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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