When the Two Second Rule is not enough

Whether I'm running basic training or advanced courses, one recurring error that occurs over and over again, even after I have explained the dangers, is following the vehicle in front far too close. The loss of forward view is another subject altogether, but maintaining a safe distance with regard to stopping distances came up on the Ride Forum on CompuServe just recently, and so some different view points on the subject were discussed.

The Highway Code says sensibly enough that you should drive at a speed that allows you to stop in the distance you can see to be clear, and at a distance that will allow you to stop if the vehicle in front stops suddenly, and quotes some following distances. The problem with quoting distances, whether in feet (which I still think in), metres or car lengths is that it is almost impossible to visualise these distances. The quoted shortest stopping distance from 30mph is 75 feet/23 metres/6 car lengths. Now if you look out the window and try to estimate that distance you'll find that you will most likely be unable to do it to within three or four metres. Now try doing that whilst driving a car/riding a bike... no chance!

Anyway the Highway Code recognises this problem, and offers an alternative - the Two Second Rule. What you do is watch the vehicle in front pass a fixed object (a lamppost, a tree, a shadow or even a seam in the road) and start talking: "only a fool breaks the Two Second Rule". That takes about two seconds to say, so if you have passed the marker before you have finished talking, you are too close. If you have finished, you are far enough back. In the wet, you at least double the Two Second Rule.

Why does it work? The rationale of the Two Second Rule is that it stretches following distance as you go faster because it is based on counting seconds. You'll find the Two Second Rule mentioned by all the riding manuals, as well as being taught on advanced courses.

However, what the Highway Code nor any of the other manuals or riding tips I have ever read fails to make clear is that the Rule is not perfect:

    * it does not emphasise clearly enough that Two Seconds is not a target to aim for - it is a MINIMUM safe distance and should be extended at all times if the traffic conditions allow
    * it relies on the driver/rider of the following vehicle being alert and on the ball
* there is a cross-over point where if the car in front hits a brick wall you could not stop even with a two second gap

Steve Kelly has kindly done some maths for me. You travel 44ft per sec at 30mph. Stopping (reaction and braking) distance at 30mph is quoted in the Highway Code as 75ft. In reality a modern bike on decent tyres can cut several yards from that. We've made two assumptions:

    * an alert rider can react in around 0.5 second
* a motorcycle can pull about 0.9G of braking force

Also, we've ignored wind resistance, the effects of poor road surfaces and all rolling friction. So here are the braking figures:

At 15mph
- In 2 seconds at constant speed you will travel   44.0ft
- Your stopping and reaction distance is             19.4ft
- Your margin for error is                                   24.6ft
- Your impact speed is                                       n/a

At 30 mph
- In 2 seconds at constant speed you will travel   88.0ft
- Your stopping and reaction distance is             55.4ft
- Your margin for error is                                 32.6ft
- Your impact speed is                                       n/a

At 45 mph
- In 2 seconds at constant speed you will travel 132.0ft
- Your stopping and reaction distance is             108.0ft
- Your margin for error is                                 23.8ft
- Your impact speed is                                       n/a

At 60 mph
- In 2 seconds at constant speed you will travel   176ft
- Your stopping and reaction distance is             178ft
- Your margin for error is                                 -2.0ft
- Your impact speed is                                       0.6mph

At 75mph
- In 2 seconds at constant speed you will travel   220ft
- Your stopping and reaction distance is             264ft
- Your margin for error is                                 -43.9ft
- Your impact speed is                                       12.5mph

At 90mph
- In 2 seconds at constant speed you will travel   264ft
- Your stopping and reaction distance is             367ft
- Your margin for error is                                 -103ft
- Your impact speed is                                       25.2mph

At 105mph
- In 2 seconds at constant speed you will travel   308ft
- Your stopping and reaction distance is             486ft
- Your margin for error is                                 -178ft
- Your impact speed is                                       38.5mph

I think you can see what is happening - at 60mph, maintaining a 2 second gap, you will crash at walking pace even if you are wide awake and brake as quickly as possible. At 90 (not an unusual motorway speed, you will have a serious accident with a big impact. At 150mph, you would hit the brickwall at a frightening and almost certainly fatal 80.2mph!

