Safe Speed Forums

Very interesting, that US article on braking waves.

It is very true that in very heavy traffic, the lanes on a motorway act as a "filtration system" - all the aggressive drivers end up in the right-hand lane with the more "defensive" drivers occupying the middle lane.

The aggressive drivers in L3 will rush forward as soon as a gap appears in front of them, then brake harshly behind the car in front when it comes to a halt again. This sets up pronounced braking waves which only serve to slow down the traffic in L3.

The more passive drivers in L2 will creep forward slowly when a gap appears in front of them and by the time they reach the car in front, this car is often on the move again. This serves to dissipate the braking wave and makes traffic in this lane move faster on aggregate.

Lane 1 is always going to be the slowest lane, as it is being disrupted by traffic entering at the onramps.

I found long ago that the middle lane of the M4 is always the "best" lane to be in when leaving London in the rush hour, but I never knew why - until now!

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“For every complex problem, there is a solution that is simple, neat, and wrong.” - H. L. Mencken

Two reasons:

1. This is a congested motorway. At times lane 2 is moving faster than lane 3 and the gap he pulled into was behing an undertaking car. So he was able to move into it first.

2. I will not change lane anyway if it means squeezing into a small gap, say two cars long, since I will then have to slow down and annoy the following driver in order to have a safe gap ahead. Drivers of German saloons are not always so fastidious.

Oh you've noticed this too have you? Its certainly no co-incidence that L3 is often the first to grind to a halt.
Pity theres no mechanism to make them stay in L3 leaving the rest of us to sail majestically by with a superior look on our faces Unfortunately they manage to push their way across into L2 and then bring that to a halt too by repeating their bad driving habits

Is this any good, as an entry level system, or is it too cheap to be true?

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

I've always found L1 to be best, certainly when I had to use the M62 to Leeds every day. The on slips didn't seem to create much of a problem, since the cars usually just made a bizaree bee-line for L3 This then creates the empty space once more. I'm not complaining however, it usually kept L1 clear, especially on the downhill bits.

L3 was and is always the slowest in my expierence and that US article pretty much sums up why - people just rush to the next traffic wave, where as in Lane 1 and 2 the trucks keep bigger gaps. Partly I assume, because it uses less fuel, and partly because they can't accelerate as fast.

When I've left Leeds at rush hour, I could be overtaken after the Bradford turn off, but meet the same car at the interchange for the M66, and I'd only be using L1 and L2(and sometimes L3 of the 4 lane stretch).

As a Sat nav system, looks good.

As an answer to tailgating...can't see how it could help personally.

Is this the same. http://www.globalpositioningsystems.co. ... ption.html

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

Oops! Wrong thread! Too many Opera tabs open!

Been wondering where that post went all afternoon!

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

quite often you see the outside motorway lane choka blok with cars , the middle lane with a vehicle every 200 metres & the left lane with practically no one in. When I'm in the inside lane & am slowly overtaking the other vehicles as they are constantly jamming on the brakes am I commiting some offence?

No. However, be very observant of:
1) cars from lane 2 coming across on you
2) Cars from lane 3 coming across either all the way to you or precipitating 10
3) Motor bikes filtering along either dotted line.

I keep speed diffeential down to about 10 - 15 mph with adjacent lanes if I am in that situation.

In crawling traffic you should not leave a 2-car gap, you should leave enough of a gap to be able to manoeuvre comfortably around the car in front and no more.

Vehicles who leave the enormous gaps in such situations are the most annoying ones to be stuck behind. It does not ease the flow of traffic, you are causing the tailback to be further behind than necessary which may go beyond the junction where someone is giong to leave the motorway.

As in any other situation is is preferable to avoid having to keep starting and stopping if you can. Leaving a big gap may allow you to smooth out the variations in speed of the traffic ahead, and keep the traffic behind you moving too.

It may also allow you do drive with the engine at idling speed, in one of the lower 3 gears, particularly with a diesel.

