Is it more efficient, in terms of getting the traffic flowing again as quickly as possible, to queue at a red light nose to tail, or to leave a gap of half-one car length between cars?

Assuming that the number of vehicles is more vehicles than can get through on one turn of the lights then nose to tail would be more efficient.

That assumes they stay nose to tail through the lights too.

Too many other variables, such as the person in front on the phone/doing make up/eating breakfast*. It moves best when everybody moves off at once, which I suppose needs a small gap, but that’ll never happen with the current standard of driving.

* That said, these people are handy to cut in front of when overtaking the queue on a bike and the lights change before you get to the front

Probably the later if people all start to move when the green light lights. But they won’t and they don’t.

I’m not sure it really matters – what does matter is how people move off from stationary, and that’s not really dependent on the gap they leave when they stop.

Doubt there’s anything in it. You still leave basically the same gap between the cars as they go through the lights anyway.
So if you stop right behind the car in front you have to wait for them to move off before you can, while if you leave a gap you can start moving at the same time or even before – but the end result is the same.

Now what really annoys me are people who seem unable to stop on the line regardless of speed and always stop half a car length over it. What’s all that about?

Not really sure …hang on a sec.. the lights have turned to green..

Does it matter? The lights change on timing or other traffic flow related systems so queuing is simply a stack waiting to move..

Not too sure what your point is.

If the lights are on sensors like the ones near where I live, nose to tail every time – the slightest gap and they go back to red again after about three cars, keep tight together and move quickly and they let through 10/12 cars. Drives me up the wall when the person in front pulls away really slowly then I get stuck in another sequence.

I’m a cyclist. I don’t stop at lights.

vinnyeh – Member

Is it more efficient, in terms of getting the traffic flowing again as quickly as possible, to queue at a red light nose to tail, or to leave a gap of half-one car length between cars?

You have to leave a one and a half car gap iirc – in the event of a breakdown the entire queue won’t be stuck.

It’s *much* more efficient if they all leave their cars at home and cycle to work instead, or take a bus.

Aha, a classic road capacity (maximum design capacity of a given roadway) versus effective capacity (the capacity available after many random influences such as the driver behaviour of individuals, changing road conditions and weather) debate. snarf snarf.

UTC SCOOT adapts signal timings to traffic requirements so it doesn’t make a difference. When I worked in that world around 50% of signal in London were on it and most signals rarely operate in isolation now. basically with effective capacity, we don’t know.

the answer is roundabouts modelled with aggressive driver behaviour 🙂 a transport planner in joke. kill me now

Whichever is better, dont forge tyres and tarmac! Otherwise if something happens with the vehicle in front you’ll find yourself a bit stuck and very pissed off! also gives the driver in front chance to not be a morron and roll backwards into you quite so easily!

Large gap every time.

Assuming all cars accelerate at the same pace (for simplicity really) we’ll use some “magic” figures

Lights change for 30 seconds
Target speed is 15m/s (33mph)
Linear acceleration of 3m/s^2
Driving distance is 7m (roughly 1.5 car lengths)

From a standing start with full distance maintained, all cars accelerate together when the lights change and maintain their distance.

So…

Maximum distance from lights for a car to get through is:

15/3 = 5 seconds accelerating :. 25 seconds at 15m/s

Distance under acceleration = 15*5/2+0*5=37.5m (0.5*(change in V)*T + initial V*T because it’s linear)
Distance @15 = 25*15=375 (V*T)

D=375+37.5= 412.5m
Based on car length of 7m/1.5 + spacing of 7m thats roughly 412/12+1 (front car)= 35 cars.

From standing start, nose to tail, the cars accelerate in sequence allowing the car infront to get clear.

To achieve a space of 7m between cars at 15m/s
Front car accelerates for 2 seconds before 2nd car moves etc down the line.

Total distance from the lights to the noise of any car is (n-1)*4.5
Distance covered by any car is ([total time]-[delay]-[acc. time])*V+[distance under acceleration] so (30-(n-1)*2-5)*15+37.5

Car 13 covers a total distance on 22.5m, it is 58.5m from the lights.
Car 12 covers a distance of 52.5 from 54m back, well let him go through on orange.

So 12 cars, so roughly 1/3rd the number.

(Edit as above though none of this works as most people can’t drive)

Neither makes the slightest bit of difference.

The in-efficiency is because the drivers aren’t paying any attention what so ever.

Despite sitting in the queue for the lights, almost certainly moaning “these Bl**dy lights, it takes ages to get through them” when they get to their turn to drive through, they dawdle along taking at least twice as long as necessary to get through.

Hmm, think there could be some sort of connection there?? 😉

If we’re doing annoyances, then the people who brake to a stop too soon, and then creep forwards again. If I have a big gap in front of me it’s because somebody has done that and I’ve given up wearing out my clutch and brakes sheeping behind them.

F1 starts should be part of the driving test, imo.

I’m doing the Vettel start the next time I’m at the lights.

