REVIEW - The 85th Percentile Speed Limit

Speed DemonI found this observation on the Grist web site: "Speed limits are set by drivers voting with their feet. That’s a problem for everyone else on the street." It was accompanied by the following video explanation of the 85th percentile speed limit setting method. Fine if you are a driver, but what if you are some other form of road user such as a pedestrian or cyclist?

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That's exactly how democratic society and legal framework is supposed to work: take the reasonable and prudent actions of the majority determined by votes/actions and implement a legal framework to mitigate the outliers. If you don't consider the actions of the majority you'll end up making laws that nobody seems to follow. The video proposes no alternatives to this "old thumb rule", and it makes one anecdotal example of a street where speed limits went up because of "votes", while ignoring the reality that many speed limits aren't being adhered to in BC or Washington (video is made by a Seattle outlet), because the limits aren't representative of the reasonable and prudent actions of the majority. The video also furthers a myth that car speed limits are "limitless" - that drivers will just drive faster ad-infinitum; that is simply not true and has been debunked, as the majority base their choices on immediate circumstances with personal safety in-mind - geometry of the road, the sight-lines, the time of day, weather and visibility. Some of those factors can be designed/altered by road engineers where appropriate, to slow the 85th percentile majority naturally.

Good laws are the reflection of the collective actions and habits of the majority as they go about their daily lives. Bad laws are the ones that are instituted against the commonly accepted practices of the society.
The reason why Seattle and Vancouver are in a crisis of hard drugs, property damage and criminality is due to the failure to mitigate the outliers. What good does it make to have law-abiding taxpayers and productive members of the society drive at 20 miles per hour (downtown Seattle and Vancouver) when hard drug users walk onto the streets with no regard for others of their personal safety in drug-induced deliriums?

Limiting and punishing the responsible participants of the society in order to enable, empower and exonerate the anti-social, nihilist and narcissist outliers leads to the filth and squalor that Seattle and Vancouver are well known for. The sooner we kick this mentally-ill inspired invert-o-philosophy, the faster we'll get back on track to productivity, progress and prosperity.

I took the point that the 85th percentile setting method only looked at drivers. Pedestrians and cyclists did not have any input unless they were a collision statistic.

My personal bias is that many drivers choose a selfish speed rather than a safe speed.

When you can't make everyone happy, the best course of action is to pick the largest group that you can make happy, and that's what the 85% rule does. The video doesn't provide a method or guidance it just asks "but what about the cyclists and pedestrians?" and makes superfluous jabs at the 85% rule that it was just a historic tradition for rural highways, but they aren't making any alternative suggestions. This video attacking the best known choice without providing an alternative is coming off petulant and uncouth.

I can't disagree with you regarding drivers making unsafe choices - I'm remembering that video you've posted of a Volvo passing you on a blind bend over double-lines, and I have to agree - absolutely selfish and scary-unsafe. But I feel that in your area there are two distinct and ultra-incompatible groups of drivers: locals 50% and transient ferry catchers 50%. Especially around the area of Parksville - scary incompatibilities in driving styles. You need a highway by-pass there, fast. Here in Vancouver/Burnaby its more of a 95% normal and 5% students / scared drivers, maybe a few outliers like stunt-driving N-drivers.

I belive engineering is the answer to making things more reasonable and safe, but land improvement costs a lot to construct and maintain; thus the engineering will naturally consider the needs of the people who are paying for it, and the majority of the people who pay, choose to ride around in cars most of the time. That's why the 85% rule considers the overwhelming majority: 85/15.

Thanks for pointing out one of the major flaws in the slower is always better camp's thinking. It's proposterous to suggest infinity is the chosen speed in the absence of a sign mandating otherwise. But this is what they do. As I suggested to Tim last week, when I first saw that video, it's doubtful to me that any of those people in the video have ever touched a steering wheel or ever plan to. 

How can anybody watch this 'sensational' video, without realizing that the producer is selecting 'facts' and determining 'conclusions' about the 85th percentile rule without some sort of reality check. Are we supposed to believe that this one street in LA has gone from a 30 mph zone to a 60 mph zone, just because each time they raise the limit, drivers simply go faster all the time - that 'feedback loop' nonsense they're expecting us to take seriously?

What a load of crap.

