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Defensive Riding, Machine Control, Mental, Progress

The Two Second Rule is not enough – revisited

I’ve just spent a half hour reading and replying to this particular blog page, which I found interesting as I’m the author of the original “When the Two Second Rule is not enough” article on in the Riding Skills section on www.survivalskills.co.uk, with the assistance of Steve Kelly on Excel!

The original Survival Skills article suggested that at “fast highway” speeds, the “Two Second Gap” rule isn’t enough, and that it should be stretched.

It came in for a bit of stick at the time, usually because people have read articles about how much better modern brakes are without really comprehending that we now go a lot faster, so it’s nice to see it revisited and objectively considered.

I’m not sure of the exact date of the original article, but it came about because of discussion on the old Compuserve Ride forum, so I’d guess probably 98/99/00 for the first version.

That means it almost certainly predates publication of the Montreal braking study that gets a mention on the blog, a study which I’ve actually read myself.

The “g” figures for braking that we came up with were based on a then-recent magazine article that found that sports bikes can actually brake at just over 1g under ideal conditions with a skilled rider.

Steve and I went with 0.9g for the calculations as being rather more readily obtainable by someone who practices emergency braking.

We also used a half second reaction time, which is often reckoned to be about the quickest that an alert rider can manage.

The blog author suggests that the Quebec study shows that both our assumptions we used were skewed towards underestimating the problem. They plumped for 0.7g and a reaction time of 0.62s.

How does that affect the figures?

To give you a couple of comparison speeds, the Bikesafer blog came up with these figures (my ‘best case’ calculations in brackets) :

30mph 66.15 (55ft)

45mph 128.38 (108.0ft)

60mph 210.05 (178ft)

As it happens, I did emphasise at some length that our first calculations were “best case scenario”, and we then went on to do a second set:

30mph 77ft

45mph 141ft

60mph 222ft

These are actually WORSE than the Bikesafer blog!

From these, Steve and I suggested that in fact it was probably more realistic to set the Two Second Gap “bar” at a speed as low as 45mph and to make sure you add another second at motorway speed.

It’s interesting to see what happens as the “g” figures and reaction delay figures get played with.

Let’s look at reaction times first.

The extension of the average reaction time to .62 of a second rather than the .5s that we used (ie from good to average), makes for virtually no difference to your stopping distance once the speed starts climbing; it results in their “average reactions rider” travelling only 3 metres (10 ft) further when stopping from 60mph than my “good reaction rider”.

But we need to look at reaction time a little more closely. Since reading an article by a UK accident investigator in the UK I’ve become aware that reaction time and braking time are only two of the three delays that occur when stopping. The third is PERCEPTION time, which is the time it actually takes the rider to realise the brakes need some use!

Rather alarmingly, this cognitive delay when the rider is simply unaware there is a situation requiring input can be up to TWO seconds.

It’s not simply lack of ‘concentration’ or ‘being distacted’ either, sometimes such delays happen because riders see the hazard but don’t simply don’t recognise the potential threat (“He MUST have seen me, he can’t possibly pull out…”), or because they are focussed on another task (maybe trying to thread through a complex junction in heavy traffic).

Although I wasn’t full aware of the rationale at the time I wrote the article, it’s actually this cognitive delay that I attempted to cover with our second set of calculations, where we set the reaction time at 1 second.

So what about improving your braking efficiency?

It turns out that neither does increasing your braking efficiency give dramatic improvements in stopping distances, certainly not enough to think you can ride a lot faster because you can brake better.

The blog author argues for a reduced braking efficiency of 0.7g (which I wouldn’t dispute as possibly being more representative of the average rider!) against the 0.9g Steve and I used. Yet though this equates to a reduction in braking efficiency by over 20%, it adds only around 9m (30ft) to my calculated distance of 54m (180ft) at 60mph.

9m IS significant – but not as significant as slowing down a little! The comprehensive calculations on the blog show that you get that 9m (30ft) back by dropping just 5mph off your speed!

And last important point. It’s not just stopping distance you need to think about, it’s the consequences of over-shooting and your impact speed.

If you start braking from 60 at the same moment as someone starts braking from 30, and the slower vehicle JUST stops, you won’t hit the obstacle doing the common sense answer of 30mph; your impact speed is actually around 52mph!!

It’s because there is a square term in the calculation of velocity, so higher speeds carry a lot more energy and so it takes far longer to get rid of the first few miles per hour.

So relatively small changes in speed have relatively larger changes in stopping distance. More proof that slowing down is better than braking harder!

What these alternative figures really demonstrate very clearly is that shaving fractions off your reaction times and having the skill to braking on the edge of tyre adhesion (or putting your trust in ABS!) isn’t the key to cutting braking distances dramatically.

Sure, you’ll make small improvements (and ANY improvement is worthwhile) but the big gains come from:

a) realising there is an emergency developing early, and

b) riding a little slower in potentially dangerous situations.

It’s good to see the article getting an airing, and quite frankly, the more people who read the sums and get a handle on the need for decent following distances at speed, particularly if they begin to believe the “go a bit faster, take MUCH longer to stop” physics, the better!


About Kevin Williams / Survival Skills

Motorcycle trainer, motorcycle author, motorcycle safety consultant, motorcycle forum moderator, former courier and ever a recreational rider. Is there a common theme here?


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