Sorry basic error – delay is 1second not 2 (final distance of 7m not waiting for a gap of 7m then move off which actually gives a gap of 24m at full speed.) So nose to tail gives 21 cars total 97.5m.

Still significantly lower than 35 though.

Big gap.
1) If the car in front has a problem a decent gap will allow you to pass easily. Gap is just small enough to stop some dimwit overtaking then squeezing in so about the length of my car, a Suzuki Jimny.

2) A decent gap allows a motorcycle to filter and slot in safely.
3) Just to annoy the person above. 😆 I often stop with a big gap or half a length from the line. Then when the vehicle behind has settled I move on. The idea is to create the big gap that I want. Getting closer to the car in front serves no purpose. Its just stupid.
” If I have a big gap in front of me it’s because somebody has done that and I’ve given up wearing out my clutch and brakes sheeping behind”. That the idea.

You should always be far enough back that you can see the rear wheels of the car in front, so that you have space to steer out if they stall or terrorists start shooting at you.

Doesn’t everyones car have Launch Control?

Assuming all cars accelerate at the same pace

everything else after that kind of falls away.

vinnyeh – Member
Is it more efficient, in terms of getting the traffic flowing again as quickly as possible, to queue at a red light nose to tail, or to leave a gap of half-one car length between cars?

suggest multiply the result by the number of people in the car?

I’m a cyclist. I don’t stop at lights.

Even in your car?

soundninjauk – Member
I’m a cyclist. I don’t stop at lights.
Even in your car?

less so I’d have thought…after all most cyclists also drive

i bore the teen antigees by saying “look there’s a cyclist driving a car”

how do you know? “got no lights on”

On a speed awareness course (I know) the suggestion was to leave a big gap when you joined a queue then keep an eye on your rear view mirror just in case the driver behind wasn’t paying attention and you needed to move forward. Once you’d a car or two’s buffer behind you then move forward to the point where you could still see the tyres of the vehicle in front.

I’ve found that when that sort of distance from the vehicle in front I’m able to start moving at the same time as they do then as my speed increases I can ease back to the 2 seconds distance.

Cars behind me were too tight to pull round when my Volvo wouldn’t re-start (turned off with the key as I could see how long the roadworks were). They seemed very annoyed I couldn’t move off, more annoyed when I realised it was due to it not starting in Drive and it fired into life in Neutral taking off just as the lights went from green to amber letting only me through!!

If everyone is using launch control then no gap

I only have 30 launch controls left due to manufacturers limit.

More launch control!

howsyourdad1 – Member

Assuming all cars accelerate at the same pace

everything else after that kind of falls away.[/quote]

For acceleration over a sample of 35 cars in the same queue, I’d think the deviation in rate won’t be that wide, certainly given you can only accelerate either at the same rate or slower than the car in front without rear ending them (esp. when nose to tail). So really, I doubt it’s a terrible assumption that they all accelerate at the mean rate.

Second it doesn’t actually matter so long as the sample is the same. 35 cars accelerating at 35 different rates it will produce the same result in terms of which is better so long as it’s the same sequence of 35 doing the same thing. The only way to get nose to tail more* effective is to make the cars in that sample obey very different constraints to the other (eg acetate faster, lights change longer, allow smaller stopping distance etc.).

The actual numbers may differ (5 cars only in total as car d is really slow of the mark for instance, or zero as car a stalls) but the result is the same, a maintainable gap is not less efficient)

Even with only one car, on a drag strip, essentially it boils down to D=V(T-d) given V is effectively constant (if acceleration doesn’t change between runs, yes ideal scenario etc.), there is never a point at which any value of d (delay) other than zero gives D max. Keeping your maintained distance when stopping at the lights makes d=0. To make the delay smaller if you’re having to let a gap open up, you decrease the maintained distance you’re aiming to achieve (which means more cars in the same maximum space in the other example), the best you can hope for is 0 delay which is zero maintained distance. In which case the hypothetical is the same and both allow (a(aT)/2+(T-aT)V)/(car length) vehicles through the lights.

Most folk in my parish view traffic lights as some kind of driver comfort break and an excuse to check some online food websites as most of them tend to be rather fat, once they have checked the food websites they then go on to look for the nearest Mr Clutch.

Tyres and tarmac is what Mrs Funk always told me. When in traffic always make sure you can see tyres and a bit of road. She was an advanced driving instructor.

But there is only so much road in which to queue before the queue starts affecting other junctions. So surely packing the cars in improves traffic flow overall?

molgrips – Member
But there is only so much road in which to queue before the queue starts affecting other junctions. So surely packing the cars in improves traffic flow overall?

Yes but more packed means slower traffic* so less distance necessary between cars…

*obviously it doesn’t.

According to the safer driver training I had to do for work last year, you shouldn’t stop at lights at all. You should be looking far enough ahead to see the lights are red and start slowing down to the point that you crawl up to the lights as they change to green.

Yes, it will wind the people behind you up something terrible and yes you will stop doing it as soon as you get out his car.

I principle I always follow when cycling 😉