Safe Speed Forums

Correct - and they would do so if all the road space up to the constriction is used. However, if more rather than less traffic merges into L1 early, the flow in L1 will be reduced by more than would otherwise be the case because L1 has to accommodate more vehicles for a longer time. So the early mergers are forcing L1 to slow more than is necessary by merging early.

'Early' is obviously when it can be done without anybody slowing down significantly. The gaps are larger and easier to adjust. When you are travelling at a walking somebody is going to have to stop to let someone in.

An early merge is just lengthing the taper to make a more gradual change so it should go smoother.

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Observer wrote:

It might be over simplifying things but I don't think it is false a larger quantity has to get through a smaller gap so a steady flow makes more sense than last minute changes of pace and direction.

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You defeat your own argument. If the traffic speed is walking pace, everybody has already "slowed down significantly". If traffic is at walking pace, it is obvious that any unused distance in L2 simply adds the same distance to the queue in L1. The effect of that on traffic further back is unknown to those at the front of the queue but it is quite likely to exacerbate congestion at best and cause a serious hazard or even a smash at worst (take the example of traffic queuing at an exit slip where the queue backs up onto the main carriageway). So the early merger is acting selfishly because he is unnecessarily lengthening the queue in L1 without considering the effect on traffic behind.

What holds true at walking pace necessarily applies at progressively higher speeds, except, as I keep saying, the merge point needs to move back to safely accommodate varying approach speeds and gaps.

Perhaps venturi flow of fluid mechanics might be a better analogy - where mass flow rate is equivalent to the traffic flow rate of x vehicles/hour. Put crudely, it means that for the traffic flow to remain constant, speed in the obstruction must necessarily increase. Obviously this can only happen up to a limit, and after that the flow rate reduces in proportion - so causing the queue to form.

It's the equivalent process to working out how many lanes are needed to support a given flow rate of vehicles/hour.

but by its nature the obstruction has an artificially limited flow rate/speed limit.

Absolutely right - and I would love to see minimum speed limits in such occasions - but I rather doubt we'll ever see them!

The point I am obviously not getting over very well is that trying to get into a queue near the constriction probably means the traffic having to stop to let you in. It may have slowed significantly but not stopped and that makes a difference. Merging while the traffic is travelling faster might keep it going for longer without stopping which must mean joining earlier and further back from the constriction.

And I don't think I am acting selfishly because while the queue is longer that doesn't mean the traffic is travelling slower. By moving further down the queue you will get in front of others but they will be penalised.

The people going straight on will always have a tendency to think they are in the right lane and others are in the wrong lane particularly if there are signs telling people to move to a lane.

Then you end up relying on the good will of other people to let you in.

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You still do not see it.

Let's say we have two streams of vehicles in two lanes approaching a constriction at the same speed ~ walking pace. Let's also say the constriction has no traffic in it, so nothing to impede the speed of vehicles once they've entered the constriction. At the point of constriction, L1#1 enters first, then L2#1, then L1#2, then L2#2 and so on. That merge doesn't require any L1 or L2 vehicles to stop (as long as all the vehicles accelerate once they have entered the constriction). All that's needed is for each vehicle to allow the one vehicle ahead and in the adjacent lane sufficent space to merge.

Hopefully you will agree that vehicles will be able to merge easily, without stopping or braking, until entering the constriction when, as stated, they will be able to accelerate away allowing the two streams to merge behind.

Now let's imagine (with the merge at the constriction continuing ahead), that someone in L2 decides to merge into L1 200m before the constriction and that everybody behind does the same (because they don't like to 'jump the queue'). That necessarily increases the length of the slowly moving line of traffic in L1 by the length of the unused space in L2. And because the traffic ahead of the new merge point cannot accelerate (the acceleration occurs only when a vehicle has entered the constriction), L1 can't cope with the increased volume so a wave of braking will ripple backwards.

All of the above will hold true if the initial traffic speed is increased - but the optimum merge point needs to move backwards.

You may think you're not acting selfishly - you just haven't understood what's happening. However, the fact is you are not helping or being courteous by merging early - you're making things worse for people behind you. But, now this has been explained, you will be acting selfishly if you fail to act on it.

what if you merge early (spotted a big gap, perhaps) then consciously - but gradually - allow a big gap to open in front of you? Or if you were already in L1 and consciously leave a big gap ahead of you for any / all cars to merge into?

(by 'big' I mean a couple of safe distances at whatever speed you are moving at, say 4 car lengths at walking speed)

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Question for those who are "miffed" by people who do not start to move-over early:

What is more efficient, and why?

