Speed limit of the street.

[gerryc]

I can't remember what thread but someone did ask how to find out the speed limit of a street as known by the tomtom map.

I was playing with demo playing my route and noticed it displayed the speed limit of the street.

So wondering if speed limit is already set by tomtom for that street? Set a route along the street and do a 'play route demo'.

 

[The Spook]

There are 2 different factors at play here, the displayed 'speed limit' and the road speed that is used in route calculation / planning. You may be mixing the two.

If you have the 'display speed limit' option active, you will see the speed limit displayed when driving around or when running a demo *If the speed limit data is present for that road*.

If the speed limit data is not present, running a demo will not display it. However, you can see the speed that the TT assumes for the road when making planning calculations. If the demo speed is set to 100%, the driving speed in the demo = road planning speed.

 

[gerryc]

I'm now using the demo as an aid in map corrections. I'm adding speed limits on roads that don't have one yet. I use the 'exceeded speed' warning functionality of the tomtom..

 

[zengshengliu]

I have a question about the speed limit
When I add a speed limit to a road like for example
35 mph, it shows up only 34 on my 920 unit
same thing if it is 25, it shows up only 24
is it the way tomtom works?

 

[dhn]

It's a software bug in the algorithm converting mph to km/hour and back to mph.

 

[toledospod]

I have a question that might be slightly off-topic, so if it is, I apologize.

I have a TT1 3rd edition and was wondering, is there a way of turning off that annoying tone that sounds when you exceed the speed limit set on a given road, e.g. on a freeway?

I live in Toledo, OH and here no-one(and I mean no-one)sticks to the speed limit, particularly on the expressways, so this drives me crazy everytime I slow down and speed up again. My wife says it sounds like the sound you hear when the fasten your seatbelt sign goes on when you're on an airplane! LOL

 

[dhn]

Preferences-->Safety Preference-->Uncheck 'warn when driving faster than a set speed'

 

[toledospod]

Thanks DHN! I had trouble finding this before, but that was because I am using a custom menu. When I switched back to the default menu, there it was!

 

[sageuvagony]

It's a software bug in the algorithm converting mph to km/hour and back to mph.

Wow, I didn't know it also affected higher end models. I set the speed limit for my local street which is 20 MPH, but afterwards it came back as 19 MPH. I thought I was crazy at first because I KNOW I entered 20.

 

[The Spook]

It's is a rounding error!

I do not believe it is not a 'bug' in the software.

It is true that it is caused by the conversion from mph to km/h (TT's native measure) and back to mph, but the resulting difference is due to rounding errors. Rounding errors are a feature of all computers, from Deep Thought to the humble TT.

When performing a calculation, where do you stop calculating after the decimal point and round the result to the last calculated number? Of course, Deep Thought has considerably more resources available for floating-point arithmetic than your TT does, so the error will be lower. But, there will still be an error due to rounding.

Of course, when talking about speed limits, you must also consider that the result must be an integer. You rarely see a speed limit of 19.8 mph.

Compounding the numerous rounding errors during the mph>km/h>mph conversion and then rounding the result means that the final figure might be 19mph or 61mph. The best fix is to let your TT speak it's native language, km/h. That way, 50km/h entered should come back to you as 50km/h. Not a great solution though I know!!

 

[dhn]

Spook.....I work for a software company as a tester. I stand by my comment that the rounding error is a bug in the software algorithm. It need not have 'worked' this way.

 

[sageuvagony]

I do not believe it is not a 'bug' in the software.

It is true that it is caused by the conversion from mph to km/h (TT's native measure) and back to mph, but the resulting difference is due to rounding errors. Rounding errors are a feature of all computers, from Deep Thought to the humble TT.

When performing a calculation, where do you stop calculating after the decimal point and round the result to the last calculated number? Of course, Deep Thought has considerably more resources available for floating-point arithmetic than your TT does, so the error will be lower. But, there will still be an error due to rounding.

Of course, when talking about speed limits, you must also consider that the result must be an integer. You rarely see a speed limit of 19.8 mph.

Compounding the numerous rounding errors during the mph>km/h>mph conversion and then rounding the result means that the final figure might be 19mph or 61mph. The best fix is to let your TT speak it's native language, km/h. That way, 50km/h entered should come back to you as 50km/h. Not a great solution though I know!!

Umm... ok? None of that changes the fact that it still shows my speed limit is 19 when it's really 20. It is a bug/mistake and maybe the formula could be modified just a bit so this doesn't happen. Something that also doesn't make sense to me is that nearly all of the roads around me show the perfect speed limit. The error only seems to happen when you manually enter in a speed limit. I tried also entering in 21 and 22, and speed limit STILL shows up as 19.

