Estimated Journey Times

[mini1400]

[leonardo1]

How can that be? Are you saying the speedo in my car is incorrect?

If so, how come the ETA shown on the TTN is more than 5 mins out from the time I actually got home? I remember the TTN saying I'll be home by 10:36 pm and yet I got home before 10:30.

My car is a new Mercedes C-class and the onboard computer tells me things like the time lapsed since the start of journey, no. of miles travelled so far, no. of miles left in the tank, miles per gallon, miles per hour and it's pretty accurate as far as I can tell.

Just curious.
_________________
TomTom Navigator 5.10 on Dell Axim 50v
TomTom bluetooth GPS
Sandisk 1Gb CF, TomTom 128Mb SD card
TomTom powered cradle

[mini1400]

[leonardo1]

I can understand your case on under-reading the speedo, why don't the manufacturers make the speedo to show the EXACT speed?

I'm sure some of you will have come cross those temporary speed indicators that are placed in city centres and some rural roads that flash the speed of your car as you drive toward it, and whenever I managed to get one of those to flash the speed of my car back at me, I noticed that my speedo indicator and the digital display on the road speed indicator pretty much match each other, i.e. if I was doing 45 mph on the 40 mph road, the indicator on the road actually flashes 45. So, where's the inaccuracy? Is it only at high speeds?

Sorry, I understand this discussion is nothing to do with TTN5 but please bear with me.

Thanks

Len
_________________
TomTom Navigator 5.10 on Dell Axim 50v
TomTom bluetooth GPS
Sandisk 1Gb CF, TomTom 128Mb SD card
TomTom powered cradle

[julianbarker]

My guess is that the speed indicators you talk about are set low to make people slow down at black spots.

As for why car speedos are not accurate - think of the factors - a simple one - a brand new tyre has about 5mm of tread. A worn one has 2mm. That is a difference in radius of 3mm = difference in circumference of nearly 2cm. That means the potental error between a new and a worn tyre is about 1% in indicated speed. The manufacturor can't be sure how worn the tyre is, so can't build an accurate speedo.

That is just one factor - there will be wear in the gearbox, differences in tollerance in various components etc. No car speedo can be accurate as it relies on a relationship betwen the speed in the geabox and that of the road wheels. AGPSr measures actual movemeent across the surface of the Earth's spheroid. That is why GPS tends to be more accurate than any car speedo. As manufacturers can't provide an accurate speedo, the provide one that deliberatly over measures speed. That way the customer is happy becasue they think their car is faster than it is, and they get fewer tickets.

[peterc10]

Another factor is that the same car model will have different size wheels depending upon the trim level. However the speedo is normally same across the range. I know that larger wheels often have "flatter" tyres but they wont be exactly the same diameter. Also the different tyre manufacturers have slightly different diameters. All adds up to mean that speedos will rarely be very accurate.

I read in one thread that GPS was accurate to plus or minus 0.1mph. There again 74% of statistics are made up on the spot.

[Robin2]

It's been on this forum many times before. All new cars must have speedos which over read between 0 and 10%. To allow for manufacturing tolerances, tyre wear etc, a typical speedo will over read by 4 or 5%.
GPS speed indication is accurate to about 0.1mph, but only of you are travelling at a steady speed and on the level. I have 2 cars, and under these conditions the speedo indication at a true 70mph is 74 on one and 73 on the other.
Robin
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TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used

[Qhunter]

I've explored this issue in TT5 in some depth. I upgraded to TT5 from TT3 as a lot of other folks have and I find I can no longer take the estimated journey times seriously. I'm also bemused that TT5 doesn't use it's own planned speeds when demo'ing it's own routes.

For example: I have recently moved and for the next few months I have a lengthy, 60 mile commute on my hands. This route, apart from the first 3 miles is all on the same road, the A47 from Fenland to Leicester. According to TT5's timings the journey should take 1hr40min, a huge amount of time for what is a simple journey. To satisfy my own curiosity I let TT run the route in demo mode to see how long it would take if left to it's own devices.

Amazingly the demo took 59minutes! I do the journey, which on the whole is made up of fast, rural single carriageway, between 1hr03m and 1hr10m depending on the number of heavies I encounter. So, just what speeds it TT using for planning as it certainly aint using the speeds it does in demo mode

Not being able to adjust the speeds for each type of road has been a huge failing on behalf of TT. I am seriously considering removing the entire thing and simply reverting to TT3. Very, very disappointed and I wouldn't recommend the TT3 to TT5 upgrade to anyone.

Whilst I'm having a moan, why have they changed the POI warnings from a time based warning to a distance based one? I used to have two warnings, one at 30seconds and one at 5 (to remind me I had arrived at the POI). This gave a good warning of the location of a camera at all speeds. Changing this to a distance based warning is a big backward step. 300yds for example is fine for a 30mph warning but useless for a 70mph warning.

Why haven't TT listened to its customers? As a PLC they need to listen to us now more than it did as a private co.

[Qhunter]

[Tom Symonds]

Always worth pointing out too that ACPO guidelines for speed cameras also allow leeway to allow for inaccuracy of speed measurement.

Camera shouldn't flash you at anything less than:

SPEED LIMIT + 10% + 2mph

ie 46 in a 40 zone, 57 in a 50 etc.

[neil01]

[Qhunter]

I was looking at the next stage in the 'milking the cashcow' as you put it. The satnav marketplace is now flushed with affordable and functioning units. When the sales start to fall the shareholders of a PLC will want to see a change of fortune much quicker than the board of a private company would.

[neil01]

Which comes back to the problem with milking the cash cow - it often kills the cow!

But you have to remember, that shareholders who get their timings right (and some are experts in doing so) can ditch their shares at the press of a button, it is the financial gain which some are interested in.

Buy in, make a killing at any cost to the long term viability of the company, get out before the S*** hits the fan.

Don't get me wrong, I was not talking about TomTom, or any company in this sector, I simply do not know enough about them as a companies - I was merely pointing out that it is the shareholders to who every PLC is ultimately accountable.

[Robin2]

[DennyL]

Coming back to the question of how Tomtom calculates journey times.

I don't see why there shouldn't be some sort of 'learning algorithm' that uses historical speed data from your previous journey and updates it as it acquires information about the speeds you are doing on your current journey. This would help to cope with the differences between speeds that we do in quiet times of the day and the speeds we do in the rush hour, and the differences between different driving styles. I can't believe it would be so complicated to implement in code. Tomtom already creates a config file, so it wouldn't be a huge step to store in it a parameter that characterizes one's speed, and once one is embarked on a journey, to extrapolate one's current speeds to estimate an ETA.

Viamichelin Mapsonic has a place to enter speeds for different classes of road, and Tomtom and Mapopolis don't. I have used Mapsonic and it was no more accurate, and one is always tempted to fiddle about with the speeds depending on the journey one was planning. My speeds very hugely depending on road conditions. I find Tomtom pretty accurate.

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