Safe Speed Forums

Does anybody know if and how often SIDS are calibrated and how accurate they're supposed to be? The reason I'm wondering is that there's a pair on the A322 not far from me, and the other day I decided to use them to try to get an idea how off my speedo is. They're on a 50 limit dual carriageway (used to be NSL ), so assuming they'd be set to display speed at 10%+2 I set the cruise to 56mph to be reasonably sure of settig the things off, and headed towards one.... but nothing at all. Surely they wouldn't set the trigger speed any higher would they? The alternative is that my speed was still too low to have set it off, so my speedo is probably reading very low. Turned back at the roundabout, flicked the cruise back on and headed towards the other.... 55 flashes at me a few times as I pass.
WTH Any ideas? The first that occurs to me is that I didn't do it right, but I've not got round to having another go and it's not so important that I'm in a rush to do it. The next thought is that the one that didn't do anything may not be working, though I've seen it activate at other times so it would have had to be a fault that particular day. Otherwise it suggests that the two SIDs couldn't agree on what speed I was doing, which would seem odd.

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Make everything as simple as possible, but not simpler - Einstein

they seem to be calibrated to go off at all sorts of speeds. One near me goes of at 25 in a 30, at least it's one of the ones that shows your speed.
Another I've been past went off at an indicated 41 in a 40.

The best thing about these indicators is watching police vehicles slam on their brakes when they start flashing.

I am concerned that they are becoming part of the furniture through overuse - especially ones that pick up the speed of vehicles a long way outside the limit that they are reinforcing. If you are in a 40 limit doing 40 and a 30 SID flashes up while you are still 100 yards outside the 30 limit the level of respect goes down rapidly.

Yeah they seem to vary tremendously. Not sure how accurate their measuring is, to be honest. I've triggered one at 25 in a 30, and the same one did nothing when I went past at 35.

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-mike[F]
Caught in the rush of the crowd, lost in a wall of sound..

I have seen one flashing permanently at an almost stationary traffic queue.

It only takes a few like that and they will lose all effectiveness.

if your speedo is 10% fast.. possible.. then indicated 56 will be more like 51.
if the SID is accurate and set to 10%+2 = 56mph then it won't trigger.


(there's one on a route i run that enjoys telling me i'm doing 47mph or so)

Yes, that was the point - I was trying to find out how off the speedo was. But having got the two different results of the pair of SIDs on the A322 for the same speedo reading I'm no wiser than before but now suspect SID accuracy as well as my speedo. Still haven't got round to trying it again theough.

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Make everything as simple as possible, but not simpler - Einstein

There was a "score board" near me a little while ago. You drive past and it flashes your speed. And I mean *FLASHES*, flash, flash, flash, flash, flash for about 2-3 seconds flashing quite rapidly. It was very distracting.

The first time I went past it I got all excited to see what my score would be but it did nothing.
The second time I was between 2 other cars and couldn't really tell which one it detected. I didn't think it saw me. I'm thinnking at this point that my car is radar proof. That time I decided I'd go round again and give it another go. I think I got up to somewhere around 40 and it said something like 34-36. I went round again later and got up nearer 50, but it was still in the 30s.

I wasn't impressed.

It's disappeared now but it seems there's still a hole to put it back if they want.

GPS is better for checking speedos.

Which reminds me I should check mine...

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Andrew.

The SIDs have gone. Maybe they were skew-wif after all.

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Make everything as simple as possible, but not simpler - Einstein

There is a DfT Advisory leaflet on SID/VAS devices, their deployment and speed settings.
They should be set at the 50th percentile speed so they activate for the fastest half of the traffic. 30 mph VAS signs that activate for all traffic are repeaters and should be altered as they are not allowed.
Don't have any accuracy figures in the spec but I would expect them to be within +/- 1 mph.

That's crazy. What if the 50th percentile is below the limit? Or do you mean 50th percentile / posted limit, whichever is higher?

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Make everything as simple as possible, but not simpler - Einstein

We've had a SID in our village for about 20 years, it has been broken for a total of about 5 of those years. Another local village has just had new LED style signs installed so we've got one of their old ones at the moment.

The activation speed had varied over the years, probably varying as each council engineer who comes to fix it fiddles with the settings. It has varied between 25 and 35mph. Currently it's set just above 30.

Gareth

The ones in Stourbridge can definitely be tripped at something like 25mph in a 30 limit. It is just a shame that the positioning is so bad as they are placed in the middle of a complex set of junctions and are either a distraction that take drivers minds of the complex dance that is required as cars have to get across 3 lanes at this point, or they are totally ignored as they are quite small and get lots in all the other signage clutter.