This site uses cookies to offer you a better browsing experience. Find out more about how to manage cookies, or dismiss this message and continue to use cookies.

# ETA calculation for TomTom 920 way off!

hudel
0 Points

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.

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

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.

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?

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.I thinkIn 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,

:D

:roll:

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,

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.

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,