(1) queueing early / merge point way back from obstruction... and the queue is probably getting longer (default UK system)

(2) signposted merge-point close to obstruction... two queues forming and both are probably getting longer (system frequently used in the Netherlands)

Taking your funnel analogy a bit further, what early merging is doing is effectively creating a second funnel above the original funnel. So the 'funnel' effect is moved back to the early merge point.
And the cocept of 'gradual' doesn't apply here (analogy = having a long, tapered funnel) because any vehicle can only be either in one lane or the other, or straddling the two.

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No you are right, I still don't see it so I am going to go away and have a think about it over night. Perhaps my brain will work better in the morning.

I would never knowingly act selfishly so I will give it some serious thought.

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Shooting is good for you and too good for some people.

Late merging makes more efficient use of roadspace, and minimises the distance over which the traffic has to travel slower.
Taking early merging to its logical extreme, imagine if the merge point was right at the beginning of the road - then the entire road would effectively be a single carriageway, with all the disadvantages thereof, and people who overtake (as they do on single carriageways) would be classed as 'queue jumpers'.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

I was having a good think about it today(nothing better to do on my way home!) and can think of 2 distinct situations where using a clear lane would help, but one where it wouldn't.

The first where it definately would help would be where 1 lane is coned off on a 2 lane dual carriageway. The throughput of the restriction is directly affected by the behavoir of those entering. By creating gaps and merging one at a time the capacity is vastly increased, maybe even so much so that it never needs to queue. All it takes is one driver to refuse to let another one in, which creates a late merge, and the late merger is moving too slow which then creates a ripple effect all the way back.

The situation I'm thinking where it wouldn't help is where two lanes approach a set of lights with L1 going straight on and L2 going right only. There, the throughput of the junction is fixed. No amount of late merging is going to increase the capacity of the junction on the red phase, and neither will it increase on the green phase if traffic has built up on the red. Therefore it really won't make a difference whether you join the back of the queue or not, since zip merging is not going to work.

Yup. It really IS that simple. Traffic would keep moving if all the road was used and all the 'holier than thous' resisted the temptation to block the one car adjacent to them on the closing lane. The problem is this - people in the lane that isn't closing are abusing their advantageous position, and their refusal to offer some of that advantage to others is the cause of the stop-starts, queue extensions and danger. As I said earlier- who are the selfish ones?

The clamour to merge early is merely a grasping attempt to get into that advantageous position, and the 'queue jumping' mentality is simply a failure to accept that someone else actually is making a better job of it than you and you see it as unfair because you did 'the right thing'. It's false morals disguising poor judgement.

Hear hear!

And with those words can I propose we declare this thread officially dead?

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Missed this one.
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Nothjing wrong with zip merging or using the road. Nothing worng with eye contact - cherry waves and smiles to helop it all along either.

By the way - R11 and JT - wise words and I agree with you!

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True, true!

My assistant (female so way better at body language) seems to be able to predict people who aren't going to let us in by their lack of eye contact.

In her book, the more rigidly ahead the stare, the less chance of letting someone in.

OK. Where would you merge here - at the first arrow (just in front of the oncoming Zafira) or at the second arrow (alongside the vehicle with it's lights on) or would you wait until the lane runs out at the brow of the hill, keep two wheels on the double whites while you finish passing the last car you can without hitting anything coming the other way, irrespective of whether they would let you in or not - after all, they cannot let you in if you are along side - only once you get in front.
Note the lack of a bend warning on the left side by the telegraph pole - it's flattened where some clown ran it down in November!


At this location, it is common for vehicles to hit the brakes hard to allow a lane two hogger to rejoin, before they get shunted into you by somebody coming the other way!! The usual speed is between 30 - 40 mph, from an NSL - if there is a bus or lorry, 15 - 20 mph is common. Eager motorists attempt to pass as many vehicles as possible before the bend to little avail.

JT's description of his merge manouvre does not seem to me to fit this pattern.

When this scenario is repeated in a similar fashion on the motorway, please explain how the vehicle forced to merge when it runs out of lane in front of the cones, is behaving reasonably, when traffic is becoming closer together as they slow?

Does the car coming up from the back, then rejoining the remaining lane at the front reduce the length of the line of vehicles back along the road?
Not when the cars in lane one have to slow even further to allow the outside vehicle to rejoin. Use of the outside lane merely fills it temporarily until they have to rejoin.
ONLY the continued flow of traffic through the pinch point SMOOTHLY will reduce congestion, and this is acheived by merging politely in good time.
NOT to do so, is akin to overtaking a line of traffic when there is an oncoming vehicle, and refusing to consider even attempting to pull in until they are about to collide with the oncoming vehicle.

NOW is everyone clear why I described this behaviour as "barging in", and not as merging?

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