The Two-Second Rule

Two-Second Rule

  

 
Many drivers worldwide have some knowledge about the 2-second rule, and some of us may actually use it sometimes; but what is its role in Defensive Driving, what will it actually do for us and what are its limitations?

Of the myriad of driving principles, the one aspect which does most to prevent a crash from occurring is keeping space in front of our vehicle. In more than a century of motorised road transport, no-one has ever crashed into a space! Yet this fundamental principle, on which a plethora of other rules are based, is often abused or ignored.

 

 

  

So why is the 2-second rule so important and why should we use it?  Well, most of us realise it takes time and distance to stop our vehicle when we are moving, but many drivers fail to recognise the length of the space we actually need. Proof, if it were needed, is given annually in 'Road Casualties Great Britain', in which it is reported that whilst there are 'only' 270,000 casualties per year, there are more than 4,000,000 insurance claims annually in UK alone; worldwide there are more than one billion crashes annually. And of course there are a significant number of unreported crashes. Excepting sideswipe crashes in which a driver does not intend to stop, it may appear to be an understatement, but generally almost all of these crashes would not have occurred if one or more drivers had been able to stop before the crash. Clearly, underestimating stopping distance is a major problem amongst the driving public.

There is a simple correlation between travelling speed and stopping distance and although there are very many aspects involved, most books simplify it all in only 2 factors.

In the first of not less than 6 factors, the driver must be able to see the hazard itself. This depends on the driver's vision, i.e. the effectiveness of the eye, which will depend upon the available level of illumination, and on the contrast of the subject against the background areas, i.e. at night how easy is it to see a dark subject against a black background? And how much longer does it take if the driver is temporarily dazzled by oncoming lights? Or is recovering from dazzle? It will also depend upon visibility in the environmental conditions, i.e. clarity of any optical correction the driver may be wearing, the clarity of both the inside and outside of the windscreen, and any atmospheric obscuration of the subject, i.e. rain or fog etc. Assuming the driver sees the hazard, the time taken for the light to reach the drivers eye can be ignored, but it can take a significant amount of time before the driver starts to see the situation developing. In addition, he/she could be distracted or looking elsewhere. Many authorities completely overlook this 'Seeing Time' factor. The next phase can also take a significant amount of time.

In the second factor, the driver's brain now has to compare the viewed scene with past experience in order to decide what to do (if anything at all). This is where an experienced driver having a larger memory bank of similar experiences, has the advantage over the novice who has to consider and calculate for all of the possible outcomes. In many books this 'Thinking Time' is given a nominal duration of just 0.7 second before this is recalculated into "Thinking Distance". Surprisingly to some, the UK thinking time of 0.7 second is one of the shortest published driver thinking times in the world. A glance at Code Rousseau de la Route from France, will show that they calculate 40 % longer at 1.0 seconds, as do many other countries worldwide, and in Australia up to 4 seconds reaction time is considered normal, especially when the range of driver distractions, and alertness / drowsiness is taken into account. Of course, if our driver is merely reacting to the brake lights of a vehicle ahead, the typical incandescent bulb will not start to emit any light until current has been flowing through the filament for around 0.2 second after the increasing hydraulic pressure has operated the switch - more time lost!!

Having done the mental calculations, and made a decision to react, we now have to add the third factor, the 'Muscular Reaction Time' into the equation, i.e. the time taken to lift one's foot from the accelerator, move it sideways, and plant it firmly on the footbrake pedal. And yes, although it doesn't take too long, this is another bite out of that very small reaction time interval !!!  Personally, I think we have all been seriously short-changed by the 0.7 second thinking time, it is unsafely short.

OK, so now we are on the brake pedal, but there is still a lot more to consider …  We may press the pedal quite firmly, but many vehicle manufacturers have decided we don't, and have come to our rescue with EBA, emergency brake assist, to increase the rate at which we press the pedal to achieve a higher hydraulic pressure in a shorter time, to help reduce braking distance. But again this is only the first of several stages of braking. However, for a moment, let's also consider that large bus or articulated lorry just behind us. That driver has seen, thought, decided, and reacted in a similar time to ourselves, and has similarly hit the footbrake, but what happens next? As his treadle valve opens we hear a hissssssssssss as the air hurtles down the pipes towards the brake actuators. Even though modern heavy vehicles have air relays at each wheel to reduce 'Air Lag Time', it can still take an additional 0.5 second before the brakes start to operate, and after starting to operate will never be as rapid as a hydraulic system. So, long after pressing the brake pedal the brakes have yet to operate.

