Safe Speed Forums

I'm absolutely certain (from personal exp) that if drivers attitudes would change from strongly territorial to "accommodating", the collision count on motorways (and all roads actually) would drop drastically. At 90 mph a car and it's occupants are travelling at 135 feet per second (multiply speed in mph by one and a half for feet per second). The so called 2 second rule, applicable in dry and clear conditions, would mean that the car should be 270 feet behind the one in front. (a sobering thought eh?) Overall stopping distance is more than this but 270 feet or 90 yards is a safe following distance. The figures at 70mph are 105 fps, = 210ft safe following and 315 ft overall stopping distance.
If we leave this safe following distance and allow other cars to enter the resulting envelope of space in front of us we will all be driving a lot more safely than as at present. Drivers regard the space in front of them as "their territory" and so as to protect it they leave a very small gap. Those drivers who are allowed to enter this envelope of space in front of us will, 90% of the time, be travelling slower than ourselves but not dramatically so and will return to lane two as soon as they've passed the slower vehicle. You, having slowed only slightly since you had the space to absorb the slower vehicle, can then return to the safe speed which you were originally doing, e.g. 70mph. Try it, it really does work. If we all adopted this non territorial attitude there would be no need for speed limits on motorways I'm sure.
Following on from "Motorway Driving" and re. safe following distances.

Below is a comment made by a member in an earlier thread and is exactly the comment made by almost everyone I speak to on the subject. It is the root of the territory problem. We should all change our attitudes before the government are gifted the excuse to install average speed cameras on all motorways in order to "reduce accidents" If there were fewer accidents they'd not have the excuse!

"The inherent problem on the M25 is that if you do leave sufficient stopping distance it is normally pretty quickly occupied by another vehicle."

It ain't a problem! Try it!

Indeed, what is more as traffic density increases, you have to go slower to maintain a 2 second gap. So speed is "Automatically" regulated to a safe level!

Accedents on motorways are often attributed to driving too fast. and speed regulation is seen as the way (By TPTB) as a way of reducing them.

Like all good lies, this contains an element of truth. but the real problem is that people are driving too close. If people maintained a safe distance (2 second gap) speed would not be an issue.

Of course speed regulation makes the situation worse. Drivers traveling at below optimum/desired speed tend to form nose to tail convoys.

As Camera disease descends on our motorways expect multiple pileups to become more common, especially in poor weather conditions!

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"The road to a police state is paved with public safety legislation"

I am lucky enough to spend most of my time on lightly (comparatively!) trafficked motorways - M74 mainly. Leaving a safe distance isn't as much of a problem as it is on (say) the M25. I also use the M6 in Lancashire / Cumbria a fair bit and pass over several areas where they have those chevrons and signs to tell you to keep at least two chevrons behind the following car.

Do we have any data for the number of accidents per hundred thousand vehicle-miles on lightly trafficked motorways (or parts of) compared to heavilly-trafficed ones? I don't know whether it would be a valid comparison because mean vehicle speeds will presumably be higher on lighter ones but some clever statistician could probably come up with an objective test to prove your theory I guess!

I also don't think it's fair to say that most drivers regard the space between them and the car in front as "their territory". It would be like saying that people standing in a queue regard the space in front of them as "their territory". They don't, of course, they just don't like the idea of waiting patiently and then having someone else force them to wait a bit longer whilst at the same time not having to wait as long themselves. It's a "double whammy" afront!

On some single carriageway "A" roads (where I also spend a lot of time) I can see how leaving a safe gap just moves you further backwards in the "queue" as there is never any shortage of people who decide that it is safe to overtake on a stretch of road where I wouldn't. It can be frustrating watching car after car overtake me only to find that they still end up following the truck I was stuck behind - just closer! When I finally get to the bit of road where it would have been safe to overtake the truck, I find myself having to overtake half a dozen cars as well (which invariably isn't possible) so instead, I see the first few cars that took "my" safe braking space pass the truck and disappear onwards to their destinations.