Now lets look at the effect of being a bit dopy at the bars - we'll double the reaction time to a not unreasonable one second.

At 15mph
- In 2 seconds at constant speed you will travel   44.0ft
- Your stopping and reaction distance is               30.4ft
- Your margin for error is                                   13.6ft
- Your impact speed is                                         n/a

At 30 mph
- In 2 seconds at constant speed you will travel   88.0ft
- Your stopping and reaction distance is               77.4ft
- Your margin for error is                                   10.6ft
- Your impact speed is                                         n/a

At 45 mph
- In 2 seconds at constant speed you will travel   132.0ft
- Your stopping and reaction distance is             141.0ft
- Your margin for error is                                    -9.2ft
- Your impact speed is                                         2.9mph

At 60 mph
- In 2 seconds at constant speed you will travel   176ft
- Your stopping and reaction distance is               222ft
- Your margin for error is                                   -45.7ft
- Your impact speed is                                       12.4mph

At 75mph
- In 2 seconds at constant speed you will travel   220ft
- Your stopping and reaction distance is               319ft
- Your margin for error is                                   -98.9ft
- Your impact speed is                                       23.3mph

At 90mph
- In 2 seconds at constant speed you will travel   264ft
- Your stopping and reaction distance is               367ft
- Your margin for error is                                 -169ft
- Your impact speed is                                         35.1mph

At 105mph
- In 2 seconds at constant speed you will travel   308ft
- Your stopping and reaction distance is               563ft
- Your margin for error is                                  -255ft
- Your impact speed is                                         47.6mph

The speed of the impact matters little at higher speeds - the important point is that now the cut-off point is actually 45mph - you will crash at walking pace if you are a bit dozy even if you brake as quickly as possible.

Average reaction time for a typical driver is actually slightly longer than 0.5 sec - around 0.7 sec I believe - and considerably longer for someone who is tired, bored or distracted. So even if you are alert 60mph is the the point at which the Two Second Rule should most definitely be stretched, and if you feel the least bit bored or tired, you should open up the gap much earlier. Don't forget, you are probably most at risk on motorways or roads which you drive repetitively!

Two objections were raised in discussion to these calculations. One guy commented: "It is well known that a modern motorcycle with good tyres can stop from 60 mph in under 110 feet; this is comparable to only a few automobiles such as Porsche, Corvette, Ferrari, etc. In the vast majority of situations where maximum braking is needed, the bike will be able to come to an absolute stop, well before the standard car or truck."

Well, I'm not entirely convinced that modern cars cannot outbrake (or at least stop in a similar distance to) a bike, but the point is that how many riders do you know who practice emergency stops? Braking a motorcycle at or even near the limit requires a degree of skill. Even the most incompetent of car drivers can get maximum braking with ABS, and even though stopping with all four wheels locked on a non-ABS car is not very efficient, it's still going to come to a stop a damn sight quicker than you will if you  get it wrong - you will be relying on the friction of leather against the road surface after you have locked the front and fallen off - been there, done that and broke an arm as I hit the back of the car!

All this, of course, assumes good traction. Our friend stated: "When the surface is less than optimal, it affects other vehicles as well. If the rider is a capable bike-handler, he/she should still be able to outbrake and/or steer past a standard car or truck, as the mass of the vehicle determines its kinetic energy when speed is equal".

The physics may be correct, and may work on a smooth wet surface but we rarely get smooth surfaces. Our roads are in a pretty poor state and the bike is much more likely to be bounced around and generally upset by road irregularities than something with four wheels. Not to mention that the consequences of stepping slightly the wrong side of the traction limit will have you on your ear.

