Smart motorway camera query

zagfles

There's a delay between the speed limit changing and it being enforced, to avoid people having to slam on the brakes when the limit changes. About 10 secs according to the RAC https://www.rac.co.uk/drive/advice/driving-advice/smart-motorways/

Car_54

bengalknights said:

General guidance is 10%+2 based on the police force operating the area, at the same time your dash will overstate speed by 7-10% anyway as they are built this way. So if you did 55 on dash you are most likely doing 49-52

if you're doing 49, and your speedo is showing 55, it is over-reading by12.2% and illegal.

AdrianC

Ant555 said:

daveyjp said:

You won't get a ticket for an indicated 55 in a 50 limit stretch.

If my speedo says 55 on the motorway then the GPS on the Waze app will almost certainly say I am doing 50 (51 at the most) - I dont know which one is most accurate though and I am sure thats why there is a small % leeway for inaccurate speedos.

In almost all circumstances, GPS is more accurate than a speedo.

Speedos are legally allowed to over-read, they are not legally allowed to under-read.

Username03725

There isn't a speed camera on every gantry with a matrix, and IME where a camera is in place they're easy to spot.

You won't be done for an indicated 55 in a 50. As per earlier posts, the guidelines suggest 10% + 2mph as the start point so you'd have to be doing 57 to trigger that, and the law that requires the speedo not to underread at all but can be up to 10% fast means that you were doing no more than 55 and more likely 1, 2 or even 3mph below that so 52-54 at most. My own car shows ~1-2mph fast at around 50, and only 2-3mph faster in the 70s, so don't rely on the 'up to 10%' aas a factor in calculating actual speed. The best way is to have a passenger monitor indicated speed on the speedo and actual speed on sat nav or phone whilst doing a steady speed (cruise control is handy here) on a longish straight road. Neither are 100% accurate but it'll give you a firm idea of the leeway.

Personally I always set the cruise to 55 - 57 or 66-68 when on motorways where a reduced limit is in place. Never seen a ticket yet for doing that.

ontheroad1970

AdrianC said:

Ant555 said:

daveyjp said:

You won't get a ticket for an indicated 55 in a 50 limit stretch.

If my speedo says 55 on the motorway then the GPS on the Waze app will almost certainly say I am doing 50 (51 at the most) - I dont know which one is most accurate though and I am sure thats why there is a small % leeway for inaccurate speedos.

In almost all circumstances, GPS is more accurate than a speedo.

Speedos are legally allowed to over-read, they are not legally allowed to under-read.

Yes and no.  There is usually a lag with GPS.  So it might measure more accurately, but not necessarily in real time.

Jenni_D

ontheroad1970 said:

AdrianC said:

Ant555 said:

daveyjp said:

You won't get a ticket for an indicated 55 in a 50 limit stretch.

If my speedo says 55 on the motorway then the GPS on the Waze app will almost certainly say I am doing 50 (51 at the most) - I dont know which one is most accurate though and I am sure thats why there is a small % leeway for inaccurate speedos.

In almost all circumstances, GPS is more accurate than a speedo.

Speedos are legally allowed to over-read, they are not legally allowed to under-read.

Yes and no.  There is usually a lag with GPS.  So it might measure more accurately, but not necessarily in real time.

But if you use cruise control to do a comparison check then you'll be at a constant (near enough, if you choose your test location suitably) speed so the lag (~1 second?) won't matter if you're gauging speedo reading vs GPS reading.

For my own car:

GPS - Speedo
70 - 75
60 - 64
50 - 54
40 - 43
30 - 32

[Deleted User]

Car_54 said:

bengalknights said:

General guidance is 10%+2 based on the police force operating the area, at the same time your dash will overstate speed by 7-10% anyway as they are built this way. So if you did 55 on dash you are most likely doing 49-52

if you're doing 49, and your speedo is showing 55, it is over-reading by12.2% and illegal.

Speedo can over-read by 10% + 6.25mph under UK (and EU) law. If you are doing 49 it can show up to 60.15mph perfectly legally. It won't be that far out of course, but tyre wear, wrong pressure etc all affect the reading, hence they set it low for safety, around 8-10% is quite normal e.g. I go past a sign saying I am doing 28mph when my speedo shows 30mph

It cannot of course under-read i.e. report you are going slower than you are