Mis-behaviour of speed limit recognition devices

[polygonum_on_google]

Just got back from a little trip in the car. It has speed limit sign recognition built-in. Most of the time, it works pretty well. But today it was going crazy. Missing some signs. Telling me I am over-limit when I go from a 20 zone to a 30 zone and was doing under 20 ? though it did ?see? the sign correctly! Not changing from indicating over-limit even when I dropped speed even further. (At no point was I actually over-limit - according to speedo and me!)

A few miles down the road, back to working entirely properly.

Now I could take all sorts of wild guesses as to the cause, and usually I would. The thing is, it misbehaved in exactly the same way, on the same bit of road, a week or two ago. Not just a straight road but a very windy route that varies from 60 through 30 and 20 several times, back up to 60, then 40 and finally 60 again. At no point in-between has it appeared to misbehave at all. On both occasions I had driver well over 20 miles before getting to this location ? and about 20 miles after ? and worked perfectly.

While these devices are merely assistants to the driver, it is no big deal. If they start to actually control speed, this could be quite an issue.

Any ideas what could be causing these issues?

[Tim+]

Just a thought, is it definitely reading signs or is it a GPS device looking up a database of speed limits? There are things that screw up GPS reception, low flying planes for one!

Tim

[polygonum_on_google]

Well, I am certain that it does read signs. For example, where a sign was not visible (fallen down, covered by a tree, etc.) it simply misses it and catches up on the next one it can see. Where a sign is advisory and not mandatory (e.g. max speed 30 mph round a bend when speed limit is 60 mph and has a black border), it treats it a bona fide speed limit sign. And it fails to recognise those areas which have street lighting but no signs as 30 mph - which is what they are. That is, in every way it seems to react as a vision-based device.

We get very few low flying aircraft (thankfully). It would be a real surprise to have low flying aircraft at the same location on two separate occasions and nowhere else. And not hear them!

[Tim+]

I wasn?t suggesting that low flying aircraft are the only cause, just one I have personal experience of as I often go running near the approach to an airport. My GPS accuracy goes way off when a plane comes in to land.

Can?t explain your problem however.

Tim

[polygonum_on_google]

Yep - I realise that - but it was one thing I could discount!

There is radar operating in the area, but even so, I find it difficult to understand how that could cause the issues. :-)

[Mr Pounder Esquire]

Not much help. But when I used to drive by the same location the hatchback on a Nissan Micra used to flip up.

[DerbyBorn]

polygonum_on_google [e-mail] wrote in news: [e-mail]:

Dirty windscreen - or reading signs off junctions.

[Brian Gaff]

Well it depends on what they depend I suppose. One might suspect that if its just optical it might get dazzled by different illumination effects in different weathers etc.

As I said the other day, I bet a lot of houses on rat runs will start having house names like 10MPH if this goes ahead. Brian

[polygonum_on_google]

Windscreen was clean and the issue only occurred on a stretch of around three miles of road. On teh first occasion, I did put it down to dirt on screen but hadn't thought much more about that. Now I don't beleieve it.

Mis-reading signs would not cause it to report that I am over-limit when I am actually doing under 20, in a 20 zone that is has recognised, go into a 30 zone, me check it has seen that sign - which it has - the number changes from 20 to 30, and continue to say I am over-limit.

[polygonum_on_google]

Yes, Brian. But it is obviously "seeing" the signs as the speed limit shown does change properly at almost every sign. (I think there was one it missed on an incredibly tight bend.)

[Roger Hayter]

RF interference a possibility?

[Robin]

I hope it's not wholly pedantic to suggest that the problem seems to be not so much mis-behaviour of the speed limit /recognition/ device as mis-behaviour of speed limit /warning/ device. Is there any chance that that does not take the speed from the speedometer alone but from that plus GPS*? (FTAOD I mean computing the speed from GPS, not using GPS to look up the location.) That could of course be tested by passengers monitoring the speed shown by GPS as you drive the section of road.

*I assume never GPS alone to deal with tunnels. But I suppose it could be "only use speedometer when no GPS" when it ought to have been "no /good/ GPS".

[soup]

<mode:tin foil hat>

See! UFOs do exist. <mode:off>

[polygonum_on_google]

Sure as hell, I didn't identify them.

[polygonum_on_google]

I'd be happy to think it is a possibility. But I can't think of an obvious source or mechanism.

[polygonum_on_google]

No - not pedantic. Agreed it is primarily the interpretation of the speed/speed limit as being over-limit. Will check the GPS-indicated speed. (Come to think of it, although I am used ot seeing GPS speed on my phone statnav, I can't remember noticing it on the in-built one.)

[John Rumm]

Could be an EMC issue - system being overwhelmed by strong RF etc...

[polygonum_on_google]

I can't think of any obvious reasons for there being high RF there. But in the absence of other explanations, it deserves more consideration and investigation.