Safe Speed Forums

If your 'body language' suggests that you are looking for the overtake, most drivers will allow you reasonable opportunity to execute it before concluding that you do not really want to. If you had four cars overtake you to fill the space, I think you must have been sending the message that you didn't really want to.

The signs of a car looking for an overtake are pretty unmistakable: closing up where the road geography suggests an opportunity may occur, positioning wide where safe for vision, etc.

If you weren't doing any of those, I'd suggest the message you were sending was that were not particularly interested, so it would not be surprising that four cars passed you.

Your 1.1 seconds may be a gap that in ideal circumstances will allow you to avoid a collision but that doesn't make it "safe". There's no (safety) margin for momentary inattention or for a truly unpredictable event (0.7s reaction time is actually quite fast). Nobody can (or should try to) drive with 100% concentration all the time so you should build in a reasonable additional margin. A smaller gap (say 1s, sometimes even 0.5s) is fine for short periods where you're fully focussed on a specific manoeuvre, but it's not safe for extended periods.

I believe that efficient traffic flow on motoways is 90% about what happens at junctions. That's where you have joining traffic and leaving traffic and, necessarily, vehicles needing to change lanes. If the lane changing is facilitated, traffic will flow more easily and faster. How do we facilitate lane changes? Simple - leave an adequate gap ahead for another vehicle to merge into. One second is not enough for that because it leaves <0.5s separation for both. Allow 2s and vehicles can merge relatively easily. The separation will be reduced, temporarily, to <1s but that can be easily corrected by minor (and temporary) throttle adjustment with virtually no overall loss of time.

There's no reason not to apply the same principle between junctions. Yes- in theory, a larger gap reduces flow capacity but only if the traffic continues to move at the same speed. The principal cause of congestion is traffic flow breakdown. Greater separation allows traffic to soak up variations in speed and will delay the onset of flow breakdown. That's much more important to maintaining flow capacity than squeezing more vehicles into the same space.

The above beliefs are based on instinct and logic, supported by observation. I'm confident that it could be demonstrated mathematically, but I'm not really up to the maths.

Just to clarify my point a little. What I was trying to say, and failing miserably, is that if you leave a gap it will get filled at which point you are driving too close again.

So I have a choice of drive too close behind the same car for a long time or drive too close behind a variety of different cars for a long time, with very small intervals of having a half decent gap from time to time. The first one gets you to your destination faster, but the principal arguments here are about safety, so I will concentrate on that.

Neither solution is particularly great, and none of them match up with the ideals that Paul and others are advocating in this thread. If you find yourself on a relatively quiet motorway where it is possible to live up to that ideal then I fully agree with Paul, sadly there's plenty of motorways where it isn't possible, especially around the midlands.

Which is safer, well if you only look at "small gap = bad" then obviously having the half decent gap from time to time is a lower exposure to risk and therefore safer, but there is more too it than that.

If you keep an eye 2 or 3 cars ahead of the one you are following, in addition to the one you are following, you can make a pretty good guess of what is about to happen. I am not one of those reps tailgating who get involved in the red light cascade as soon as the lead car drops it's speed by 0.1mph. It's trivial to see that cascade coming and ease back to the new speed before the car in front has begun to break.

Being behind the same car for a long time has other advantages too, you get to observe their driving behavoiur and range of reactions to various events, as well as those in the queue ahead, you can soon anticipate most of what they're going to do before they even know they're going to do it.

There is, of course, always the risk of being involved in a pileup that may have been avoidable if you had a larger gap available to you. I have never been in that siuation, so do not know if observing so far ahead will actually help to avoid it. I would like to think (hope) that you would see the sudden stop forming ahead and take some kind of evasive action, either pre-emptive braking or a dive into a different lane.

So, does the safety gains of these small advantages negate the increased risk of losing your short periods of having longer gaps. I don't know. I'm not a statistician and would have no idea of how to assign a value to two very different kinds of risk reduction. However with my approach the level of risk is relatively constant, wheras with the gap->filled->slow->gap approach the risk is constantly shifting up and down which I find more stressful.

Some other related points:

Regardless of approach used would have to do VERY early evasive braking to stop the guy behind from running into the back of you. I know that Paul advocates leaving a double sized gap when you have a tailgater but if you accept that any large gap will get filled, leaving a double sized gap will just result in 3 cars filling it.

