Recommended speed signs

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bikeman2, Nov 22, 2:03am
sorry if this has been done before but its been bugging me for a while

the recommended speed signs on corners.does anyone know how they work out what corners should get them and if so what speed they should be!

i have noticed some recently that are signposted 55 but can quite comfortably go round at 100 and see clearly, no side roads etc. but then a couple of ks down the road there is another 55 which really is a 55 and not nice at all to push it much past that.

just curious if there is some kind of "formula" nzta use or whether the guys just have a handful of signs to use and put them where they look like they could be useful

cheers
rayzor14, Nov 22, 2:15am
No idea on formula but I can tell you that one thing easily overlooked is the impact of weather conditions on some corners of this nature.
I know a LOT of corners in the central north island that can be driven round much quicker than the signposted recommendation - but try doing it in the wet at even 10% over that recommendation and say goodbye.
How they arrive at the recommendation is anyones guess though.
db.price, Nov 22, 2:19am
The suggested speed is notmally based on 2 things.For cars this is the recommended corning speed in the wet.The signs are also for trucks and is the speed recommended for them in any condition.

Certainly MOST of these corners can be handles at 10-15km more than the recommended limit in good dry conditions
bikeman2, Nov 22, 2:24am
ah yeah thats a good point, i suppose I was just thinking of them in the dry but your right when its wet etc you slow down. and hadn't considered the truck side of it (or tourists in motorhomes)

I suppose they are good also not because they set out a speed but for people not paying attention or unfamiliar with the road they think "oh s**t better take this one easy"
saxman99, Nov 22, 3:35am
They seem to cater pretty much for the "worst case scenario". (pissing rain at night in a top-heavy van with no suspension or brakes)There is a trader on here called Tintop who seems to know a fair bit about how it's done - I think perhaps he is on a team that works out the recommendation for each corner.From memory he talked about using various bits of hardware attached to a vehicle to measure lateral loading, g force, incline, angles etc etc.So there is some kind of formula that is used, although I agree with #1, sometimes it's hard to see how they they arrive at it.
jono2912, Nov 22, 4:06am
^^ What he said.
rob_man, Nov 22, 4:08am
I convert them to mph and they're about right.
fordcrzy, Nov 22, 4:56am
in an MX5 you DOUBLE the posted speed and its about right.:)
kazbanz, Nov 22, 7:18pm
The speed sighns that really confuse me are the 95km/h ones
ontwowheels, Nov 22, 7:57pm
agreed, i saw one of these last week, and as I went round the corner I thought 'what the hell, this is at least a 97kmh corner'
jenny188, Nov 22, 8:07pm
in the old days (60's) someone from the AA drove round the same corner 3 times while his passenger watched the swing of a pendulum. From this an average swing was calculated and a safe speed was derived at. Now days I don't know. Maybe axial tilt from Google Earth map!
crzyhrse, Nov 22, 9:23pm
They place a pallet of 200-series blocks on the roof of a Combi van then drive around the corner until there's no body roll. At least, that's what they seem to do.
romulan7, Nov 22, 9:25pm
An accident blackspot near my location had the cornering speed dropped from 85 to 65kph after the death of a motorist who hit a truck.It seems to have worked there have been no serious incidents on that bridge since.
pauldw, Nov 22, 9:36pm
I remember touring the South Island years ago and all the signs seemed so conservative. Then we struck a 15mph sign at Denniston and found that the "corner" was equivalent to a tight U-turn. Some signs need a "we really mean it" marker.
a.woodrow, Nov 22, 9:49pm
Damn I was going to post about these too 15k corners they aren't kidding
tuttyclan, Nov 22, 11:32pm
Why do they put up open road signs (speeds up to 100k) for a gravel goat track with tight twisty corners.Do they want us to be rally drivers or something.
richard198, Nov 22, 11:34pm
"It's not a target."
therafter1, Nov 23, 12:11am
That is my problem with them, consistency, or should I say, lack of consistency, you will get half a dozen in a row and you can safely double the advisory speed, and then you will get one where the advisory speed can be about right, or even on the limit for the corner, either get it right or get rid of them.

At the end of the day I do not see the point of them anyway as I drive to prevailing conditions and to my own personal limitations. I also spend most of my time avoiding main roads and use secondary roads, most of which are devoid of yellow lines and advisory speed signs for corners and I seem to manage O/K on those, so why the necessity for them on the ???main??
richardmayes, Nov 23, 12:12am
The issue is what the highway designers call "SPEED ENVIRONMENT"

On a hill road where you have 40km/h to 60km/h bends every 50 metres for 20km, they're not going to put a speed advisory on every one, even though the speed limit is 100 it's assumed that after a wee while you'll figure out that it's not a fast road.

However, on a super-highway where there are wide shoulders to the sides of the lanes, and most of the curves are 300km/h curves, anything a bit tighter gets an advisory sign even if its for 95km/h. This is so that the slightly tighter one doesn't catch you out.
richardmayes, Nov 23, 12:16am
PS: And FWIW I think having these signs is MUCH better than not having them.

On a clear day, when I'm driving my own car by myself, I can usually do a much better speed through most highway curves than what the advisory sigh suggests.

However, I recently drove a large motorhome on an unfamiliar highway, in the rain, at night.
On that trip it was EXTREMELY helpful that the really serious corners had speed advisory signs that could be read from hundreds of metres away!
big.b-lil.c, Nov 23, 12:52am
i have a idea that its the speed that generates .3g
pup2, Nov 23, 1:02am
They put a device in the car that measures g force. They then drive round the corner lowering their speed until they only pull a certain number of G's. Can't remember the G force number. Simple really. But yea, some corners do seam tighter than others when they both have the same speed posted
rob_man, Nov 23, 1:02am
I remember a film we were shown at school and there was a box they had on the front seat of an A40 Somersault which they were using to establish the advisory speed.
I wonder if they still use the same combination.
gunhand, Nov 23, 5:31am
I also got caught by 70kph signs that I could do an easy 120kph plus around on the old Katana 11OO years back. There were 4 in a row and the top one was really really off camber, not tight at all.I fell off. And according to a guy sitting at a lay by who wandered over I certainly wasnt the first. Since been fixed. And as some have said some signs really do mean what they say, especially up the west coast.
clark20, Nov 23, 5:43am
They are for the "speeders" ie they can't put 100 as it is the limit so they show 95 so you don't get the 130k brigade falling off.