Hi everyone,
Since the issue of the "85th percentile" comes up consistently in discussions around speed limits (and speed limit reductions), I thought others might be interested in reading my letter below in case it doesn't get published in the Winnipeg Free Press.
For reference, it was in response to Mr. Clark's letter which is pasted below as well.
cheers, Beth
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*Letter to the Editor (sent Tues, Dec. 21'11)*
In response to Mr. Ted Clark's Dec. 21 letter "Grant speed limit is result of spurious reasoning", the use of the 85th percentile in defence of higher speed limits is a circular argument. The speed at which motorists travel is primarily influenced by road design. A street that is wide, straight and flat encourages driving faster. To argue that the speed at which 85% of motorists are travelling at (or below) on a road is therefore reasonable and acceptable is an argument that feeds itself. It does not take into account adjacent land uses or other road users, such as pedestrians from surrounding houses and apartment buildings crossing Grant to reach the shopping centre or students walking or cycling to Grant Park High School. OurWinnipeg, the City's new long-term vision, recognizes the critical connection between land use and transportation. I suspect that it was an early acknowledgment of this connection that compelled councillors and the provincial highway traffic board to set a 50-km/h speed limit on Grant. If motorists are driving above the desired speed limit because the road design encourages it, the answer should be to address road design.
Beth McKechnie
* * * * * Grant speed limit is result of spurious reasoning
The real scandal on Grant Avenue in the vicinity of Nathaniel Street is not the accuracy or otherwise of the police radar speed measurements.
The real scandal is that the legal speed limit has been set far too low, leading to annoying and inappropriate enforcement.
Years ago, that particular Grant speed limit was set much higher. City councillors then proposed that it be lowered to 50 kilometres per hour.
Traffic studies showed, however, that the speed limit should be at least 60 km/h. Nevertheless, the committee members decided on the 50-km/h limit "in the hope that drivers would then go 60."
This unfounded conjecture was explicitly articulated at the time by the area councillor, and the other councillors then succumbed to his flawed reasoning. I don't know whether this was recorded in their minutes at the time.
I, as the city's then-director of streets and transportation, was there, and I have a distinct recollection today.
For reasons unknown to me, the provincial highway traffic board (whose ultimate approval was needed then if not now) went along with this spurious reasoning.
In some American states, local governments are required by statute to conduct proper and valid engineering and traffic speed studies before modifying speed limits on local roads.
If the outcomes of such studies are not respected, the senior government may overrule the inappropriate limit or even withhold transportation-related financial grants to the junior (urban) government.
Central to such studies is measurement of the speeds at which motorists are typically travelling. The most appropriate limit is usually a speed close to the "85th percentile" -- the speed at (or below) which 85 per cent of the motorists are driving.
The highly reasonable related presumption is that 85 per cent of motorists are inclined to behave in a reasonable and prudent manner and therefore enforcement directed at more than 15 per cent of motorists is likely to be impractical and unpopular.
Unfortunately, the automatic equipment now being used by police has made it practical to target a large number of responsible motorists, whose indignation is thus understandable.
The city should conduct proper traffic studies (taking into account not only the 85th percentile speed but also geometric design, crash data and the functional classification of the roadway) on Grant between Stafford and Kenaston, and then should scrupulously abide by the outcome.
TED CLARKE
Winnipeg