This doesn't work in practice.
Every day I travel along a road with a wide, sweeping bend where I can see all the traffic for at least half a mile ahead. When the traffic's crawling along you can see someone's opened up a large gap a long way ahead, and they're consequently travelling at a steady very low speed. But five or six cars back, they're stopping and starting again, and so someone else opens up a large gap and the process just repeats itself.
All that that's achieved is to push the tail-end of the queue much further back, where it's likely to block junctions needlessly.
The only time it helps to open up a larger than necessary gap is if the traffic ahead slows to a short stop and starts moving again - but then your timing must be such that you catch up with the traffic just as it starts moving.



Trouble is, particularly with a high-geared car, driving at idling speed may be too slow for the driver behind to be able to get their foot off the clutch.

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

Don't you mean low geared car?

If you leave enough space you can anticipate the traffic stopping and leave a gap for traffic energing from side turnings

Also, if the car following you has to slip the clutch, he could avoid it by leaving a larger gap ahead of him.

Braking and clutch slipping are symtoms of following too closely. The size of the gap makes very little difference to the time you will arrive at your destination. It is less stressful too!

Well, well, I do seem to have caught everyone's attention with my original post. The fact is that tailgating is rife on motorways. Of course if one keeps to the 2-second rule, the gap between vehicles can reduced at lower speeds, so a 2 car gap can be sufficient, it all depends on the speed.

What is completely daft is a one car gap at 70 mph, (yes I see this all the time !!), in this case the time gap is measured in 10ths of a second. No matter how good you think your reactions are they are not sufficient with these sort of times if somebody up front slows down fast.

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Good manners maketh a good motorist

Yes



What about the people behind you who want to turn at the junction, but have to wait for you to crawl 100 yards before they can?



If I can do 8mph but he can do no less than 10mph without slipping his clutch, he has two options - either slip his clutch, or he's going to catch me up and have to stop, which is the very thing I'm supposedly trying to prevent.



Perhaps for you, but think about this: if just 30 cars each leave a gap which is 2 seconds longer than necessary, the traffic behind them is delayed by a full minute - and more traffic is piling in from the rear all the time. And if we're talking 300 cars doing the same then we're up to 10 minutes. And in that 10 minutes another 300 cars have joined the queue.

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

I find when I leave a nice comfortable gap somebody cuts in because they think my lane is moving faster.

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

I'm just thinking out loud here rather then claiming it to be a fact, but aren't the delays caused when people stop and start, if you can keep some, even just a trickle of speed on the delays will be reduced? For example if you slow down in advance of the queue, but not stop when the queue moves on it's going to be quicker and easier to get the car going if it's already moving then if stationary? It's just a case of pressing the accelerator, as opposed to the various permiatations of brake/clutch/handbrake/gas/back into gear that maybe needed if stopped. I know the difference for one car is only small(it doesn't take that long to get a stationary car going) but once you add that up for every car in the queue will mount up.

This is an excuse often used by tailgaters. I agree it can be a little disconcerting when another car reduces the safety gap you have carefully allowed but, imo, that's because of an incorrect attitude. I have developed the attitude that I regard the gap in front as an open invitation for anybody else to use. Why?

1. Do I own the road in front of me? No.

2. Is it reasonable for me to expect other road users not to use the road in front of me? No - although I would like them to do so with reasonable consideration for the effect it has on me. Usually, another car taking advantage of the gap in front of me has no noticeable effect on my driving at all - and the bigger the gap I have left, the less likely I am to be affected.

If I leave a comfortable gap (at least 2 seconds) I am not at all inconvenienced by another car filling it. All I need to do to re-establish my comfortable gap is ease off the throttle slightly for a second or two. No big deal.

If all drivers left decent gaps on motorways, I am quite sure we would see reduced congestion, especially around junctions where most problems occur.

If you have traffic passing a certain point along the road at a steady rate of 1 car every 2 seconds, and a couple of miles further ahead the gaps have opened up so the traffic is passing a second point at the rate of 1 car every 4 seconds, the traffic is piling into that section of road at a higher rate than it's leaving that section, so something has to give and, usually, everything just grinds to a halt.

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