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Stopping Distances
tealady
Posts: 26,268 Forum Member
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My maths course came up with this in a chapter on modelling:
Most cars with brakes in good condition can achieve a braking force corresponding to a = −8ms −2. The minimum requirement for passing the annual UK vehicle test corresponds to only a = −5ms −2. The UK HighwayCode gives a table of stopping distances which are based on an acceleration of about a = −6.56 ms−2 .
The stopping distances in the highway code haven't changed for at least 30 years but cars, brakes and tyres have improved.
So what are the real stopping distances and does the "2 second" rule need to be called the 1.5 second rule (or other) ?
Most cars with brakes in good condition can achieve a braking force corresponding to a = −8ms −2. The minimum requirement for passing the annual UK vehicle test corresponds to only a = −5ms −2. The UK HighwayCode gives a table of stopping distances which are based on an acceleration of about a = −6.56 ms−2 .
The stopping distances in the highway code haven't changed for at least 30 years but cars, brakes and tyres have improved.
So what are the real stopping distances and does the "2 second" rule need to be called the 1.5 second rule (or other) ?
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They did a test on this subject on Top Gear a few years back.
http://www.youtube.com/watch?v=KGkKDaYd3Mo
The conclusion - if you are going to walk out in front of a car - make sure its a fast one
I stick to around 2 car lengths behind vehicles and the 2 second rule.
I suppose I have thought about it more, as whenever I leave a gap on the A12 or M25, someone always pulls in to it, so I am left wondering if I can stop in time.
2 car lengths is dangerously close.
That's what you should do on escalators too.:D
I was on one yesterday at a tube station and it nearly ended in a close call at the bottom of it:D.
Here's the table of stopping distances from the 1946 edition of the Highway Code. Look familiar?
The upshot of the figures being quoted in your maths course is that the Highway Code gives a stopping distance from 70mph of 315 feet (about 20 Mondeos) in perfect conditions. With "good" tyres and brakes, and again in perfect conditions, you might be able to stop in 270 feet - or about 3 Mondeos less. But to pass the MOT, your brakes only need to be able to stop you within 390 feet, which is 6 Mondeos more.
At 30mph, the official distance is 40 feet, but the question of whether your brakes / tyres are "good" or "just legal" still makes a car-length's difference either side of that.
To conclude - if you're relying on stopping distances to come to a halt with a few inches to spare... you're too close.
The industry is moving more and more toward 'eco' tyres. These have been shown in a number of studies to provide inferior grip to mainstream tyres due to the inferior grip due to decreased rolling resistance.
For example, a recent 5th Gear showed an expensive Continental eco tyre taking 7m longer to stop from 40mph in wet conditions than a fairly bog-standard budget Matador tyre.
death rates have declined consistantly for decades
we need to take the human out of the loop
funny that insurance has rocketed because of shister lawyers tho
Some cars have - our Volvo has a pedestrian detection system that has cameras to detect both pedestrians and cars in front. If it detects a collision is likely and the human behind the wheel either hasn't braked or isn't braking hard enough - it will take over.
Such systems will likely be common place in a few years.
(x/20 + 1) * x
so at say 60mph
60/20 =3
+1 = 4
4 * 60 = 240 feet
they actually use the wrong distance. at 70mph it's 315 feet. i guess they were taking off the reaction time.
Yep - they weren't reacting to danger - just braking at a pre-defined point, therefore the thinking/reaction distance doesn't come into play.
Common sense suggests that road conditions have deteriorated by roughly the same amount that cars have improved so the same rules of thumb can apply.
There was an article about speed in a DeLorean in the Guardian last week - https://discussion.theguardian.com/comment-permalink/83315054 and some one claimed that a BMW 318d will stop in 36m.
I had a quick google to see if there had been any real world tests done, and I found an article on auto express unfortunately back in 2011.
The Mercedes S320 CD stopped in 47.7 meters. I have no idea really about cars and whilst they don't seem to be in a totally different class the Mercedes takes an around 32% greater distance to stop. Strangely the BMW X6 an SUV has a shorter distance of 45.7 meters.
Then a more recent article on super minis (2012) says that the VW Polo stops in 34.2m.
I can't find anything newer than that. Now the tests must include thinking distance, but would the 36m stated for the 318d be excluding that? The weight of a Polo must be a lot less than the BMW, can breaking systems really make up for the weight difference?
It looks like the weight of the Mercedes is 1955kg vs 1,505kg of the BMW so maybe that's what makes the difference, but I have no proof of the 36m stated for the BMW318. The Vauxhall Insignia is a similar weight to the BMW but I would not imagine it is in the same group as the BMW as it is only a family car.
There's some joke in there about the DeLorean having the longest stopping distance of all as it was slowing down in 1985 even though it started off in 1885, but I can't really be arsed making it work and it's not that funny anyway.
I nearly did likewise - at the top of it..!
A very good demo of this was shown on a programme David Coulthard made together with Guy Martin. Coulthard arrived at the track with an F1 car and Martin brought his race bike along.
The braking distance test from 100mph to a stop proved interesting. Both came fairly close but the F1 car beat the bike on that.
Look what happens with the many supercars flying around on the roads. Well engineered but driven by unskilled drivers with poor reaction times.
Personally I don't understand why the majority of drivers drive so close. They're like sheep driving around in little flocks. There's little advantage in driving close to the car in front unless there's the imminent opportunity to overtake. Personally I don't drive around in these little convoys that most people seem to favour. I always leave a gap of two, three or even four times the distance most people seem to drive at. I find it makes for a more relaxing drive. My attention is not solely focused on the rear end of the car in front. I can see past the car and take in what's happening further up the line or elsewhere on the road. If something happens I'm not dependent on stopping distance. I've got time to consider what to do. Very often I don't have to take any evasive action of break at all because of the increased distance.