since the recalc. It you are say a mile from your destination and are, and continue to travel, close to the speed limit for that road your ETA would be spot on.
    Regards,
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