Speed limits

jrwc · Occasional Visitor Joined: Jan 02, 2008 Posts: 23

When travelling on the bypass of local towns I often hear the speed warning. I'm travelling at about 45mph on a 60 or 70mph zone and the TT One XL shows the speed limit to be 30.

Anyone know a reason for this?

GerryC · Posted: Fri Feb 01, 2008 6:40 pm Post subject: Re: Speed limits

DennisN · Posted: Fri Feb 01, 2008 6:45 pm Post subject: Re: Speed limits

NukeThemAll · Posted: Sat Feb 02, 2008 7:37 pm Post subject:

I concur with The Experienced Ones on this site regarding this subject. Just been to Bournemouth and back today and I'll be turning off the over-speed warning. It relies on TT's data of road speed limits, which is as flaky as a factory of 99's (ask your Dad if you don't understand the reference...) Seriously, the data is totally misleading much of the time - I also wonder if it picks up the road under or over the one you're on and thus 'thinks' that a short section is 30mph when you're on the bypass overhead which is a dual carriageway. More Bongs than (insert appropriate drug paraphernalia reference here.....) and few of them useful.
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GerryC · Posted: Sat Feb 02, 2008 10:13 pm Post subject:

NukeThemAll · Posted: Sat Feb 02, 2008 10:27 pm Post subject:

GerryC - an interesting assertion regarding TT's use of a speed limit when it does mapping. I thought it didn't use localised speed limits for the simple reason that it would add enormously to the calculation; I assumed TT's routing algorithm uses just a few pre-set road types with associated 'best-guess' average speeds.

If you're right - so that the TT routing is indeed using very localised - and often very incorrect - speed limits, it might explain some of the anomalous routings that have been the subjects of many messages. Of course, as chip speeds increase and more speed limit data becomes available (and thus better estimate of transit time for any one section of the route) I would expect the software to indeed progress to this type of routing method.
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Oldboy · Posted: Sun Feb 03, 2008 8:25 am Post subject:

Andy_P · Posted: Sun Feb 03, 2008 11:58 am Post subject:

So each section of road has a data packet that is "time to travel" and a totally separate packet with "speed limit" (maybe), AND there are "road categories" (9 reduced to 3 by TomTom) which TT use to calculate their journey times?

It would be really interesting if someone from TeleAtlas could write an article about the way these sorts of things work in the map data.
I suppose TeleAtlas would say it was commercially sensitive information, but I'm sure a general overview wouldn't hurt them.

DennisN · Posted: Sun Feb 03, 2008 12:24 pm Post subject:

I can't help thinking how good the navigating would be if TT used all 9 categories instead of just 3 - that seems like a ridiculously limited number - even I can classify roads to better than 3 categories ...

Motorways & big dual carriageways.
Good A roads.
Poor A roads.
Decent "other" roads.
Poor "other" roads.
Residential estate roads.
Downright awful narrow lanes.
Access, but not through route (eg service areas).

That's how I do my own algorithms when I'm flying solo without TT GO, on roads I know.

I think it would be far more relevant if TT could write an article about why they reduce the data from 9 to 3 and how the navigation software deals with it (not very well!).
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xda · Posted: Sun Feb 03, 2008 2:03 pm Post subject:

GerryC · Posted: Sun Feb 03, 2008 2:05 pm Post subject:

I'm only making my assumptions on what I have observed. It could well be that the roads in central Birmingham are at an incorrect "road type" which would allow 40-50mph if possible (ie the straight bits).

It has been while I have been using the demo function that it appears to take notice of the speed limit shown for the road. This could be entirely co-incidental though.

An example of what I mean can been shown by planning a route past Birmingham Airport from N52.45103 W1.76031 to N52.44588 W1.74546. The speed gets up to 40mph then changes to 60mph just after the Main Road junction. I doubt if the road type changes at this point though this could be just the demo being shown and not the route calculation. Confused

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Oldboy · Posted: Sun Feb 03, 2008 5:10 pm Post subject:

Andy_P · Posted: Mon Feb 04, 2008 1:48 pm Post subject:

gslewin · Occasional Visitor Joined: Jun 18, 2006 Posts: 1

I have a tt720t and have noticed that the overspeed alerts when driving past a refuge/island junctions no matter what speed the main road is set at as the limit is set to 30mph whilst passing the junction.

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