Sunday, May 24, 2009

2-second rule not enough for highway speeds

My friend Hamayar, who hails from Wales, sent me this link, from the UK Survival Skills site. The site is pretty cool, although some of the articles need translation to driving on the right. The referenced article makes the point that the two-second rule provides a following distance in which the bike sometimes can't stop in time to avoid a vehicle stopping in front. His calculation is basically that the two second rule is insufficient at speeds of 60 mph and above, probably because the distance covered during reaction time is greater at these speeds.
Food for thought, epsecially for group riders on the highway.
My take is that the calculations aren't conservative enough. He uses 0.9 G of braking force for the motorcycle braking force, while the Quebec Promocycle Foundation did a study that found more like 0.75G for a bike without ABS but an experienced and skilled rider. The Quebec site has more on rider reaction time also, which makes the half second response time look a bit skimpy. The average seems to be consistently a bit over the half-second, and is actually worse for women. The average reaction time looks more like 5.3 seconds, and a large number of the braking intervals sampled were longer than this average.
The Quebec study suggests that both the assumptions the Survival Skills author used were skewed toeards underestimating the problem.
The lesson is clear. The two second rule is an absolute minimum at speeds up to 45 MPH - or maybe less - and seems to be less adequate at highway speeds or greater.
Has anyone got a calculator who can apply the Quebec numbers to the Survival Skills computations? I bet that would be a scary set of numbers.
In the meantime, we should probably rethink our following distance calculations for highway speeds, and add a liberal (or conservative) dose of extra following time to the two-second rule. For group riders and lead riders, perhaps something to mention at the next safety briefing. Looks like inexperienced riders might want to add even more time cushion.
In the meantime, y'all enjoy the Promocycle and Survival Skills websites.

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Anonymous Kevin Williams / Survival Skills said...

Good Day from the UK

I've just come across a link 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, with the assistance of Steve Kelly on Excel!

I'm not sure of the exact date of the 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 you've mentioned (which I am aware of).

The "g" figures for braking were based on a magazine article that found that sports bikes can actually brake at just over 1g under ideal conditions with a skilled rider. We went with 0.9g for the calculations as rather more readily obtainable by someone who practices emergency braking.

Even so, I did emphasise at some length that our first calculations were "best case scenario", and we then went on to do a second set where we suggested that in fact it was probably more realistic to set the Two Second bar at a speed as low as 45mph!

Some other comments which I hope you will find constructive.

You've chosen to use an average reaction time on braking of .62 of a second based on that study, rather than the .5s that we used.

Realistically, such slight variations in reaction time makes relatively LITTLE difference to your stopping distance once the speed starts climbing - the difference of 0.1 second between your calculations and mine actually results in your "rider" travelling only 3 metres (10 ft) further when stopping from 60mph.

Neither does the reduced braking efficiency of your 0.7g (which I wouldn't dispute as possibly being more representative of the average rider!) have quite the effect that people might expect because the added distance is linear with change of speed. Your reduced braking efficiency adds around 9m (30ft) to my calculated distance of 54m (180ft) at 60mph.

Significant - but not as significant as slowing down a little! Your own figures show that you get that 9m (30ft) back by dropping an insignificant 5mph off your speed!

The other key point is that since reading an article by an accident investigator in the UK I've become aware that reaction time and braking time are only two of the three delays in 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 distactions either, such delays happen because riders don't simply don't recognise the potential threat ("He MUST have seen me..."), or because they are focussed on another task (maybe a complex manoeuvre in traffic).

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

Anyway. Glad the article was of interest, and the more people who read the sums and get a handle on the need for decent following distances, particularly the "go faster, take MUCH longer to stop" physics, the better!

March 18, 2010 7:40 AM  

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