The whole idea behind the 85th percentile concept is that there is an optimum safe speed that most drivers will drive at, regardless of the limit that has been posted (or not posted, if it's the usual 50 kmh city / 80 kmh hwy limit).

I took the point that the 85th percentile setting method only looked at drivers. Pedestrians and cyclists did not have any input unless they were a collision statistic.

Of course it looks at drivers. It's the speed LIMIT and typically, they're the only road users capable of reaching, never mind exceeding that limit. As for pedestrians and cyclists? Well, most pedestrians who get killed in BC are in clearly marked crosswalks, never even noticing (until it's far too late) that they're about to get hit. And most cyclists don't give a damn for the speed limit; in the last few years, I've driven senior's buses around Stanley Park many, many times. Lots of radar there, no reason to hurry, 30 kmh zone clearly marked. On every occasion pretty much, cyclists - either singly or in packs - have passed my vehicle, often travelling significantly faster on the downhill sections. They don't care about the rules applying to everybody - and neither do the cops running the radar, who never ticket cyclists for speeding (or pretty much anything else so far as I've seen).

What good does it make to have law-abiding taxpayers and productive members of the society drive at 20 miles per hour (downtown Seattle and Vancouver) when hard drug users walk onto the streets with no regard for others of their personal safety in drug-induced deliriums?

Well, let's think about this from the point-of-view of the traffic engineers. Shall we?

And let's suppose that certain areas / intersections feature really highly in statistics on pedestrian deaths. Which is what happens, and what traffic engineers are expected to address.

Consider Main & Hastings, here in Vancouver, for instance. Oddly enough, that was the first intersection in BC to get traffic lights, almost a century ago. These days, it features as the #1 spot in this entire province for pedestrians getting killed or injured; other intersections on Hastings, both sides of Main street, have elevated numbers of pedestrian deaths.

So if you're the traffic engineer trying to reduce the number of collisions, injuries and deaths, reducing the local speed limit to 30 kmh for a few blocks seems like the best solution you've got, in order to do your job. It's not unlike the 30 km/h School Zone in the 500/600 block West Keith in North Vancouver (except that the pedestrian collision rate there was never actually an issue - the imposition of that zone was arbitrary, and probably unnecessary). This is also a multi-lane arterial road, but I don't hear any comments about how the rest of us are massively inconvenienced by the existence of Thomas Aquinas High School students. Heck, they don't need drugs to distract them They have phones to do that ... 

OK, so let's go back to that video. That producer guy claims that most of the collisions occur with traffic turning at cross-streets. This in itself is remarkable! The fact is, most 2-vehicle collisions, most of the time, are rear-enders. These are caused by drivers following too close (which seemingly never gets ticketed) or not looking sufficiently far ahead, or being distracted, etc.

So what makes Zelzah Avenue in LA so dangerous, in terms of crashes? Remarkably - according to the narrator/producer - it's 'turning cars' (and here we get an illustration of some fool in a nice pale blue car turning directly into the path of the evil oncoming red car, even though in fact left-turning drivers generally don't make this suicidal mistake).

But guess what? You find these 'turning cars', most of the time, at intersections. Turning into the cross-street - or from the cross-street. And what do we see on Zelzah Avenue? Answer: multi-lane traffic, with a cycling lane just to the right, and then the parallel parked cars at road edge. And by my reckoning, that's about as much as a traffic engineer can do, in order to create horrible sight-lines for drivers.

This type of situation also puts cyclists at great risk, due to drivers opening a car door right in front of them.

I think LA may need better traffic engineers. And they had better be smarter than the fool who created this video.

The whole idea behind the 85th percentile concept is that there is an optimum safe speed that most drivers will drive at, regardless of the limit that has been posted.

It is acknowledged by the NHTSA that the 85% rule is outdated as it is a rule of thumb guide based on rural highways in the 1930s

What good does it make to have law-abiding taxpayers and productive members of the society drive at 20 miles per hour (downtown Seattle and Vancouver) when hard drug users walk onto the streets ..

Could it be possible that saving the lives of some pedestrians who stray off the sidewalk is a good thing? Categorizing all such people as hard drug users devalues them - my experience is that more are drinkers.

I don't understand the idea that reducing speed limits is a punishment. If you will just comply with the damn rules then you won't get punished. Same as traffic congestion that slows you to a crawl - it's an incovenience not a punishment


Going painfully slowly for no apparent reason feels like punishment to me.