When the brakes do operate, we now have the situation where the brake pads or shoes now start to rub against the disc or drum. But again, we need more time. It may be stating the obvious, but first we have to slow down the wheels, before the tyres can slow down the vehicle. Even here, there are many variables, wheels need time to slow down due to the inertia of a rotating wheel. A small diameter alloy wheel carries less energy than a large diameter or steel wheel, energy which must be absorbed by the brakes, then, ultimately, as the wheels slow down, the vehicle may slow, depending on surface friction. Have a close look at any set of skid marks made by a braking car; look closely and you will see that for the first several meters the marks are very feint - the 'shadow' skid mark, and gradually darken as the wheel slows, the 'Wheel Deceleration Time'.

After all this preceding time we eventually have full braking performance. We may not want to lock the wheels; what we are attempting to achieve is the condition in which the wheels are slowed to a rotational speed of about 10% of the roadspeed, to give maximum braking performance, similar to what ABS may achieve. All we need to worry about now are the many variables in the friction characteristics of the road surface, (not pure friction) variables in the vehicle's suspension and tyres, plus other environmental factors, e.g. the weather, and the always overlooked gradient, but that's another story!

But what about the weight of the vehicle? There is a common misconception that heavy vehicles take longer to stop than lighter vehicles. Whilst there are certainly differences between vehicles in terms of suspension design and tyre compound etc, notwithstanding air lag time, and provided a vehicle is not overloaded beyond its maximum gross weight, the braking distance of a modern vehicle does not depend upon weight whatsoever. (Ancient vehicles may have had undersized brakes). Braking distance is dependant primarily on the square of roadspeed, the friction between the tyres and the road, and very little else, although there are innumerable minor variables. If weight was a factor then 40 tonne trucks would be forever running into the rear of one tonne cars when both stop at red traffic signals. And why is it that a 2 tonne Mercedes luxury saloon can stop in a shorter distance than a 1 tonne family hatchback? Weight, although giving greater momentum forwards also provides greater downforce to increase friction by a similar amount, so is absolutely neutral in all stopping equations.

Anyway, back to the essence of this article: Defensive Driving, safe following distance, and the 2-second rule. If we assume that in modern traffic situations we are very often following another vehicle, a primary need is to be able to slow down or stop behind that vehicle if it should slow or stop, and to accomplish this without hitting its rear whilst maintaining a moderate level of safety. Many books worldwide tell us "never to get closer than the overall stopping distance" in a given table. But this itself presents a boatload of problems. Whilst many driving instructors and a few other drivers can undoubtedly recall all the stopping distance figures in metres, feet, and car lengths, just exactly how accurately can anyone actually measure such distances? I would be quite surprised if any reader of this article can accurately guess the distance this moment from their nose to the wall opposite; and I am certain no-one would guess the dimensions of their living room if they wished to buy carpet. Even for small distances we don't trust our guesswork, we would accurately measure. How then are we to accurately measure distance which may be 10 or 30 times the aforementioned dimensions? How much error would be acceptable? And how are we to accurately measure dimensions when we are travelling at speed? What figure should I use if I am travelling at e.g. 73 km/h? Or at any other speed which is not an exact multiple of 10 ? What figure should I use if my speed is increasing or decreasing? - I find I rarely travel at a constant speed in typical traffic!  Some Highway Codes say we can measure in 'car lengths', which are stated to be 4.0 metres long, but is this any easier to measure? I think not.

 

 

Enter the 2-second rule. We now have a reasonably reliable means of ensuring a safe following interval between us and the rear of the car ahead. Firstly though, a few limitations. The 2 second rule works at all speeds above 10 km/h (6 mph) with no maximum limit, but below this speed use the Queueing Distance Rule. The 2-second rule does NOT give a distance equal to the stopping distance. What it does give us is REACTION TIME, remember that myriad of events stated earlier. We must however make an assumption - that the driver ahead will take a similar distance to stop as ourselves, (not always the case as the following vehicle may certainly have a different stopping ability to the vehicle ahead, e.g. may be on a different quality of road surface). Some may say this is not acceptable, only the full stopping distance will suffice, but as stated earlier, we cannot measure stopping distance. Some may quote the situation where a head-on crash occurs and the vehicles stop dead in almost zero distance, affirming that we need no less than the full stopping distance between us and the vehicle ahead. However, let's just consider the risk of a head-on crash happening directly ahead of us: Consider the number of times you have used the footbrake to slow behind other traffic today, possibly 4 times per minute, multiplied by the number of minutes you spend travelling each day, multiply that by 365 to give the number of times you brake per year, multiply that by the number of years you have been driving. Divide that figure into the number of occasions the car directly ahead of you has been involved in a head-on collision and stopped dead. It's quite a rare event.