Having been rear ended twice (neither on a motorway or as a result of anything relating to the situations described above I should point out) I think I would go for the dive into another lane option first, which is why I try to avoid staying alongside other vehicles for too long. Always have at least one escape route planned. I'm sure Paul would at least agree with me on this point

I think it's barely "half" rather than "most"! I certainly take your point and it was pretty obvious that I wasn't interested in overtaking on this ocasion but it's all relative. I have, however, often been in a situation where I AM looking to overtake and have suddenly found another car leap into the space between me and whatever I'm following. I might have THOUGHT that my positioning was such that it was obvious, but there are plenty of people who are happy to leave much smaller gaps and overtake on much shorter straights and I guess that to them, I must have looked like someone who wasn't going to overtake!

Also the speed of the vehicle you're following is a big clue. If I had been following a tractor, I should think most drivers would have expected me to try to overtake. This was an artic driving (other than up the hills) pretty much on his 56MPH limiter (naughty boy!) and I wouldn't have gone much faster than that in the conditions at the time anyway.

That's a pretty broad brush you've got there sir!

I agree there will be such people out there and that (on the face of it at least) the Safespeed message will suit their ends but surely that's a reason for decent upstanding and virtuous folk TO support the SafeSpeed rhetoric? If not, it will be hard to differentiate it from the "anarchy" rhetoric.

It's possible to support the kind of behaviour I was describing AND support the SafeSpeed message.

One of the things SS supports is an increase in real traffic police and improved driver education.

If SS got their way on these two then suddenly cars cutting in to your safe stopping distance would become much less of a problem, so people would not feel the need to close up the gap in order to maintain progress.

Currently SS have not gotten their way. People do cut in, so people choose to drive too close for the reasons I have already gone into in far too much detail

Lane changes also happen throughout the length of carriageways, but there will of course be a high concentration at junctions. I’ve observed the propagation and amplification of the domino effect from a lane changer up ahead (yes I really do look ahead ). Inclines are another source of flow disruption. My commute takes me though many hills (motorways, A roads), there is almost always bunching on the inclines, even though they’re really gentle; drivers need to understand that they should put their foot down to compensate (just like on entry slip roads).


That makes sense for drivers needing to change into a left lane (for those wanting to exit the carriageway), but not to a right lane (to overtake); if there’s not an adequate gap to the right then there’s no reason to change into it (which funnily enough contradicts kenp’s original sentiment).


Yes, but this is what causes the said disruption to flow at exit junctions.
What do you do if multiple vehicles want to change left? What if someone behind wants to change left too? Perhaps it would be better to minimise flow by concentrating and temporarily accepting a more reasonable 1.1 second (minimum) gap?


So what are you advocating exactly? I think we both know that 99.99% of drivers will happily sacrifice part of the 2 second gap if it means not facing restrictions - they already do.


I believe we’re past the point where peak time traffic capacity/flow would benefit from 2 second (minimum) gaps. If the 2 second gap was enforced then we’ll be taking a huge knock elsewhere.


I don’t know why everyone is jumping on my statement of the 1.1 second gap. It can be safe so long as the driver is on the ball; if the driver isn’t then we’ve got bigger problems anyway. I don’t advocate that that drivers should strive to keep to a minimum gap, but in some circumstances trying to maintain 2 seconds can also be detremental (in terms of road capacity). Heck 1.1 seconds is so much better than what the majority of drivers routinely do anyway
I see no benefit of trying to maintain a minimum gap of 2 seconds regardless of conditions.

There may be some people out there who indeed think that way (joyriders) but I'm not sure you reaslise that SS also calls for alternatives which will reign in any form of bad driving (including inappropriate speed) - which actually work against these particular people's desires.

Besides, I’m sure there are criminals who want to hide behind various privacy acts but that doesn’t mean the acts themselves are a bad thing.

The central point, which you haven't seen (or if seen not addressed), is that a smaller gap only increases capacity IF the traffic continues to flow at the same speed. If the smaller gap causes a flow disturbance or breakdown, the notionally improved capacity is either reduced, or completely eroded, or negative. My observation is that if the gaps are so small (say <2s) as to inhibit lane changes, that does cause flow disturbance and breakdown to occur sooner rather than later.



I answered that in the earlier reply. It's not "safe" because there is no safety margin. It's acceptable for short periods. As far as capcity goes, see the comments above.