He went on: "I don't think the Two Second Rule results in such dire consequences as the calculations imply. One is assuming a fixed object towards which one is braking. In reality, emergency braking is initiated in response to a vehicle ahead, also braking. That vehicle must decelerate from its velocity just as you do, so the Two Second Rule simply provides adequate reaction time to initiate your braking in response to the emergency ahead."

That's fine in theory...
......until something unexpected happens:

    * in 1995 it was a car in front of me that for no apparent reason drifted off to the right, dipped its right front wheel in the gravel drain, swung into the armco in the centre of the motorway and came to a rest upside down in front of me - that stopped pretty quick
    * in 1998 (with my brother driving the van and me asleep in the passenger seat) it was the car in front of us colliding head on with a car coming the opposite way - that stopped almost dead (no serious injuries by the way in either case)
* in 2000 (and the month before I wrote this) it was a car that suddenly lost a wheel and spun round in the road in front of me

THESE are the situations the Two Second Rule is designed to rescue you from. Three incidents in five years means having to apply the Two Second Rule is not exactly uncommon.

It is true that generally roads where you are travelling much over 50mph are likely to be in lighter traffic, wider and more open (M25 excepted), so that your as view improves, you are reacting not to the car in front, but several cars in front of that (at least on a bike, with the better view, you should be!)... Still, that does not allow you to cope with the emergency where the wheel falls off and the car DOES stop almost dead!

It was also pointed out that 40+ year old drivers suffer from slower physical reaction times than 17 year olds. That's true - but studies have shown that they more than compensate for the longer reaction times by recognising the danger far earlier, so their overall reaction times and stopping distances are shorter. Much of the ability to recognise a real emergency comes with experience.

"Rider overreaction to a perceived emergency is a greater hazard in many situations than the actual emergency itself" was another comment. I can't argue with this having done that myself and fallen off on several occasions. Riding experience allows you to sort the real danger that you DO need to react to from the merely potentially dangerous, and braking practice allows you to get the most out of your brakes when you need them.

As Mac pointed out: "The "dead stop in the road" or "car in front hits a brick wall" scenario would initially seem to be somewhat rare, in actuality it is very common.  What I am referring to is when a car pulls out from a side street or a car turns across traffic in front of you.  Both of those situations present you with an obstacle in front that is at a dead stop in relation to your speed.  These are the situations where obstacle avoidance and emergency braking practice pays off.

"Yes, I realize that the two-second rule has no bearing on the above situations. Just thought I'd point out that one shouldn't get so involved with counting seconds that they don't see the car about to transform itself into that dreaded "brick wall".

It's actually a very good point, and another reason for stretching that Two Second gap - sit too close to the vehicle in front and all your concentration is directed to the brake lights on that car. Drop back, you're not so worried about running into it, and have time to scan to the sides and the rear and generally know what else is happening in the world around you!

Which brings me neatly to another point - alternative strategies - there is often space to avoid collision by swerving if the two-second rule was being implemented. So practice collision avoidance too as just as an important a way out of trouble as braking. However UK basic training does not emphasise steering as a way out of trouble - I do talk about it in general terms on the advanced courses, but maybe I need to improve my courses here. Just like the Vanishing Point discussion last month this shows up the dangers of defensive riding strategies being considered in isolation.

You may be surprised to learn that the Two Second Rule doesn't work so well as you go faster. I knew this but hadn't bothered to do the maths - so in the event I was a bit startled to find that the cross-over point was so low and I shall emphasise this point on my courses in future. As Steve said: "I'll think about this a lot more when I'm next travelling at 85mph along a motorway".

In conclusion, as a general riding or driving rule the Two Second Rule works well enough and all riders should apply it, whilst remembering, as I point out on my courses, remember that 2 seconds should be considered a MINIMUM safe distance, not a target to aim for, and should be extended where possible below 60mph, and most definitely over 60mph!

 

Thanks to Steve Kelly for doing the mathematics.