For sure, motor vehicle laws ought to reflect conditions.

I've lived on the north shore here for many years. There are two arterial roads - Dollarton Highway, and Capilano Road - which used to be managed under the auspices of Provincial Highways. Each were zoned at 60 km/h, which was pretty much in that 85th percentile of normal traffic speed.

Eventually, maybe 20 years back (from memory) the province transferred the responsibility for both of these to the District of North Vancouver.

Not long after, and at no small cost, improvements were made - particularly to Capilano Road. Road widening enabled the addition of separate bicycle lanes, traffic lights were upgraded (or added) to facilitate vehicle movements at intersections, and the sudden dogleg in the 2300 block became an easy curve, with vastly improved sightlines for drivers. Awesome!

Oh, and the other thing they did was to change the speed limit on this vastly improved multi-lane arterial from 60 km/h to 50 km/h. Good grief ...

It probably won't surprise anyone to know that average speeds on that road really haven't changed much, over the last four decades. It's just that these days, we all have way better cars (better tires, better brakes, etc) than we had in the 70's. Plus, the road has been vastly improved.

Except for the speed limit ... 

I would argue that the 85th percentile approach makes a lot of sense, for a number of reasons.

First of all, speed matters.  If it didn't, we'd all still be walking everywhere and using oxen to move heavy loads.  The first person to train a horse had a need to go faster and farther.

Second, motorists act out of self interest.  I don't think it's fair to call it selfishness.  The vast majority of motorists, and I mean well over 90%, typically drive in a manner that's appropriate for conditions.  They intuitively consider road width, shoulders, grades, curves, driveways, sight lines, other traffic, pedestrians, cyclists and other factors in choosing their speed.  Quite reasonably, they want to get to where they are going as quickly as possible with a moderate level of risk.

Third, all movement involves risk, starting with getting out of bed in the morning.  We have to accept a certain amount of risk as part of living.

Fourth, the 85th percentile principle involves observation of normal motorist behaviour.  It is not arbitrary.  It means that, for example, if the 85th percentile speed on a roadway is 91 km/h, the speed limit should be set at 100 km/h.  The result is that well over 90%, and perhaps close to 100% of motorists will drive in accordance with the speed limit most of the time.  This is important.  They will naturally do this without having to use cruise control or constantly monitor their speedometers to avoid a ticket.  

Speaking of speedometers, their primary purpose is speeding ticket avoidance.  They are not safety devices.  Think of driving on a snow covered road; you are too busy evaluating and responding to conditions to look at the speedometer most of the time.  Nobody has to tell you that the speed limit may be too fast for conditions.  Safe driving requires dramatically more judgement than looking at a number on a dial.  Thankfully, this level of judgement is not beyond the realm of most human beings with some training and experience. 

Fifth, posted speed limits, like most laws in civil society, should be an indication of the norms of behaviour.  Arbritrary rules get ignored; reasonable ones are respected by the vast majority of people.  One example might be an extremely low speed limit in an overly long posted construction zone on a highway.  Driving 50 km/h feels odd and silly when there are no workers around and the road has already been repaved.  

My basic conclusion - the vast majority of motorists regularly exceed speed limits, not because these motorists are acting unreasonably or in an unsafe manner, but because speed limits are arbitrary, and don't make sense in light of normal human behaviour.

Using the 85th percentile approach is reasonable, it balances productivity and risk, and it's democracy in action.

So what about pedestrians, cyclists, and children in playgrounds?  The answer is that every roadway has effective design speed, whether deliberately set or simply as a result of choices made when it was first built. Traffic engineers deliberately set design speeds for highways - it's really just a process of reverse engineering from speed surveys.  They know how motorists will behave given lane and shoulder widths, sight lines, grades, and curves.  (BTW, the normal design speed for divided four lane+ highways in North America is 130 km/h.  It's why 100 and 110 km/h speed limits are widely ignored.)

This design logic can also be applied to urban streets.  If you want traffic to move at 30 km/h or less, make the paved cross-section of the street a minimum 6 metres wide, and people willl naturally drive that slowly without getting frustrated and without looking at their speedometers.  Use crosswalks, different paving textures, landscaping, and other features to create an attractive urban environment.  Work with normal human behaviour rather than against it, and the need for speed enforcement will be diminished.  In fact, with few customers the traffic enforcement officers and speed camera contractors won't have much to do!