So I agree that people don't like other drivers pulling into the gap in front of them but I'm not sure I'd agree that it's just a "my territory" argument.

Trouble is 'game theory' would apply, i.e if everyone left wide gaps the individual driver could exploit them to his or her advantage, and we end up where we started.

You're clearly convinced that you're right and that leaving the correct gap wouldn't work. Why don't you try it for once, you obviously don't at the moment. To disagree that a correct gap isn't a good thing to have is plain daft. It is and it is workable. I apply it every day and it really has kept me out of at least four collisions over the years, two actual in front of me and at least two which almost occurred due to the drivers ahead driving too close. Get rid of your "theories" and just try what I've said.

You have a valid point, but the problem is that some drivers definition of a ‘safe distance’ is wildly inappropriate, sometimes greater than 5 seconds (yes, even when in the outside lane); these are the drivers who also greatly amplify the effects of congestion.

The two-second rule only applies to drivers who don’t look beyond the brake lights of the car in front. I endeavour to keep a minimum of one second and, more importantly, I look ahead.

I stopped getting stressed about drivers pulling in front of me a long, long time ago, my journeys are much more relaxed and enjoyable (unless I get a pratt who forces me into a crash evading manoeuvre).

sorry Smeggy one second or 35 yards at 70 mph simply isn't a safe distance to follow. To be safe all drivers should look ahead as far as possible during the routine scanning all around and it isn't correct to say that "The two-second rule only applies to drivers who don’t look beyond the brake lights of the car in front". It is a good rule and applicable in dry and clear conditions. I find that 3 seconds in the wet works for me and not four as many advocate. As you imply, if you are always "aware" then the thinking time before applying the brakes can be reduced slightly thus reducing the stopping distance if required. Why don't you try the 2 sec. rule coupled with what you seem to already have, i.e. a tolerance towards drivers who wish to enter the space in front of you, for a week and see how you feel then?
I'm not trying to win a point here, I genuinely am convinced that it is a teritorial thing which leads to drivers leaving insufficient space between their vehicle and the one ahead. Ken

Gosh, you're tetchy. How about some manners?

How did you come to the conclusion:

a. That I don't leave adequate gaps?
b. That I endorse not leaving adequate gaps?

Sorry to johnnytheboy if I came across wrongly, I interpreted your quote "if everyone left wide gaps the individual driver could exploit them to his or her advantage, and we end up where we started. as just what it implied i.e. that you were against leaving what you call wide gaps and what I call safe following distances. I can only respond to what is in front of me and if your views are different to how I read them please let me know what you think about safe following distances,braking and overall stopping distances.

I would like to add that I said that as a minimum gap (following on from your clause of favourable conditions). I believe it can be a safe distance so long as I’m aware of the conditions ahead that would force the car in front to react. In fact, several months ago I mathematically confirmed the feasibility of using such a gap using real-world, worst case figures. The figure I ended up with is 1.1 seconds at 70mph. I can post the derivation if anyone wants to view it.


I partly agree. There will also be drivers who are half asleep, slipstreaming or just plain ignorant.

Did you allow for a second or so of 'unavoidable inattention' associated with (for example) mirror and speedo checks? When I've looked at it, I've found that we need 1 seoncd for unavoidable inattention and 1 second to react. And even that can end up on the tight side if there's a differential in braking performance.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I aim never to need to brake on motorways or main roads, unless something exceptional and unexpected happens, or I need to slow down to very slow speed. A two second gap is too short for this, at two seconds you have to respond quite quickly and positively to any speed changes from the car in front. I'm confident that I can drive safely with a gap of less than two seconds but I prefer not to spend hours poised over the brake. Personally I don't feel comfortable with a gap of less than one second. I can anticipate what the car in front is going to do and avoid running into the back of him as long as I stay alert, but there's no room for mistakes or loss of concentration. I know that sooner or later I'm going to make a mistake, and I prefer not to put myself in a situation where a momentary mistake could cause a collision.

