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Keep your distance chevrons.
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Border_Collie
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PostPosted: Wed Feb 05, 2014 9:57 pm    Post subject: Keep your distance chevrons. Reply with quote

Out today and noticed the advisory 'Keep 2 chevrons distance apart'.

On a motorway where most people are doing 70+, I thought, 'not much stopping distance in the case of an emergency'. So I counted and found there are 6 chevrons from start to finish between the 100yd/100 metre, markers. So 5 segments = 20 yards apart x 2 = 40 yards.

The highway code shows 105 yards stopping distance, good brakes/tyres dry road etc (I know modern day cars stop quicker than that) so 2 chevrons apart appears poor to me.

e.g. at 70 mph it's 70 feet thinking distance and 245 feet stopping distance. 70 feet is just over 21 yards i.e one chevron, which leaves less than 20 yards to stop from 70.

I wonder who thought 2 chevrons was safe? Driving
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M8TJT
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PostPosted: Wed Feb 05, 2014 11:47 pm    Post subject: Reply with quote

They probably thought that it was a lot safer than the distance that a lot of people tail gate at ;=0
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MaFt
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PostPosted: Wed Feb 05, 2014 11:56 pm    Post subject: Reply with quote

You're looking at it the wrong way. You're making the assumption that the car in front would suddenly stop dead. In reality their car in front would stop more gradually - with both cars still moving at the same time. In general as well, there will be more than 2 chevrons between them as the 2nd driver will need to be behind a chevron in order to see it before it's covered by the bonnet.

If we assume that both cars are travelling at exactly 70mph, with exactly 2 chevrons (42yards) between them - at this point in time there is 0mph difference in speed; they may as well be both sat stationary. All of a sudden a badger magically appears causing the 1st car to slam their brakes on. There is a 21 yard delay (thinking distance) before the 2nd car slams their brakes on which leaves a 21yard gap. Both cars are slowing down but, most importantly, they are both still moving forward. So that an approximately 21 yard gap stays there if they slow down at roughly the same rate.

Hope this makes sense. I can't be bothered trying to draw it :/

MaFt
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M8TJT
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PostPosted: Thu Feb 06, 2014 12:16 am    Post subject: Reply with quote

But what about the slogan "Only a fool breaks the two second rule". Quite clever as it's a variable distance that takes into consideration your speed. Now 70mph = 102ft/sec. 2 secs = 204 ft = 68 yards. So you need to be more than 3 chevrons apart to stick to the 2 second 'rule'. Surprised
So 2 chevrons is safer than tail gating, but too close for real safety.

I see a problem with your analysis:
When cruising at 70, people are not concentrating as much as perhaps they should be therefore it takes a bit longer to react blowing your 70 ft out of the water . Your first driver goes into a full emergency stop to miss the errant badger (instead of leaving a nice bit of road kill) and the following driver does not realised this in the usual reaction time and don't go into a full emergency stop (unless they have auto brake assist) until it'd too late. So not only starting to brake later than he might, he doesn't brake hard enough soon enough. Bent fenders.
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Border_Collie
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PostPosted: Thu Feb 06, 2014 1:20 am    Post subject: Reply with quote

Quote:
Both cars are slowing down but, most importantly, they are both still moving forward. So that an approximately 21 yard gap stays there if they slow down at roughly the same rate


Think about it MaFt. If the first car brakes he has already started slowing down, hit my brakes and it's down to 60 very quickly, the car behind brakes and by the time he hits 60 I'm down to 50 or less, and the slower you're going the quicker you stop, so the car behind will always be going that much faster (Bang).

Put a third car in the line or 4th etc and TWO chevrons is nowhere near enough. I'm sure the 'traits' in France are further apart but could be wrong as I haven't checked any measurements.

In my opinion, for safety, it should be 4 chevrons or even better 5.
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PaulB2005
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PostPosted: Thu Feb 06, 2014 7:47 am    Post subject: Reply with quote

The chevrons are supposed to be 40m apart so 2 chevrons equals 80m between vehicles. I make that 88 yards which is over the 68 yard (2 second rule) needed at 70 MPH calculated below.

Whenever I've gone though these chevrons areas I've found two chevrons seems to be a larger than "normal" gap between cars. Where are these chevrons you saw Border_Collie?

Quote:
Chapter 5 of the Traffic Signs Manual has this to say about the markings and associated signs:

"VEHICLE SEPARATION MARKINGS

22.39 The chevron markings to diagram 1064 (see figure 22-9) are prescribed for use on motorways only.

22.40 The marking is intended to remind drivers to keep a safe distance from the vehicle in front and has been shown to be beneficial in reducing accidents. Chevrons are placed at intervals of 40 metres,ensuring that a safe distance is kept between vehicles travelling at 70 mph as long as drivers keep a minimum of two chevrons apart.
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DennisN
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PostPosted: Thu Feb 06, 2014 8:43 am    Post subject: Reply with quote

Y'know, every time one of these subjects comes up, I marvel at the way you SmartyPantsSouthernerSofties do sums. I can just about work out that at 60mph I do a mile in a minute, but two seconds? No way (I'm already there and Devil take the man with the flag!). Confused
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PaulB2005
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PostPosted: Thu Feb 06, 2014 9:38 pm    Post subject: Reply with quote

This page shows the sort of thing I recall it being like -

http://www.geograph.org.uk/photo/2111433

Doesn't look too close for me.

But I did mis-remember* how it works and I did say...

Quote:
The chevrons are supposed to be 40m apart so 2 chevrons equals 80m between vehicles. I make that 88 yards which is over the 68 yard (2 second rule) needed at 70 MPH calculated below.


