The whole point of the amber is to give you time to stop safely before the red.
So how does the maths work out?
Taking 30mph as an example, you are travelling at 44ft per second.
Assuming deceleration is even then your average speed over the braking distance is 22ft per second.
Stopping distance from 30mph on a good road always used to be quoted at 75ft in the Highway code.
I believe that included a one second reaction time (someone may confirm), so knock off 44 feet, since the reaction time is at unreduced speed.
That gives a braking distance of 31 ft.
31 divided by 22 is 1.4 seconds.
Add back your one second reaction time and you get 2.4 seconds minimum on a good road.
On a wet road or at higher speeds you need more time.
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We need some system of showing how long you have before the lights change.
What we already have works. It's people who decide to "jump" the lights who are at fault/the problem. They should take driving lessons.
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it's a lack of awareness that cause problems. A green light can only do one thing, change to amber, then red. If you're approaching a green light and it's been like it for a while...what's it going to do...change.
If you're unsure how long it's going to take to change, slow down a bit to give yourself more leeway.
The problem arises because people actually speed up to try to 'make' the light before it changes, which if they're unlucky means they get caught out at a higher speed than they should be, meaning either a lucky escape barrelling through too fast..or... doing a red light and increasingly nowadays they're covered by cameras.
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Interesting to compare driving in Florida recently to driving around at home, in Cheshire.
In the US, the lights generally had very long cycles - if you see a green ahead then chances are you'll get through it. In the UK, you seem to have to stop at pretty well every set of lights you come to.
Lifting off, as suggested earlier, can make some intelligent lights on fast main roads change - they look for a gap in the traffic.
In Chester itself (and I lived in leeds for a while and that was the same) many of the lights are on extremely short cycles - amber seems to flash through very quickly, and the lights in the other direction are changing to green as the first set are coming to red, so you almost get synchronised driving through junctions.
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Here's some sums.
v_mph=[10 20 30 40 50 60 70 80 90 100]'; v_ms=v_mph.*(1609.3/3600); a=0.65*9.81; % assume 0.65g braking t=v_ms./a; % time taken to brake s=(v_ms.^2)./(2*a); % distance taken to brake [v_mph round(t) round(s)]
ans =
10 # 1 # 2
20 # 1 # 6
30 # 2 # 14
40 # 3 # 25
50 # 4 # 39
60 # 4 # 56
70 # 5 # 77
80 # 6 # 100
90 # 6 # 127
100 # 7 # 157
Hashes added to make the table at least come close to being aligned.
All of these numbers assume maximum braking of 0.65 * g, which is a reasonable value for the co-efficient of friction between locked tyre and road under normal conditions.
As the stopping time is linear in v, the stopping time from 100 mph is only 10 times that at 10 mph, and so, for UK legal speeds a 6 second amber delay should be sufficient for all traffic lights.
What is more difficult is that the stopping distances are proportional to v^2. This means that you need to be making your decision whether to stop or go much further away from the lights at high speed. Small errors and delays in making this judgement at speed will have a large effect upon your capability to stop.
One of the odd effects of decelerating on the run up to lights is that it effectively lengthens the period of time in which you need to react quickly to a sudden change to amber.
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'a sudden change to amber.'
but this is one of the prerequisites of motoring - it's not a sudden change, it's a 50% chance of change and you should be driving accordingly.
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But whatever the chance is, the problem is STILL that the ambers are uncalibrated, and not standardised, and can be misused for a supposed road safety stealth tax while actually making the roads more dangerous.
But at least the government can point to another group of motorists who support its laws, or, rather, their misuse of OUR laws, regardless of the effects or consequences.
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how can it be a problem? amber means STOP - end of story.
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I was caught for this a few years back. In a 50mph limit the amber period was 2.8 sec.
I tried to tell the court that this interval was inappropriate for a 50mph area. The magistrates disagreed and told me that all drivers should approach every green light as if it was just going to change. Going by Highway code stopping distances I believe that this means in wet weather we should all drive up to a green light at 24mph. Is that really what they want us to do?
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Thanks, Number Cruncher.
I think that supports my original point, that the amber period needs to be related to the speed limit.
If I read it right then in a 50 limit (4.39 secs.) the time needs to be double that for a 30 limit (2.14 secs).
I also note that you use a coefficient of friction for a locked tyre and road i.e. an emergency stop.
Surely times should allow for a controlled stop?
And allow for a wet road, not dry, so add maybe 50% to the times?
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>>i.e. an emergency stop
That co-efficient is very close to that used in the highway code to calculate the braking distances - it's a reasonable starting point, which can easily be exceeded on a dry road.
I'm not a probability expert, but, I'm sure the probability of a green light turning amber ... now, is not 50%. Of course, that it will turn to amber in the near future has a very high probability.
A green light does suddenly change to amber, there's no warning! Unless you now the timing and phasing of the lights beforehand, and you saw when the light turned green, you can't know when that sudden change back to amber will happen.
I agree that amber means stop - no debate there.
What amazes me is how often we get this decision during the approach to traffic lights right, as there's obviously a point where if you see the lights change to amber you still can't stop, no matter how quickly you apply the brakes, and you would be better off keeping going. We tend to be pretty good at making that judgement call.
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I think many traffic lights now use some inteliigence/sensors - it's not just fixed timings.
So it's difficult to predict when the lights will change - it may depend on other traffic arriving at the intersection.
NC makes a good point about the decision we all make:
I make a judgement as I approach - If it changes to amber before I pass my 'braking marker' then I stop - else I go through. That 'braking marker' is certainly speed related.
I never go through the lights anywhere near red (unless I'm fast asleep :-) )
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I do the same as you, Steve, I can't understand what's so difficult about it... its only a basic judgement, if you know your car then its easy.
We have a set of lights like that near to me where two A roads join, you can be going along and if someone coming along the other road hits the sensor first you have to stop!
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Traffic Lights seem to be purely advisory these days.....people seem to go whatever the colour and its getting worse...
Green = Go ( speed up in case they change)
Amber = Go Faster to get through before they go red
Red = **** it , just another two then....
Particularly annoying for me are the lorry / bus drivers who stick the nose through on the amber and by the time they get the tail over the junction the lights have changed and they have delayed the cars on the other carriageway.
Pure selfishness IMO.
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Why not just have two lights, but the green flashes increasingly rapidly in the countdown to red? Like the timer on a camera.
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Seeing as you mention it, I would be in favour of cutting out the red/amber phase, as on the Continent, (i.e. go straight from red to green) which should stop the takeoff before green merchants and reduce the risk of meeting someone late in stopping on the amber on the other road.
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I really do think the amber light is a throwback to the days of more sedate driving and probably outlived its usefulness - who's going to start a campaign for red/green only then we can consign that 'amber gamblers' phrase to the dustbin of history?
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