Jeremy
....actually, the UK version (of the 1/7th rule) is, or rather was, V85
+/-7mph (effectively, as that's what the wording of DfT 1-93 implied), V85
being the 85th percentile speed. The latest DfT guidance on speed setting
on primary roads is now mean speed, with no tolerance, DfT Circular 01/2006.
So, basically, the British use the same procedure......unless, of course,
you live in Surrey where it's based on a call-my-bluff game between highways
officers and local politicians.
(Sorry, but we just had this with a stretch of the A24 locally....and that's
actually how it was arrived at.)
A complicating problem is that certain local authorities will have used
traffic models to estimate a "delay time" incurred by reducing speeds on a
particular stretch of road (because traffic is moving slower) and then
insert an economic cost into the arguments. I haven't yet seen the
calculation presented in written form, but know that our local transport
director has used the argument with councillors.
Cheers Jeremy
John
-----Original Message-----
From: Cycling and Society Research Group discussion list
[mailto:[log in to unmask]] On Behalf Of Jeremy Parker
Sent: 17 January 2008 20:44
To: [log in to unmask]
Subject: Re: 20mph speed limit research[Scanned-Clean]
I note that in the USA the "default" urban speed limit seems to be
fairly universally 25 mph, not 30. That's just an impression, not
based on any perusal of all the states' laws. I don't know whether
the "Uniform Vehicle Code", a sort of moral example to encourage
sensible state laws, has any numerical value. I suspect not. The 25
mph value has been around for a long time. It was long established
when I first went to the USA forty-four years ago. All the fifty
states must have had their own history of how their laws evolved.
That 25 mph is generally set by state law, but, just as with the
British 30 mph, a street might be given a different limit. It seems
to be the standard practice of American traffic engineers to choose a
speed limit by observing the speeds that motorists actually follow,
and then setting the limit to turn one driver in seven into a
criminal. There's some handwaving about standard deviations involved
with that, to make it sound scientific, but it does strike me as
being an odd way to go about things
Jeremy Parker
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