And looking at the above it's clear the cars are really 40 meters apart - the distance between 2 chevrons. A look at Google Earth and it's ruler tool has confirmed they are 40 meters apart, so that's 44 yards if I haven't mangled my maths too much which is less than the 2 second rule but looks plenty in the picture above......

I'm scratching my head because I know something does not add up but not sure what... I think.....



* make a right mess of things......
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DennisN
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PostPosted: Thu Feb 06, 2014 11:19 pm    Post subject: Reply with quote

I tried counting them on the M5 today, but lost track. I thought it might be about 5 or 6 between the 100 metre markers, but I'm really not sure. I can't find the site on Google maps.
But there's a set on M4 west of junction 16 (Swindon West), there's a fairly pronounced bend. But they don't show up on overhead satellite view, only on Streetview. I have no idea how to measure the separation distance. Maybe this link will work, who knows. Might the number of lane markers help? CLICK ME

EDIT to add... Road markings diagram 1005.1 seem to be the ones to consider for distance measuring - a "set", being a white line and the gap to the next white line, is 9 metres. I make it 4 or 4.5 sets, which would support the 40 metres between chevrons. Tomorrow, I might think about learning how to do the softysouthernerfancypants sums to do with speed other than 1 mile per minute.
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DennisN
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PostPosted: Thu Feb 06, 2014 11:45 pm    Post subject: Reply with quote

PaulB2005 wrote:
And looking at the above it's clear the cars are really 40 meters apart - the distance between 2 chevrons. A look at Google Earth and it's ruler tool has confirmed they are 40 meters apart, so that's 44 yards if I haven't mangled my maths too much which is less than the 2 second rule but looks plenty in the picture above......

I'm scratching my head because I know something does not add up but not sure what... I think.....
I guess "they" mean two chevrons as in "have your bonnet on one and see two more ahead of you" - that would be 80 metres.

Thus (red V means chevron - I can't use the right thingy on my keyboard).... CARV gap V gap VCAR [For those not capable of following the technicalities of this diagram, I have no idea how to make it intelligible :-)) ]
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PaulB2005
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PostPosted: Thu Feb 06, 2014 11:47 pm    Post subject: Reply with quote

The "100 meter marker" is a misnomer. Now the are usually 500 meters apart. Where road side furniture stops placement on the 500 meter mark they can reduce the distance to 300 or 400 meters.

Looking at the StreetView I believe it reinforces the official information that the distance between chevrons is 40 meters and a safe distance between vehicles travelling at 70mph. You can see this in Dennis's photo as the middle lane car behind the StreetView car is just going over a chevron and the StreetView car hasn't yet gone over one. The gap between the two vehicles seems fine to me and further than most drivers are behind me on the motorway.


Last edited by PaulB2005 on Fri Feb 07, 2014 12:00 am; edited 1 time in total
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PaulB2005
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PostPosted: Thu Feb 06, 2014 11:55 pm    Post subject: Reply with quote

DennisN wrote:
PaulB2005 wrote:
And looking at the above it's clear the cars are really 40 meters apart - the distance between 2 chevrons. A look at Google Earth and it's ruler tool has confirmed they are 40 meters apart, so that's 44 yards if I haven't mangled my maths too muc which is less than the 2 second rule but looks plenty in the picture above......

I'm scratching my head because I know something does not add up but not sure what... I think.....
I guess "they" mean two chevrons as in "have your bonnet on one and see two more ahead of you" - that would be 80 metres.

Thus (red V means chevron - I can't use the right thingy on my keyboard).... CARV gap V gap VCAR [For those not capable of following the technicalities of this diagram, I have no idea how to make it intelligible :-)) ]


I fear not. The road side sign simulates the view from your windscreen. Only two chevrons between you and the car in front so its 40 meters. See http://www.blueschoolofmotoring.com/Driving-Lessons-Keeping-Space-2-Second-Rule/
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DennisN
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PostPosted: Fri Feb 07, 2014 12:00 am    Post subject: Reply with quote

The 100 metre marker I'm talking about is the little sticks/pegs at the edge - marked 525.1, then the next 525.2, 525.3 etc. They also carry a little symbol of a phone and an arrow showing which way to walk to the nearest one. The main larger, taller ones (500 metres apart) are marked 525.1, then 525.6 (or something similar). On reflection, I think I must have been trying to count between 500m markers. (I know of these thingies because I used to have to buy and stock the little numerals etc for us to use to replace any damaged ones). (Thirty five years ago? - seems like yesterday! Rolling Eyes )
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DennisN
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PostPosted: Fri Feb 07, 2014 12:20 am    Post subject: Reply with quote

PaulB2005 wrote:
I fear not. The road side sign simulates the view from your windscreen. Only two chevrons between you and the car in front so its 40 meters. See http://www.blueschoolofmotoring.com/Driving-Lessons-Keeping-Space-2-Second-Rule/

I did have only two chevrons between my cars (the first one has its bonnet at the start of the chevron - I didn't know how to draw it on top of the mark which really should have been under the letter R), second car has its back bumper over the start of the second). The way I see the picture in your link is, in the middle lane, two cars are each in the middle of the chevron gap, so they are two gaps/sets apart as in my drawing ("drawing"? well, diagram or summink), therefore 80 metres.

Why am I arguing? - I'm a technical dumbo!!


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PaulB2005
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PostPosted: Fri Feb 07, 2014 12:35 am    Post subject: Reply with quote

Ah i see. However I see the sign as you are about to drive over one chevron and the car in front has just passed over the second chevron. Ie 40 meters apart.

Funnily enough this discussion / argument is repeated on many driving forums. And I still can't find a clear and definitive answer.
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