On 12/02/2011 23:07, tracyinoz@xxxxxx wrote:

/And no, 'differential velocity' does not 'cause' a collision: one 
driver not seeing another vehicle is a 'cause', two cars trying to 
occupy the same space at the same time is a 'cause'; differential 
velocity it is a factor in the likely severity given the realisation of 
the collision in the first instance (save in a veeery small percentage 
of cases) - see also response to Nicholas Mc Guire's email.
/
I accept that speed may not be a root factor in most collisions. But my 
point was about safety in general, and safety involves both the 
probability and the severity of a hazard. The "5%" statistic is a 
dangerous one if not put in context. And while we are  being pedantic,
1. by the laws of physics, I see no mechanism apart from differential 
velocity that can be the immediate cause a collision, though there may 
be other root causes, and
2. cars cannot "try to occupy the same space"  (unless they have some 
incorrectly programmed intelligence) - it is the drivers do this.

Apart from this I agree wholeheartedly with your comments about average 
speed cameras that, at least subjectively, reduce the differences in 
velocities between cars, donīt cause hard braking, and hence improve, at 
least subjectively, the safety on the roads.






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