Monday 10 September 2012

Speed limit proposal based on bad statistics.

Winnipeg City Council is discussing lowering speed limits to 40 km/h in residential areas. The justification for this includes a study conducted in Edmonton that appears to show a great benefit to lowering the speed limit. It's even baked right in to the proposal before council.

 The proposal includes the following:

AND WHEREAS the City of Edmonton recently reduced speed limits to 40 km/h in several residential neighbourhoods with a 25% drop in severe collisions;

THEREFORE BE IT RESOLVED THAT Council ask the Winnipeg Public Service to consider lowering the residential speed limit in Winnipeg to 40 km/h

From the study:
However, these reductions were not significant, as the 95% confidence interval included zero, implying no change or no effect. Generally, when a confidence interval is very wide like this one, it is an indication of an inadequate sample size (i.e. short “After” period) and implies poor precision. Consequently, the results of the collision analysis were inconclusive and additional research will be required to substantiate the impact of the pilot project on the number and frequency of collisions.
(emphasis mine)

That's a big problem with research related to public policy. Give a politician a study that says "implying reductions in predicted collision counts of 25% with a 95% confidence interval of -81%, 77%" and the politician will read "blah blah blah 25% reduction in collisions! blah blah blah."

I read "95% confidence interval of -81%, 77%" and see "these results are as useful as a tampon dispenser in my garage".

Some may also point out that the study showed a statistically significant decrease in operating speed in the study areas with the 40 km/h speed limit, but the decrease only averaged 3.95 km/h. Assisting with the decrease were the following factors:
i) a pre- and postcommunication plan;
ii) installation of new speed limit signage and setting up speed display boards, dynamic messaging signs and school dollies;
iii) implementing community speed programs (i.e. Speed Watch, Neighbourhood Pace Cars and Safe Speed Community vans) and
iv) using covert photo-radar trucks.
It was the result of a full-out blitz to increase awareness and reduce speeds, with a modest result of 4 km/h.

Look, I don't want kids to get run over by cars any more than you do, but this isn't the answer. Most people already drive a reasonable speed in residential areas. This isn't going to change that. What this will do is lower speed limits on collector streets where 50 km/h is a reasonable speed, but are deemed "residential", providing more opportunity for the police to set up radar traps to ticket drivers who are driving in a responsible manner.

What we need in this city is a common-sense approach to setting speed limits. It is insane that Kenaston Blvd has the same speed limit as Valour Rd. Set speed limits at levels that reasonable according to industry standards (ie. 85% percentile) and adjust as necessary for special cases like school zones. Let's not create misguided legislation based on inconclusive data and misinterpreted studies. That never turns out well.


One Man Committee said...

Great bit of insight. However, I think the penultimate paragraph is dead on when it comes to the reason why this detail won't sway anyone in the end.


reedsolomon.matr1x at said...

I say we lower the speed limit on residential streets to 5 km, and increase the fines to $5000 for every 10km over the limit.

Because somebody has to think of the children.

Honestly now. How ridiculous.

Riverman said...

Depends on the neighbourhood I guess. Where I live (Glenelm) shortcutting through at full throttle is a local sport. I'd say 30 km/h is fast enough.

cherenkov said...

OMC: ya, probably ..

Reed: studies have shown that getting hit by a car at 2.5 km/h is less 30% dangerous than getting hit at 5 km/h.

Riverman: Would decreasing the speed limit on Hespeler to 40 or 30 help with your short cutting problem?

bwalzer said...

All we really have to do is actually respect the existing limits. Lets face it; in Winnipeg the posted speed is treated as a minimum with the actual maximum speed 10 km/h higher. Just try to stay at or below the speed limit and Winnipeg and you *will* experience hatred...

Riverman said...

"Would decreasing the speed limit on Hespeler to 40 or 30 help with your short cutting problem?"

I believe what the city is proposing to do involves residential, not arterial streets.

Anonymous said...

@ Riverman - Where the heck is Glenelm?
-The whole world needs to slow the hell down.
Bicyclist that has been hit by a car 8 times.

Riverman said...

Anon - try a map. ;-))

cherenkov said...

River: Hespeler is set at the default residential 50 km/h, no? It has houses on it. The proposal is pretty vague, but I would not assume it precludes arterial or collector streets, as those are also often residential.

Riverman said...

No, I don't believe the action will involve arterial streets such as Hespeler. It is a Metro route, a truck route and a bus route. Nobody plays street hockey on Hespeler.

Gustav Nelson said...

A standard speed limit should be set, but focus less on enforcement so police resources aren't being tied up and as an alternative put up speed bumps.
Speed bumps are considerably more effective at controlling speeds a majority of the time, even to those who do not worry about radar traps.

Riverman said...

Council approves plan to ask public works to consider reducing residential-street speed limits to 40 km/h. Vote is 11-3.

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