On Tue, 25 Feb 2003 21:33:51 +0000, "Kit Wolf" <
[email protected]> wrote:
>Whenever I see a speed camera sign, I think of the safespeed website, see red, and cycle past
>calmly at a reasonable speed. I used to enjoy trying to tease the gatsos.
LOL! Here's an interesting site, though, linked from s*f*sp**d -
<http://www.tfhrc.gov/safety/speed/speed.htm>
The author analyses a lot of data from various sources and concludes that changing urban speed
limits makes little difference, increasing highway speed limits increases accidents, and the safest
speed is the median speed or slightly below, which, on a 30mph road, would mean travelling at -
guess what? somewhere below 31mph[1].
I noticed in particular figure 8, showing (amazingly) a rise in the accident rate with increasing
speed, and a drop with decreasing speed.
Also some tables showing, generally, speed limit reductions leading to reductions in crashes, and
speed limit increases leading to rises in crashes.
Also tables showing quite strongly that traffic calming significantly reduces crash, injury and
fatality rates, but that some cagers compensate by driving more dangerously on adjacent streets.
The summary is quite measured:
"There is evidence that crash risk is lowest near the average speed of traffic and increases for
vehicles traveling much faster or slower than average[1]. The occurrence of a large number of
crashes involving turning maneuver partly explains the increased risk for motorists traveling
slower than average and confirms the importance of safety programs involving turn lanes, access
control, grade separation, and other measures to reduce conflicts resulting from large differences
in travel speeds.
"When the consequences of crashes are taken into account, the risk of being involved in an injury
crash is lowest for vehicles that travel near the median speed or slower and increases exponentially
for motorists traveling much faster. One of the major concerns in all of the studies is the travel
speed before the crash. Emerging technology used in mayday, vehicle tracking, and adaptive speed
control systems provide the opportunity to accurately and continuously capture travel speed. This
technology should be applied in improving our understanding of the relationship between speed, speed
variation, and safety.
"When a crash occurs, its severity depends on the change in speed of the vehicle at impact. The
fatality risk increases with the change in speed to the fourth power[2]. International research
indicates the change in injury crashes will be twice the percentage change in speed squared, and
fatal crashes will be four times the percentage change in speed. These relationships are based
mainly on speed limit and speed changes on high-speed roads. More research is needed to assess their
applicability to low-speed urban roads[3].
"In general, changing speed limits on low and moderate speed roads appears to have little or no
effect on speed and thus little or no effect on crashes. This suggests that drivers travel at speeds
they feel are reasonable and safe for the road and traffic regardless of the posted limit. However,
on freeways and other high-speed roads, speed limit increases generally lead to higher speeds and
crashes. The change in speed is roughly one-fourth the change in speed limit. Results from
international studies suggest that for every 1 mi/h change in speed, injury accidents will change by
5 percent (3 percent for every 1km/h). However there is limited evidence that suggests the net
effect of speed limits may be positive on a system wide basis. More research is needed to evaluate
the net safety effect of speed limit changes.
"Most of the speed related crashes involve speed too fast for conditions. This would suggest that
variable speed limits that adjust with traffic and environmental conditions could provide potential
benefits[4].
"Despite the large number of references concerning traffic calming, very few reports include results
of a systematic evaluation. In many cases traffic volumes as well as speed are reduced. As a result
of the traffic diversion, crashes may be migrating to other roads. More research is needed to assess
the system wide impacts and permit comparisons to be made among individual as well as combinations
of traffic calming measures. "
[1] average speed on a 30mph road is 30.9mph, average on a 40mph road is 35.8mph.
<http://www.transtat.dft.gov.uk/tables/tsgb02/4/download/41302.xls>
[2] the word *change* is omitted in Smith's hilarious 12mph page.
[3] i.e. the entire 12mph page is complete bollocks :-D
[4] I said that! see
<http://www.chapmancentral.com/Web/public.nsf/Documents/Safer_Roads_Manifesto> Guy
===
** WARNING ** This posting may contain traces of irony.
http://www.chapmancentral.com (BT ADSL and
dynamic DNS permitting)
NOTE: BT Openworld have now blocked port 25 (without notice), so old mail addresses may no longer
work. Apologies.