So we eventually recognise that for almost all situations a following distance will suffice that can be simply measured as a reaction time. But how do we do this?  Well the 0.7 second reaction time as published in the UK Highway Code may be fine for playing your favourite space invaders or driving game on your computer, but it certainly is NOT sufficient in the real world whilst hurtling along your local High Street in your car. A minimum interval of 2 seconds is however suitable in IDEAL CONDITIONS, providing we add more time whenever driving conditions are not ideal. Firstly we may consider the environmental conditions, e.g. the light, weather, road surface, sightlines, gradients, etc. For each of these which are not ideal we must add at least one second. Then we may consider the speed and condition of our vehicle, if anything is marginal e.g. tyres which are legal but well-worn, we must add another second. Lastly ourselves, if we are not concentrating 100% or if we are fatigued or drowsy, or if we allow ourselves to become distracted by anything inside or outside the car, if we have a headache or any other ailment, again we must add at least another 2 seconds to our following distance. For example, if we are driving home at 60 mph, after sunset, down a slight gradient, after a long day's work, we should add another 5 seconds to the 2 second rule making a 7 second interval.

 

 

 

So how do we actually use the 2-second rule ?  Many references suggest using a road sign, which is fine providing it is not too far from the road edge for parallax to be a problem. Better still is a line across the road, e.g. the shadow of an overbridge, or the line at a pedestrian crossing. It is important that you (and your learner driver) can see and define the reference point before the vehicle ahead passes it. Then of course we recite the well-known phrase "Only a fool breaks the two second rule", but that of course only gives the 2 seconds we need for ideal conditions - don't forget to add the extra seconds for adverse conditions. It may be better to slowly count "one-thousand-and-one, one-thousand-and-two, one-thousand-and-three, …" etc.

However, what may surprise many driving instructors is that the pupil will typically recite the rhyme, and claim to be 2 seconds behind the vehicle ahead regardless of when the front of our car reaches the same point, either earlier or later !!!!
Of course the pupil must be assessed and an ideal way of doing this is to add from time to time the following exercise into your training routine: When in free-flowing traffic at a reasonable speed of maybe 80 km/h (50 mph) or more, ask him/her to follow the vehicle ahead at the same speed as that vehicle and to adjust their following distance until they are exactly 2 (or 3 or 4) seconds behind. Ask the pupil to inform you when they think they have achieved the requested distance. Let the pupil find their own reference points while you assess using your references.

All driving instructors aim to teach driving as a life skill, but it is vital that we teach how to keep a safe separation distance using a method we can all measure that will work in the real world. Just remember - "Space is your friend" no-one has ever crashed into a space. The 0.7 seconds thinking time in the Highway Code is woefully inadequate, but if it is extended to a minimum of 2 seconds in ideal conditions, and longer still for any adverse condition, we can substitute braking distance for reaction time to ensure a safe following space in all traffic conditions.

  


Table 1 below identifies the ideal conditions under which the 2-second rule works.

                      Conditions for a 2 second interval
Environment  -      daylight, clear weather, warm dry clean level surface   -   2 sec 
Vehicle          -      all new / good condition, < 50 mph                                  -   2 sec
Driver            -       fit, alert, good sight, concentrating                                  -   2 sec


Table 2 below identifies some examples of the non-ideal conditions demanding time in addition to  2 seconds. However, there are many other conditions.


                Conditions                   additional seconds
     Environment -

 Night - good street                                  1
 Night - no street lighting                          2
 Air temperature below +5º C                  1
 Falling rain                                                2
 Fog                                                            2
 Uneven road surface                               1
 Wet surface                                              2
 Loose gravel, sand, or dust                    2
 Muddy or greasy surface                         3
 Snow or ice                                               5
 Downhill < 5 %                                          1
 Downhill > 5 %                                          3
  
      Vehicle -

 Windows not clean                                   1
 Speed > 80 km/h, (50 mph)                    1
 Tyre tread < 3 mm                                    1
 Tyre inflation unchecked                          2
 Suspension system worn                         2
 Leaf-spring suspension                           1
  
       Driver -

 Eyesight improperly corrected                                4
 Fatigued or drowsy                                                   3
 Suffering a mental distraction                                  3
 Suffering any ailment                                                2
 In-car distraction, e.g. adjusting the radio              2
 Scenery distraction                                                   2
 Navigation distraction, e.g. reading direction sign, GPS, or map,        2 sec

 

 
 © Keith Lane  B.Sc.DE. 2006