 

[LostFool]

I would like to know why when you correct a speed limit it does not correct the displayed speed limit. When I drive from Lufkin, TX south on US 59 to Livingston. The displayed speed limit is showing as 45MPH all the way from one end to the other even though the actual speed limits vary from 70 down to 35 (passing through small towns in between). I have tried going in and correcting each section and the only thing that changes is the trip calculating speed. I have my speed limit warning turned off when I go through this section as it is constantly going off!!
If this has to be sent to TOMTOM as a feedback correction instead of correcting on the map and sending as a mapshare it is going to be a very long report email.

 

[gerryc]

I would like to know why when you correct a speed limit it does not correct the displayed speed limit. When I drive from Lufkin, TX south on US 59 to Livingston. The displayed speed limit is showing as 45MPH all the way from one end to the other even though the actual speed limits vary from 70 down to 35 (passing through small towns in between). I have tried going in and correcting each section and the only thing that changes is the trip calculating speed. I have my speed limit warning turned off when I go through this section as it is constantly going off!!
If this has to be sent to TOMTOM as a feedback correction instead of correcting on the map and sending as a mapshare it is going to be a very long report email.

I verified my speed corrections to a street near me. And it seems to work.

Prepare a route from one end of the street to the other end. Then play a route demo. This will display the speed limit corrections you have set at different portions of the street.

 

[Ricochet]

I would like to know why when you correct a speed limit it does not correct the displayed speed limit. When I drive from Lufkin, TX south on US 59 to Livingston. The displayed speed limit is showing as 45MPH all the way from one end to the other even though the actual speed limits vary from 70 down to 35 (passing through small towns in between). I have tried going in and correcting each section and the only thing that changes is the trip calculating speed. I have my speed limit warning turned off when I go through this section as it is constantly going off!!
If this has to be sent to TOMTOM as a feedback correction instead of correcting on the map and sending as a mapshare it is going to be a very long report email.

I've painstakingly made dozens of speed limit corrections in & around my local area. And other than TomTom's occasional habit of misapplying corrections to adjacent streets, they definitely show up for me in the status bar (when that option is selected).

I have a ONE 3rd & a GO 720, both display my changes with Nav8.

R?

 

[sageuvagony]

I verified my speed corrections to a street near me. And it seems to work.

Prepare a route from one end of the street to the other end. Then play a route demo. This will display the speed limit corrections you have set at different portions of the street.

I do, and it shows 19 instead of 20.

 

[Ricochet]

I do, and it shows 19 instead of 20.

Oh, that. That is the result of the well-documented "rounding error."

For some reason, TT appears to be converting your mph into kph for storage & then converting them back to mph for display. During this process, "sig figs" are lost.

R?

 

[rickier55]

Based on what I'm seeing, the issue is agravated by the fact that while it is a rounding issue, it's a rounding issue related to increments of 5 KPH (rather than 1 KPH as you might expect).

I suspect the unit is doing calculations in increments of 5 KPH, then converting to MPH. This makes since for speed limits in most of the world, but doesn't work well here. since increments of 5 kph results in speed limits in increments of 3 MPH here in US (ie. 24, 27, etc).

This problem could be fixed in a number of fairly easy ways, if they choose to do so. They obviously addressed it in storing/displaying the speed limits that come with the maps, but did not carry that forward to map corrections algorythms.

Rick

 

[tukatz]

I made a few corrections on local roads before taking my 920 out for the first time. I was surprised to see the limits showing up 1MPH slow. I was even more surprised when I started seeing limits on roads for which I had not entered corrections. Mapshare corrections? Those limits were also 1MPH slow. I'm pretty sure I have the same mapset as I had on my 910 and it did not show any of those limits. It also did not pick up mapshare corrections.

 

[Ricochet]

Based on what I'm seeing, the issue is agravated by the fact that while it is a rounding issue, it's a rounding issue related to increments of 5 KPH (rather than 1 KPH as you might expect).

I suspect the unit is doing calculations in increments of 5 KPH, then converting to MPH. This makes since for speed limits in most of the world, but doesn't work well here. since increments of 5 kph results in speed limits in increments of 3 MPH here in US (ie. 24, 27, etc).

This problem could be fixed in a number of fairly easy ways, if they choose to do so. They obviously addressed it in storing/displaying the speed limits that come with the maps, but did not carry that forward to map corrections algorythms.

This makes a LOT of sense! Cause the math doesn't work out for all speeds when using "1 mph" increments--we should be seeing more variant, than just -1 off.

My guess is that this feature either wasn't tested fully, or was deemed "close enough" for those English unit-users (we should have all switched to the metric system, right? ).