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Peter Humphries (and a green V8S)

Hello Mole, I'm a mechanical engineer and would like to see your calculation but I'd be surprised if it were one which could take into account for only one example, your being behind a car (at best or a large van even worse) on an uphill stretch of motorway where forward visibility isn't as good as with a straight and level stretch of same. All of these rules etc. are subject to discretional and ongoing change by the way but with, in my opinion anyway, 2 seconds as a minimum. What do we gain by reducing this gap to a dangerous 1 second anyway? Theis all applies to dual carriageways in particular and with single carriageways and for example national speed limit, we should either safely pass the vehicle ahead having considered everything ahead such as concealed entrances, junctions road markings etc. etc. or leave sufficient space between our car and the vehicle ahead for an overtaking vehicle coming from behind, perhaps driven by either a more competent driver than ourselves or even one in more of a hurry, after all we aren't always late for an appointment and needing to get to 'b' as uickly as possible.

Some of us are!

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Peter Humphries (and a green V8S)

I assumed a significantly unfavourable difference in brake performance (1.0G of the lead to 0.8G of the following).

As for factoring unavoidable inattention, not really. I almost never check the speedo (I use it as a reality check, there are more important things to watch) and I don't let myself be distracted by conversation or button fiddling; I did allow 0.7 sec realisation/reaction time. I guess I used my own standards in the derivation.

With the realisation/reaction time modified to 2 seconds (I’ll be horrified if I ever let my reaction time suffer so greatly, especially when I’m leaving a shorter gap), the overall gap required to prevent a crash rises to 2.4 seconds @ 70mph.

That's my opinion also - BUT we ALSO have to continuously observe the Safe Speed rule (always being able to stop comfortably in the distance that you know to be clear). It doesn't matter if other vehicles are in your 'safe braking zone' you physics guarantees that they will have gone before you get to them. And so it is following at 2 seconds. The car in front is within your 'safe braking zone' but he can't brake significantly harder than you can and will gone before you get to him.

Particularly obstructive (to vision) vehicles in front can make you drop back to preserve knowledge of your safe braking zone.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

true! But then we should be alert and looking for an overtake position as soon as it safely presents itself, closing the gap as necessary just before passing and having done a final check of mirrrors and scanned well ahead. Sometimes it is better or even necessary to go to the offside to get a good view ahead before committing to an overtake. Ken

Agreed. Except. If you factor in a one second mirror check during which time your vision ahead is compromised, we'll end up at the same place.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Hell, yes. Position for vision. ALWAYS (unless it is unsafe to do so).

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

The uphill example doesn’t make sense to me. Anyway, there are things you can do to increase your own visibility: position in the lane, looking through the rear/front windscreens of the car in front. If all else fails, drop back


There are advantages, for example at entry slip roads where a quick squidge of the throttle will open an escape route for the vehicle in the left lane, but overall there’s nothing to gain.

Here’s the derivation:


The distance between two vehicles must be, at minimum, the potential difference in stopping distances.

Some reasonable assumptions:

The driver of the lead vehicle recognises a hazard which the driver of the following car does not (there could be multiple reasons)

The lead vehicle has better brakes (1.0G) than the following car (0.8G) (an unknown variable to the driver of the following vehicle)

The following driver is observant enough such that they know the lead vehicle will not instantly come to rest.

The driver of the following vehicle needs a little extra time to determine how hard the lead vehicle is braking before they react (0.7s)

Both vehicles are travelling at legal motorway speeds (70mph, 31.3m/s)


From the application of the brake, the lead vehicle will stop in:
((31.3)^2) / (2 * (1 * 9.80665)) = 49.9 meters

The following vehicle will stop in:
(31.3 * 0.7) + ((31.3)^2) / (2 * (0.8 * 9.80665)) = 21.9 + 62.4 = 84.3 meters

The difference in braking distance is (or the gap between both vehicles should be)
84.3 – 49.9 = 34.4 meters (or 8.6 ‘average’ car lengths)

The time distance between the two vehicles (rear of lead to front of following) should be at least:
34.4 / 31.3 = 1.1 seconds.